METHODS USED FOR CATARACT EVALUATION: DOCUMENTATION OF CATARACT
AND ITS EFFECT ON VISIONREFERENCES
Careful evaluation of the lens is an essential part of a
complete eye examination. Cataracts (Figs. 1, 2, 3, 4), in
particular age-related or senile cataracts, are among the most
common ocular finding in older patients and accounts for up to 50%
of blindness worldwide.1-3 In the United States, among Medicare
beneficiaries, cataract is the most common condition for which eye
care services are sought, accounting for 45% of visits to eye
doctors.4 In addition, cataract surgery is the most frequently
performed surgical procedure among 30 million Medicare
beneficiaries.4 With the prolongation of human life by advances in
medicine, increasing numbers of patients with age-related
disorders, especially cataracts, are to be expected.
Fig. 1. Cortical cataract using (A) direct illumination (B)
retroillumination. Cortical cataracts usually start in the lens
periphery and encroach into the visual axis (and interfere with
central vision) only in later stages.
HYPERLINK
"http://www.eyecalcs.com/DWAN/pages/v1/ch073b/002f.html" \t
"_blank" Fig. 2. Brunescent (brown) nuclear cataract using (A)
narrow beam slit illumination and (B) retroillumination. The
cataract is best seen by direct slit illumination. In
retroillumination, as seen in this image, the outline of the
nuclear cataract may be seen due to its increased refractive index,
although the cataract does not cast any shadows. Because of its
central location, this cataract may cause distortion of images
early in their development.
HYPERLINK
"http://www.eyecalcs.com/DWAN/pages/v1/ch073b/003f.html" \t
"_blank" Fig. 3. Posterior subcapsular cataract (PSC) using (A)
direct illumination and (B) retroillumination. PSCs usually start
centrally and extend toward the periphery. For this reason, they
interfere with visual function, causing glare disability early.
Fig. 4. Mixed cortical-nuclear-posterior subcapsular
cataract.
The primary purpose in managing a patient with cataract, as
stated in the American Academy of Ophthalmology's, Preferred
Practice Pattern for Cataract in the Adult Eye,5 is to improve
functional vision and the quality of life. Currently, the only
effective treatment of cataract is surgical removal with, in most
cases, insertion of an intraocular lens. The indications for
surgery are: when the cataract-impaired vision no longer meets the
patient's needs, and the anticipated benefits of surgery outweigh
the risks. No single test adequately describes the effect of
cataract on a patient's visual status or functional ability.5 It is
important then, that each clinician be well versed in the current
techniques of cataract assessment (such as cataract detection,
documentation, and monitoring methods), and the assessment of the
total effect of the cataract on the patient's daily needs and
quality of life (such as visual function and functional impairment
tests).
Noncataractous lens changes occur with normal aging6,7 and the
clinical difference between early cataract and age-related change
is often not clear-cut. The difference becomes more obvious as the
cataract progresses, hence the need for regular follow-up
examinations in the elderly. Most clinicians tend to vary in their
subjective grading of cataractous changes. For this purpose, newly
developed clinical cataract grading systems (see below) using
standard cataract photographs help clinicians to document the
progress of cataracts better and to compare their assessment with
that of other clinicians. These grading systems are especially
useful in clinical research studies as well as for publication of
clinical reports.
Once a cataract is diagnosed, the clinician should determine its
overall effect on visual function and the well-being of the
patient. These assessments will serve as the basis for a decision
whether to recommend any treatment such as cataract surgery. A
careful inquiry should be made into the patient's daily,
occupational, leisure, and social activities and document any
cataract-related impairment. The recent introduction of functional
disability measures, such as the NEI VFQ-25 and VFQ-14 tests (see
below), allow more objective and accurate measures of the
functional disability caused by a cataract on a particular
patient.
This chapter aims to summarize currently available methods for
the evaluation of cataracts for clinicians and to describe new
promising methods being developed and tested. Tremendous
technological advances in the field of computers, photography, and
imaging in the last 10 years have revolutionized ophthalmology,
especially in the diagnosis and treatment of eye disorders
including cataracts. Developments in recent years have helped
standardize various methods used for the documentation and
monitoring of cataracts as well as the documenting the effect of
cataracts on a patient's visual function and quality of life. Table
1 gives a list of these methods, which are then discussed in detail
in the text.
Table 1. Methods used in Cataract Evaluation: Documentation of
Cataract and Its Effects on Vision
A. Visual acuity/function tests
1. Snellen/ETDRS acuity charts or projectors
2. Glare and contrast sensitivity tests
3. Potential acuity tests
a. Pinhole Aperture
b. Entoptic Phenomenon
c. Macular function tests: Potential Acuity Meter (PAM),
clinical interferometers
4. Tests for refractive distortions in the lens: Resolution test
target projection ophthalmoscope (acuityscope, Oqual)
B. Functional Impairment/Quality-of life tests
1. NEI VFQ25
2. VF14
3. Others: Short Form 36 (SF-36), Activities of Daily Vision
(ADV), Sickness Impact Profile
C. Clinical examination and documentation of physical lens
changes
1. Clinical examination with hand-held light, slit-lamp
biomicroscopy and ophthalmoscopy; and accessory devices
2. Standardized clinical grading and photographic systems
(comparing a patient's cataract with standard photographs)
a. Lens Opacities Classification System (LOCS) III Clinical and
Photographic Grading system
b. Wisconsin Clinical and Photographic Cataract Grading
system
c. Wilmer Clinical and Photographic Cataract Grading system
d. Oxford Clinical Cataract Grading system
e. Age-Related Eye Disease Study (AREDS) Cataract Grading
System
f. Other systems such as the Japanese CCRG Cataract Grading
system and the World Health Organization Cataract Grading
System
3. Specialized slit-lamp photography/imaging
a. Scheimpflug slit lamp imaging and densitometry systems
b. Retroillumination imaging and area analysis systems
c. Others: Laser slit lamp, sequential color imaging and
analysis
D. New methods under development:
1. Quasielastic or dynamic light scattering (QELS or DLS)
2. Magnetic resonance imaging (MRI) and nuclear magnetic
resonance (NMR) spectroscopy
3. Wavefront technology
4. Raman spectroscopy
5. Autofluorescence
6. Optical coherence tomography
Back to TopMETHODS USED FOR CATARACT EVALUATION: DOCUMENTATION
OF CATARACT AND ITS EFFECT ON VISION
VISUAL ACUITY TESTING
Snellen Charts and Projectors
Since its introduction in 1862, the Snellen test chart has been
the clinically preferred standard used to measure visual acuity,
and thus also used initially to assess the effect of cataracts on
visual function. It remains the gold standard used to measure
minimum separable and legible acuity,8 to measure the effect of any
abnormal state of the eye, as well as for measurement of the
effectiveness of medical and surgical intervention in diseased
states of the eye.
Recently, the need to be able to obtain measurements of visual
acuity values that can be used for statistical analysis for
research studies has led to further standardization and
modifications of the Snellen chart. One such modification for such
purposes is the Early Treatment Diabetic Retinopathy Study or ETDRS
version.9 It was designed to have a geometric progression of the
letter size of test letters and standardized lighting of the chart.
Visual acuity scores are expressed as the logarithm of the minimal
angle of resolution (LogMar), which is linear, meaning, it
decreases by 0.1 unit for each lower line on the chart. Most
current clinical eye research studies use the ETDRS visual acuity
chart measurements as a major end point.
As recently as four decades ago, as a result of high
complication rates, cataract surgery was deferred until a patient
did not have much to lose in terms of visual acuity, should
intraoperative or postoperative surgical complications occur.
Patients were advised to wait until their cataract was ripe and
their Snellen visual acuity dropped down to 20/80 or worse before
contemplating surgery. With the development of safer techniques
such as the intracapsular cataract extraction (ICCE) technique
using the Cryoprobe, and later the extracapsular cataract
extraction (ECCE) technique using automated irrigation-aspiration
(I/A) devices and implantation of an intraocular lens (IOL), the
indication for cataract surgery was lowered to a Snellen acuity of
20/40 or worse coupled with anticipated improvement of vision. In
most states, a minimum requirement for an unrestricted license to
drive a vehicle was a visual acuity of 20/40 and a patient could
potentially lose his/her driver's license should his/her Snellen
acuity drop below this level. Hence, the cataract needed to be
removed to allow the patient to continue driving.
Currently, with the development of even safer techniques such as
phacoemulsification followed by implantation of an IOL (resulting
in lower complication rates and superb postoperative vision), the
indications for surgery have further changed. The American Academy
of Ophthalmology (AAO)'s Preferred Practice Pattern for Cataract in
the Adult Eye states that there is no single test that adequately
describes the effect of cataract on visual status and functional
ability. Cataract/IOL surgery is therefore indicated when the
vision no longer meets the requirements of the patient, and when
the expected surgical benefits outweigh the risks. Hence, not only
the level of visual acuity but other considerations such as the
ability to perform daily tasks of living, ability to perform work
or avocations, and good quality of life in general are equally
important in deciding when cataract extraction should be
performed.
Snellen-type visual acuity tests measure the eye's ability to
resolve fine detail at high contrast but do not adequately describe
the ability to see large but low contrast patterns such as faces or
nearby objects. A cataract may affect the results of the Snellen
acuity test minimally, and yet a patient may already experience
difficulties in daily activities such as driving or walking
especially in bright sunlight or at night, or have difficulty in
their line of work, such as lawyers or accountants who need to read
fine print accurately or architects who need to see fine lines in a
line drawing.
Recently, a number of tests have been proposed to document
changes in visual function that are not detected by the Snellen
visual acuity test. Among the important ones are contrast
sensitivity testing and glare testing.10 A number of glare and
contrast sensitivity tests have been devised and continue to be
further refined, especially with the use of computer monitors.
Potential acuity tests such as the Guyton-Minkowsky Potential
Acuity Meter (PAM) and clinical interferometers are used mainly to
determine macular function independent of media opacities such as
corneal, lens, and vitreous opacities. The Resolution Test Target
Projection Ophthalmoscope was developed to document distortions in
vision that are not adequately determined by the Snellen acuity
test. The following are discussions of each of these.
Clinical Contrast Sensitivity Tests
Contrast sensitivity is a measure of the amount of contrast
required to detect or recognize the target. Cataracts increase
intraocular light scatter causing a reduction in retinal image
contrast, and a subsequent decrease in contrast sensitivity. In
general there are two types of devices used for contrast
sensitivity testing.10 The traditional devices consist of sine wave
gratings, which are patterns of alternating light and dark bars
produced and controlled by computers, wherein the spatial
frequency, contrast luminance, field size, and attenuation at the
edge of the field are either modifiable or fixed.11 Examples of
this are, among others, Optec 3500 Vision Tester (Stereo Optical
Co., Inc., Chicago, IL), B-VAT PC system (Medtronic Solan Co.,
Inc., Jacksonville, FL), Smart System 20/20 (M & S
Technologies, Inc., Chicago, IL), and CSV-1000E Contrast
Sensitivity Instrument (VectorVision, Inc., Arcanum, OH). The
second type of devices are based on photographically reproduced
sine wave gratings (such as the Arden plates and the Vistech vision
contrast test system) or variable contrast optic types (such as the
Regan letter chart, the Vistest picture test, the Pelli-Robson
letter chart, and the Melbourne edge test),12Hess and Woo13 first
reported contrast sensitivity function loss in patients with
cataracts. They suggested that early cataracts cause high-spatial
frequency loss, whereas more advanced cataracts produce both high-
and low-spatial frequency losses. Drews-Bankiewicz et al.14
documented correlations between early nuclear cataracts with loss
of contrast sensitivity in the intermediate and high spatial
frequencies (4 to 16 cycles per degree). Lasa et al.15 found
significant contrast sensitivity loss only in advanced cortical and
posterior subcapsular cataracts. This loss was also correlated with
decreased Snellen visual acuity. Adamsons et al.16 however, found
that contrast sensitivity scores were lower for all patients with
lens opacities than for clear lenses at high frequencies only, and
all lens opacity groups scored similarly with each other. Recently,
Kuroda et al.17 found a significant correlation between increasing
lens density (using the Scheimpflug camera to measure lens density)
and loss of contrast sensitivity in nuclear and cortical cataracts
at 12 cycles per degree.
Glare Testing
Glare sensitivity refers to the change in visual function caused
by the presence of a glare (light) source in another part of the
visual field. In general, glare can be divided into either
discomfort or disability glare. Discomfort glare causes a
photophobic sensation without measurable effects on visual
function, whereas disability glare causes reduction in visual
function because of the presence of a bright light source.18
Disability glare is a specific type of glare caused by light
scattered by the ocular media and is the type of glare that is
commonly tested by the devices that have been developed to document
glare.
Glare testing is helpful in documenting glare disability
especially for those patients who complain of glare when driving at
night and having difficulty with oncoming headlights, or having
difficulty reading road signs when there is bright sunlight.19
Examples of these devices include the Brightness Acuity Tester or
BAT (Marco Ophthalmics, Jacksonville, FL), the CSV 1000HGT
(VectorVision Inc., Arcanum, OH), and the Optec 3500 Vision tester
(Stereo Optical Co., Inc, Chicago, IL).
Potential Acuity Tests
Pinhole acuity8 is easy and quick to perform and is often used
when visual acuity is less than 20/20. It can give useful
information, especially if one obtains a good reading. However, a
poor pinhole acuity result does not necessarily mean poor macular
function because the decrease in retinal illumination produced by
use of the small aperture degrades the image as it reaches the
macula. Recently, Melki et al.20 studied a standardized method of
pinhole testing and found it relatively reliable in estimating the
visual outcome after uncomplicated cataract surgery without
coexisting disease. Another version of the pinhole test described
by Hofeldt and Weiss21 uses a specially illuminated near card
(Mini-Illuminated Near Card, Gulden Ophthalmics, Elkins Park, PA).
They found that it was useful in predicting postoperative acuity in
cataractous eyes with comorbid disease.
Various tests of visual discrimination such as the ability to
perceive light coming from various quadrants of the visual field
(light projection), to perceive the orientation of the streak from
a Maddox rod, and to discriminate between two light sources versus
one source of light do not depend on macular function. Even color
perception is not a valid measure of macular function, since cone
receptors are present in the peripheral retina. The
electroretinogram and visual evoked potential are not specific for
macular function. These tests do not correlate well with visual
acuity in the presence of amblyopia or macular degeneration.
The entoptic phenomenon test is traditionally used in mature
cataracts to test for gross retinal function. This phenomenon is
created by sweeping a small light source such as a hand-held light
from side to side against the eye, usually shining the light
through the lower or upper eyelid pointing toward the macular area.
The strips of photoreceptors beneath the retinal blood vessels do
not have time to adapt when the shadows of the blood vessels move
rapidly from side to side, and the shadows become visible as a
branching pattern embracing the macula. Perception of the vascular
shadows described as seeing veins or vines is a time-honored test
for visual function in mature cataracts, but it is not specific for
macular function and therefore does not correlate well with
postoperative visual acuity in the presence of localized macular
disease.
Entoptic phenomena are poorly quantifiable and it is difficult
to assign a numeric level of expected visual acuity based on the
response of a patient. Using the bluefield entoptoscope, Sinclair
and coworkers22 tested 136 eyes prior to uncomplicated cataract
surgery. They obtained up to 94% correct prediction of good foveal
function and at least 75% correct prediction of poor foveal
function. However, Murphy,23 showed less success with bluefield
entoptoscopy, especially in dense cataracts. False-positive
predictions of potential acuity have also been reported in the
presence of macular disorders.
In the presence of dense mature cataracts, if there is serious
doubt as to the status of the retina or optic nerve, and thus the
outcome of cataract surgery, it may be necessary to perform
additional tests such as ultrasonography and computed tomography
(CT) scanning.
The following two devices were designed as macular function test
devices independent of media opacities in the cornea, aqueous,
lens, or vitreous: the potential acuity meter and the clinical
interferometer. In actual practice, however, both are effective
only in mild to moderate cataracts where there is no macular
dysfunction or disease state. However, they have proven to be very
useful within these limits, esp. in predicting surgical outcomes in
questionable cases, as discussed below.
The Guyton Minkowski PAM (Marco Ophthalmic, Jacksonville, FL)24
projects a standard Snellen chart through a 0.1-mm diameter
aperture. This projected chart is directed through small windows in
the cataract onto the macula and the patient reads the chart from
20/400 to 20/20. It uses a low-cost incandescent lamp and has a
field of vision of 6 degrees.
In the clinical interferometer, beams of coherent light from two
point sources are directed to the clearest area of the lens into
the retina. Interference fringes in the macula are formed wherever
the two beams overlap and by varying the width of the interference
fringe pattern, visual acuity can be determined with the Snellen
equivalent from 20/660 to 20/20, independent of the optics of the
eye. Clinical interferometers use either red helium neon laser
light or white light from an incandescent source such as a Xenon
Halogen lamp, with a field size ranging from 1.5 degrees to 8
degrees. An example of this is the Heine Lambda 100 hand-held
Retinometer (Lombart Co., Norfolk, VA).
In actual use, both the PAM and clinical interferometers
underestimate as well as overestimate the potential vision in
certain conditions.25 The PAM tends to underestimate the potential
acuity in advanced, dense cataracts without any clear zones or
openings through which to project the Snellen chart. This is not
necessarily a disadvantage in such cases, because the actual
postoperative acuity may be much better than predicted, to the
pleasant surprise of both patient and surgeon.
On the other hand, clinical interferometers tend to overestimate
potential vision in patients with macular disease, such as macular
degeneration and amblyopia. In this device the retina has to
distinguish the two overlapping coherent light beam fringes as two
distinct lines. There may happen to be a few good cones that are
situated at just the right position to distinguish these two lines
as separate and give a good potential acuity reading. However,
there may not be enough cones to recognize a Snellen letter of the
equivalent level of acuity. This may result in an overestimation of
the potential vision resulting in an unexpectedly poor
postoperative vision.
Both the PAM and clinical interferometers are therefore useful
to determine the potential acuity and possible outcome of surgery
in patients with mild to moderate cataracts. However, they perform
poorly in dense opacities, which they cannot penetrate, so that the
patient cannot perceive the test objects. Caution should be used
when interpreting results from patients with macular disease and
amblyopia.2628Tests for Refractive Distortions in the Lens
RESOLUTION TEST TARGET PROJECTION OPHTHALMOSCOPE.
Localized refractive distortions may occur during the
development of a cataract, which may distort vision to such an
extent that the patient is incapacitated. This may be of major
importance to patients with special visual needs, such as surgeons,
accountants, bookkeepers, and architects. A device to document
these distortions by projecting a series of parallel lines into the
retina, which decrease in size to correspond to a Snellen
equivalent numerical unit of measure, was developed by the U.S. Air
Force29 and modified by Lobo and Weale.30 The Oqual (Zeiss Meditec,
Dublin, CA) is mounted on an ophthalmoscope and the examiner views
the projected lines in the fundus, grades the degradation of the
projected chart image, and documents the degradation of the image
using the Snellen equivalent unit.
Recently, wavefront technology,17 using the same idea but more
sophisticated methods, promises to be a useful tool in documenting
and quantifying the existence of these refractive abnormalities. If
the ocular media do not appear opaque enough to explain visual loss
(in the presence of an otherwise normal cornea, retina and optic
nerve), such refractive lens changes may be responsible and may
indicate the need for lens extraction (see under New Methods).
FUNCTIONAL IMPAIRMENT/QUALITY-OF-LIFE TESTS
During careful history taking, the patient usually volunteers
typical complaints associated with cataracts, such as painless,
progressive loss of vision, difficulty seeing in bright sunlight or
at dawn and dusk, increased glare from incoming headlights when
driving at night, and difficulty reading road signs. As a result of
the vast improvement of cataract and intraocular lens surgical
techniques, with the resultant decrease in the complication rate of
cataract surgery, criteria for cataract surgery have changed in
recent years. These changes allow for earlier surgery if the
cataract interferes with the patient's occupation and other
activities, combined with the determination (using the potential
vision tests described above) that the patient would indeed benefit
from cataract surgery. Hence, any visual difficulty with daily
activities, as well as the patient's occupational, leisure and
social activities, should be inquired about and noted down. A brief
statement should be entered into a patient's chart to summarize
these discussions, such as: The patient's visual function has
decreased to a level that interferes with the patient's ability to
carry out normal daily activities. In addition, given the mental
status and physical abilities of the patient, there is reason to
expect some surgical benefit in function and personal comfort
and/or activity. This can be added to the consent form also, with
the statement that possible complications with eye surgery were
discussed with the patient. These discussions with the patient and
their proper documentation in the chart play a crucial role in the
decision making in cataract surgery as well as in litigation when
there is a problem as to the outcome of cataract surgery.
Recent research has also revealed the interesting observation
that in older patients, cataracts may play an important role in the
causation of car accidents,31 falling accidents resulting in
fractures,32 and mortality and functional decline.33 Hence, earlier
visual rehabilitation through cataract/IOL surgery is becoming not
only an option but a necessity.
Recently, more detailed and standardized methods of documenting
these visual and functional disabilities have been developed,
especially for clinical research purposes.
National Eye Institute Visual Function Questionnaire: 25 Items
(VFQ-25)
The National Eye Institute Visual Function Questionnaire: 25
Items (VFQ-25) was developed under the sponsorship of the National
Eye Institute with the goal of creating a survey that would measure
the dimensions of self-reported, vision-targeted health status that
are most important for persons with chronic eye problems.34 The
survey measures the influence of visual disability and visual
symptoms on patients' daily visual functions as well as other
health domains such as emotional well-being and social functioning.
It consists of a base set of 25 vision-targeted questions
representing 11 vision-related constructs, plus 1 general health
rating question. It takes approximately 10 minutes for an
interviewer to administer. There is also a self-administered
format. The VFQ-25 forms (revised 2000), as well as additional
information, can be obtained from the website of the National Eye
Institute (www.NIH.GOV).
Visual Function Questionnaire14 Items (VF-14)
The Visual Function Questionnaire14 Items (VF-14) was developed
by a team from the Johns Hopkins University Hospital and Georgetown
University Hospital as a measure of functional impairment caused by
cataract and provides information not conveyed by visual acuity or
a general measure of health status. It has also been shown to be
sensitive and reproducible.35,36Others test such as the Short
Form-36 Items (SF-36),37, 38 Activities of Daily Vision (ADV),39
and Sickness Impact Profile (SIP)40 are other measures of
functional impairment that have been used in clinical research on
cataract patients.
OPHTHALMOLOGIC CLINICAL EXAMINATION
Hand-Held Light Examination
Field eye examinations, such as done in epidemiologic studies
and surveys, can be facilitated by the use of a hand-held light in
conjunction with a head-mounted or spectacle-mounted magnifying
loupe, ideally inside a darkened room (to promote mydriasis).
Because one is looking mainly for visually significant cataract,
information obtained with this method, together with a Snellen
visual acuity measurement, is the usual end point for these
studies. More sophisticated equipment such as an ophthalmoscope and
a hand-held slit lamp biomicroscope will be needed if a more
precise classification and grading of the cataract is needed.
Ophthalmoscopy
The use of the direct ophthalmoscope's built-in +10 lens both
with direct illumination and retroillumination using the light
reflected from the fundus allows the detection of opacities in the
lens, especially in field situations. However, the two-dimensional
monocular view, the limited magnification and the short working
distance makes ophthalmoscopy inadequate for thorough cataract
evaluation. The indirect ophthalmoscope may also be useful in
making a gross assessment of the clarity of the media as one looks
at the fundus. Experienced clinicians performing indirect
ophthalmoscopy can often gauge the amount of vision loss from the
haziness in the ocular media that they observe. These are gross
assessments, however, and are inferior to information obtained from
the slit-lamp biomicroscope.
Other Accessory Devices
Ultrasonography (A and B scan) is used routinely to obtain
measurements (axial length of the eye, anterior chamber depth, lens
thickness) needed for intraocular lens power calculation. In
completely opaque, mature cataracts, it is also useful in
determining the status of the vitreous and retina. Specular
microscopy of the lens epithelium (originally developed for the
cornea) is also being used to study age-related changes41 as well
as changes in the lens epithelium in special cataracts such as the
myotonic dystrophy cataract.42 Confocal microscopy (originally used
in vivo on the cornea) is also being tried on the lens in vivo, and
promises to be useful in the future.
Slit-Lamp Examination
The optimum way of examining the lens clinically is using the
slit-lamp biomicroscope through a widely dilated pupil. This
instrument provides a three-dimensional view of the lens. One can
focus on specific areas of the lens from different angles, and at
the same time vary the location, direction, and intensity of the
illuminating beam independently. The following techniques can be
used: (1) direct focal illumination using either a wide or narrow
beam; (2) retroillumination; and (3) others including specular
reflection, indirect illumination, diffuse illumination, and use of
the light reflected from the iris and posterior capsule.
In direct focal illumination, the slit beam is positioned
directly on the area being studied. One can use various
configurations of the slit beam, but the most useful way is a
narrow beam to produce the cross section of the lens. The light is
slightly attenuated by passage through the cornea and undergoes
refraction, reflection from the surfaces (zones of discontinuity),
scattering, absorption, polarization, and fluorescence.
Vogt43 first used slit-lamp biomicroscopy using the narrow beam
(0.1- to 0.5-mm thick) to study the lens and described the various
zones of discontinuity denoting the layers of the lens fiber cells
laid down during a patient's lifetime. These zones represent
lenticular growth periods and aid in determining time of origin of
opacities. Because of the narrow depth of focus of the
biomicroscope, the examination begins at the anterior capsule and
gradually focuses deeper to see the various layers. The zones of
discontinuity are the shells of the nucleus and cortex that are
concentric to each other (Figs. 2, 5, 6, and 7).
Fig. 2. Brunescent (brown) nuclear cataract using (A) narrow
beam slit illumination and (B) retroillumination. The cataract is
best seen by direct slit illumination. In retroillumination, as
seen in this image, the outline of the nuclear cataract may be seen
due to its increased refractive index, although the cataract does
not cast any shadows. Because of its central location, this
cataract may cause distortion of images early in their
development.
Fig. 5. Scheimpflug slit-lamp photographic images of: (A)
normal; (B) cortical; (C) nuclear; and (D). Posterior subcapsular
cataracts. The Scheimpflug method allows for slit-lamp imaging
wherein the entire lens is in focus. The gray scale on the left of
each image was built-in to aid standardization of the image during
densitometry (for objective quantification of the opacities).
Fig. 6. Automated densitometric analysis of a digital
Scheimpflug slit image of a normal 50-year-old lens. Because of the
characteristics of the lens layers, the location of the cortex and
nucleus can be automatically detected by special software, and with
automated densitometry, mean optical density values can be easily
and quickly obtained for analysis. Thus, automated detection,
classification, and grading may be possible. Note the concentric
layers of lens fibers in the cortex and the nucleus.
Fig. 7. The Lens Opacity Classification System II (LOCS II)
photographic grading standards. N = Nuclear photographs. Stage 0 =
normal; IIII = various stages of nuclear cataract. For nuclear
opalescence, the average opalescence across the entire nuclear
region is used. An opalescence that is less than or equal to
Photographic Standard 0 = grade 0; if the opalescence is less than
or equal to Standard I, the grade is 1, and so on. For Color
Grading of the nucleus, only the N2 standard is used. ( ). P =
Posterior subcapsular photographs. 0 = normal; IIII = various
stages of posterior subcapsular cataracts (Chylack LT, Leske MC,
McCarthy D, et al: Lens opacities classification system II [LOCS
II]. Arch Ophthalmol 107:991, 1989. Copyright 1989, American
Medical Association with permission.)
The lens nucleus can be divided into embryonic, fetal,
infantile, and adult. The embryonic nucleus is a clear central zone
found between two cotyledons that make up the fetal nucleus. These
cotyledons are similar to mirror halves of a peanut. The infantile
and adult nuclear zones lie over this. The sutures classically
described as y-shaped are points of convergence of the anterior and
posterior tips of the lens fibers and may vary in shape.4445 Recent
research suggests that abnormalities in suture shape, which reflect
abnormalities in lens fiber development and/or maturation, may
signify a predisposition to the development of cataracts later in
life.46 The nucleus increases in thickness and density with
increasing age. The lens cortex lies between the nucleus and the
capsule and varies in thickness. It is usually clear although some
isolated dots may be present normally. The lens capsule is thicker
in the front than in the back and is the basement membrane of the
lens epithelium isolating the lens from the rest of the
eye.46Direct focal illumination using the narrow beam is useful not
only in studying the anatomy of the lens but also in examining
minute opacities to localize their position and estimate their
size. Nuclear cataracts tend to scatter light so that narrowing the
beam prevents the washout effect and allows examination of details
as well as enhances patient comfort. The Scheimpflug camera (see
below) was developed to increase the depth of focus of images
obtained with the slit lamp (Figs. 5 and 6) and is ideal for
documenting slit images of the lens, and especially nuclear
cataracts.4748 It uses a fixed narrow beam and obtains reproducible
images in which the whole-lens thickness is in focus. These images
can be examined by densitometry for statistical comparison with
other images of the same patient taken over time to document and
track changes.
The broad beam is useful for examining cortical cataracts
especially spokes and water clefts (Fig. 2), which tend to be large
and irregular. It is also useful in posterior subcapsular
cataracts, particularly in the early stages, which can be detected
by irregular grainy reflection from the otherwise mirror-like sheen
of the posterior capsule. Abnormalities in size and position of the
lens are also assessed by using either broad or narrow beams.
Retroillumination uses the light reflected from the fundus to
highlight opacities. This is very useful in examining cortical and
posterior subcapsular cataracts (Figs. 1, 3, 7, and 8). However,
some cortical water clefts and early posterior subcapsular
opacities may not be easily seen with retroillumination if they are
not dense enough to cast shadows or only refract the
retroilluminate light; these can be best seen with the broad beam.
Using retroillumination on nuclear cataracts does not give much
information because these usually do not cast shadows. However, as
seen in Figure 2B, a change in the refractive index can be seen
toward the center. Usually, the outline of the nuclear cataract can
be seen as a result of a magnifying lens artifact. This is also
easily observed during direct and indirect ophthalmoscopy. Several
retroillumination cameras (modified slit lamps with the slit beam
fixed for retroillumination and depolarizers built in to remove
corneal reflex from the image) have been developed and are being
used for documenting cortical and posterior subcapsular
cataracts.49, 50
Fig. 3. Posterior subcapsular cataract (PSC) using (A) direct
illumination and (B) retroillumination. PSCs usually start
centrally and extend toward the periphery. For this reason, they
interfere with visual function, causing glare disability early.
Fig. 8. Follow-up retroillumination photographs of an eye with a
cortical cataract, obtained at various intervals. A. First visit
(1-21-87); B. 1 year later (1-27-88); C. 22 months later
(11-14-88); D. 28 months later (12-4-89); E. 35 months later; F. 41
months later (6-18-90). With such photographs one may be able to
plot the progression rate of a cortical cataract, and aid in
performing longitudinal studies. Note the central opacity, which is
out of focus and represents a small posterior subcapsular
cataract.
The surfaces of the anterior and posterior capsule may be
studied using specular reflection. A bright reflex or shagreen is
usually seen as the beam is moved from side to side across the
surface of the lens. When examining the lens epithelium, for
example, this can occur when the observer focuses on the lens
surface and the angle of incidence of the beam is equal to the
angle of reflection. The clinical specular microscope developed for
the corneal endothelium has also been especially adapted for study
of the lens epithelium.41,42In summary, slit-lamp biomicroscopy is
the most useful method for clinically detecting and localizing lens
opacities, determining their extent and density, and monitoring
changes over time.
SLIT-LAMP PHOTOGRAPHY OF THE LENS AND GRADING OF CATARACTS
Slit-lamp photography has been used to document anterior eye
segment disorders, including abnormalities and opacities in the
lens, since camera attachments to slit lamps became available.
Variables to consider in its use in the lens include the limited
depth of field, the variabilities in light intensities with the
slit beam, limits of magnification with corresponding limits on the
area that can be photographed, limits in the angle of the slit beam
used, and limits imposed by pupil size. The advent of digital
cameras has made lens documentation even more useful. The examiner
can check the image quality while the patient is still on the slit
lamp. Patients and their families are often grateful to see the
lens pathology during the consultation. This facilitates their
active participation and cooperation in any discussion and decision
making, especially if intervention is required. These images can
also be sent electronically to distant tertiary centers for quick
consultations or stored in disks immediately for easy transport as
well as inclusion in a patient's chart.
Recently, cataract classification systems have been developed
that use carefully selected slit lamp photographs of cataracts as
standards for comparison with the patient's cataracts. These
include the following: the Lens Opacities Classification System
(LOCS) version I,51 version II (Fig. 7),52 and version III,53 the
Wisconsin Cataract Grading system,54 the Wilmer Cataract Grading
System system,55 the Oxford Cataract Grading System system,56 and
the Age Related Eye Diseases Study (AREDS) Cataract Grading
System.57,58 These systems are similar in that they provide lens
photographs or films showing various severities or grades of
cortical, nuclear and posterior subcapsular cataracts to be used as
standards, which a clinician can then compare to the patient's
cataract as seen directly on the slit lamp. For nuclear cataracts,
slit photographs of the lens are used, and for cortical and
posterior subcapsular cataracts, retroillumination photographs are
used. Instructions are provided for the clinical use of the
systems, specifically what borders or cutoff points are to be used
for using each standard image. Figure 7 shows the LOCS II standard
photographic plate. Another recently described system that was
designed to be simple and easy to use on the slit lamp, especially
for field cataract assessments, is the World Health Organization
cataract grading system.59Another way of using these classification
schemes is to obtain slit-lamp photographs of a patient's cataract
following a specific photographic procedure described by the
authors of each system. These photographs can later be read by a
reading center or by the clinician, comparing the patient's
photograph with the standards. With the advent of digital cameras
and the possibility of automating most of the photographic
processing of the images, this method may become easier with time.
At present, it is only used in clinical research studies and is
expensive, cumbersome, and impractical for regular clinical
use.
Modified Slit-Lamp Photography
Several instruments have been developed to convert the cataract
image into numbers in a more sophisticated way. These use either
35-mm film or digital cameras to capture the images, which are then
digitized onto a computer. In nuclear cataracts, densitometric
analysis of the cataract image is then performed to convert the
values into optical density units. In cortical and posterior
subcapsular cataracts, the area occupied by the cataract can be
measured in square mm. These values can then be analyzed
statistically.
SCHEIMPFLUG CAMERAS.
Slit lamps modified along the Scheimpflug principle (Fig.
9)47,48 can obtain lens images with enough depth of focus so that
the entire anterior chamber from the cornea to the posterior
capsule of the lens are in sharp detail (see Figs. 5 and 6).
Usually, the slit beam is set at 45 degrees away from the image
plane of the camera, which is focused on the lens parallelepiped
(Fig. 9). The available charged-coupled device (CCD) cameras, which
are supplied with computer hardware and software, use a slit beam
with a fixed thickness and level of illumination and obtain images
that are reproducible, easily stored in portable disks, and easily
analyzed using built-in densitometers. The operator can manipulate
the software to designate which area to analyze and the average
density is expressed either in optical density units or gray
scale/intensity values. Among those available currently are the
Nidek EAS-1000 camera (Scheimpflug Unit, Nidek Tech, Inc.,
Pasadena, CA) and the Oxford Scheimpflug camera (Marcher
Enterprises, Hereford, U.K.). Figures 5 and 6 show Scheimpflug
images of a normal lens and various types of cataracts obtained
using a Scheimpflug camera, and two ways to analyze these images to
obtain the mean density of various areas within the lens.48,50
Longitudinal studies, such as for following the progression of
nuclear cataracts, can thus be conducted in an objective and masked
fashion.48
Fig. 9. Scheimpflug principle: When an object plane (slit beam),
objective plane (camera lens) and image plane (film plane)
intersect, the result is a photograph with a deep depth of focus.47
On most Scheimpflug slit cameras, the slit beam and charged-coupled
device (CCD) camera are at 45 degrees angles to each other, and the
anterior eye segment (cornea to lens posterior capsule) is in focus
in the resultant image.
RETROILLUMINATION CAMERAS.
Retroillumination cameras49,50 obtain images of cortical and
posterior subcapsular cataracts as shown in Figures 1B, 7, and 8.
These are useful for both cross-sectional and longitudinal studies.
Various manual or automated methods have been developed or are
being developed for the analysis of these images to determine the
size of opacities. Usually, an artificial mask with a chosen
diameter is used to standardize the area of interest in the image,
and either a percent area or area in square millimeters is
determined. Because of the variability of the background light (the
images are shadows of the cataracts using light that is backlighted
from the retina or optic nerve), densitometry is unreliable. Among
the methods used include computer planimetry, counting boxes,
manual and automated edge detection, and automated area analysis.
This method is also being used to study IOL decentration and
posterior capsular opacification after cataract surgery. Examples
of this type of device are the Nidek EAS 1000 retroillumination
unit (Nidek, Pasadena, CA), Oxford retroillumination camera
(Marcher Ltd., Hereford, UK), and Topcon CTR (Topcon Medical
Systems, Paramus, NJ).
Other Specialized Slit-Lamp Imaging Systems
Various innovators have devised specialized slit-lamp cameras
with improved acquisition and image analysis of cataracts. The
Laser Slit Lamp (Bausch and Lomb Surgical, San Dimas, CA)60 was
developed to measure the density of nuclear cataracts. The
Sequential Color Cataract Imaging System,61 provides three lens
images for analysis: (1) saggittal sections of the lens, (2)
retroillumination images, and (3) images of opacities obtained by
direct or side illumination. These devices promise to be useful in
clinical studies of the lens and cataracts but may need further
development and standardization.
In summary, the use of slit and retroillumination imaging
coupled with a computerized analysis system is presently the state
of the art in objectively measuring cataracts. However, because of
the expensive equipment, the extra time, space, labor, and effort
required, and limited practical clinical use, these systems are
presently used mainly in clinical research settings. The clinical
judgment of the ophthalmologist based on the history and eye
examination remains the standard in clinical practice.
NEW METHODS UNDER DEVELOPMENT
New technologies are being applied in the study of cataracts and
show great promise for providing new insights into the cataract
problem, as well as new devices that a clinician can use with
cataract patients in the clinic. In this section, the following
devices are discussed: quasielastic or dynamic light scattering
(QELS or DLS), magnetic resonance imaging (MRI), spectroscopy,
wavefront technology, Raman spectroscopy, autofluorescence, and
optical coherence tomography (OCT). These new noninvasive
techniques add to the armamentarium available to cataract
researchers. Steps being taken to develop these devices for
noninvasive clinical use in vivo aim for their possible use on
patients for diagnostic purposes in the near future.
Quasielastic or Dynamic Light Scattering
The intense scattering of light by a cataract arises from a
change in the interaction and organization of the constituent
particles/lens proteins mediated by lenticular stress. These
changes in interaction and organization are reflected in altered
motional dynamics (translational and rotational diffusion) of the
lens proteins in the cytoplasm. The investigation of lens protein
dynamics is being successfully accomplished by the use of DLS.62DLS
is an established laboratory technique to measure the average size
or size distribution of microscopic particles (3 nm to 3 m)
suspended in a fluid medium in which they undergo random Brownian
(or thermal) motion. Intensity of light scattered by the particles
from a laser beam passing through such a dispersion will fluctuate
in proportion to the Brownian motion of the particles. Because the
size of the particles influences their Brownian motion, analysis of
the fluctuations in scattered light intensity yields a distribution
of the diffusion coefficient(s) of the suspended particles from
which average particle size or particle size distribution can be
extracted. In these experiments, visible light from a laser is
focused into a small scattering volume inside a sample. The
scattered light is collected using a photodetector (photomultiplier
tube or an avalanche photodiode) and is processed via a correlator
that yields a time correlation function (TCF). For dilute
dispersions of spherical particles, the slope of TCF provides a
quick and accurate determination of the particle's translational
diffusion coefficient. This can then be correlated to the size of
particles in the solution via a Stokes-Einstein equation, provided
the viscosity of the suspending fluid, its temperature, and its
refractive index are known.62-66 The distribution of particle size
obtained from the tissues examined (such as the lens) can then be
plotted (Figs. 10 and 11).
Fig. 10. Data obtained from a cold-induced cataract study using
calf eyes. Data on the left show the size distribution of lens
proteins as the cold cataract appears (as the temperature of the
calf lens is lowered) showing the shift from small to large
molecular weight proteins (data obtained from the dynamic light
scattering [DLS] device) versus Scheimpflug slit-lamp images of the
same calf lens as the temperature is correspondingly lowered and
the cold cataract appears. The DLS device picks up a shift in
protein size much earlier than the Scheimpflug camera shows the
appearance of the cataract.66
Fig. 11. Comparison between distribution of particles in a
normal human lens versus a nuclear cataract obtained clinically (in
vivo) on patients using the NASA-NEI clinical dynamic light (DLS)
device.64,66Clinically, DLS can be used to study cataracts
noninvasively at the molecular level. It is safe and fast to use in
early cataract evaluation because of the very low laser power
(50100 W) and short data acquisition time (5 seconds). In a
cold-induced cataract model experiment in which the cataract was
simultaneously monitored with both the DLS device and Scheimpflug
camera (Fig. 10), the DLS picked up subtle changes in the lens
quicker (23 orders of magnitude earlier) than the Scheimpflug
camera.66 The DLS measures the Brownian motion of the crystallins
inside the lens. The major proteins that can scatter light in a
human eye lens are -, -, and -crystallins. The -crystallins are the
largest molecules (molecular weight, 106 daltons) and they induce
the greatest amount of light scattering in a DLS measurement. When
these protein molecules aggregate, they give rise to lens
opacities.
For clinical use, DLS probes can also be integrated with
slit-lamp, Scheimpflug, and autofluorescence instruments because of
the current modular design. Data obtained from patients in clinical
studies have shown good reproducibility.65,66 For clinical
purposes, it has been suggested that the log 10 mean particle size
be used as a clinical end point for this device.66 However, more
studies are needed to further understand the wealth of data
obtained with this device. Its application in eye research is just
being explored, and much information can be obtained not only on
the lens but also on the cornea,67 vitreous, and
retina.68,69Magnetic Resonance Imaging and Nuclear Magnetic
Resonance Spectroscopy of the Lens
As any other human tissue, the lens contains carbon and hydrogen
atoms in which protons spin around their nuclei in random
directions. On application of a magnetic field, these microscopic
magnets are aligned in a particular (north-south) direction (higher
energy state). On turning off the magnetic field, the microscopic
magnets return to their original random state (lower energy state).
The frequency of rotation is equal to the energy of a photon
(normally a known radio frequency) that would cause the nuclei to
flip between these two energy levels. This provides measurements of
relaxation rates between different energy states of the nuclei in
relation to the applied excitation photon field. Because they are
dependent on the hydrogen nuclei densities in the tissue, the
relaxation rate information can be translated into images.
MRI provides the ability to probe the chemical and metabolic
status of the lens noninvasively. Thus, in response to normal and
pathophysiologic conditions, areas such as lens metabolism, ion
concentrations, the state (bound versus free) of lens water, and
metabolite and macromolecular motional dynamics may be
investigated. Valuable biochemical and biophysical information
pertinent to the factors that govern lens transparency, and
conversely, the medical condition of the cataract can thus be
studied.
MRI has been used to image the eye but problems have been
encountered including poor resolution; limited access to the
surface coil and poor resultant magnetic signal (because of the
location of the eye within the bony structure of the orbit); motion
artifacts (caused by microsaccadic eye movements, breathing and
heartbeat); and the presence of high susceptibility gradients
around the eye. Recently, Lizak et al.70 used a special technique,
magnetization transfer constant enhancement (MTCE), to enhance the
lens image successfully and study diabetic and galactosemic animal
models of cataract, and have applied it to clinical use. MTCE takes
advantage of the magnetic interactions between water and
macromolecule hydrogen atoms.
Preliminary clinical studies suggest that cortical lens changes
can be better observed with unenhanced magnetic resonance images,
whereas nuclear lens changes are better observed by the addition of
the MTCE preparation pulse (Fig. 12). MRI, therefore promises to be
an imaging method independent of optical imaging that will allow
clinicians to monitor metabolic processes in the lens.70
Fig. 12. Sagittal section images of a nuclear cataract. M0:
Magnetic resonance image (MRI) of a patient's eye with a nuclear
cataract, taken in vivo and noninvasively. Scheimpflug: Image of
the same nuclear cataract (LOCS II nuclear opalescence grade 2)
taken using a Zeiss Scheimpflug slit lamp camera (optical/digital).
Ms: MRI image of the same eye with magnetic transfer contrast (MTC)
enhancement (see text).
13C nuclear magnetic resonance (NMR) spectroscopy of the intact
lens, on the other hand, has provided information about the
production, turnover, and inhibition of sorbitol by aldose
reductase inhibitors. Proton NMR spectroscopy of 13C-labeled
metabolites offers the ability to monitor the reactivation and
dynamics of the hexose monophosphate shunt (HMPS), a pathway
important for the maintenance of the lens redox state, in real time
and noninvasively.7131P NMR spectroscopy allows the monitoring of
the phosphorus-containing metabolites, thereby permitting the
real-time assessment of lens tissue metabolic response to
pathophysiologic conditions. Important metabolites, such as
adenosine triphosphate, phosphomonoesters, and phosphodiesters may
be monitored. Furthermore, intralenticular pH may be measured.
However, no clear correlation between phosphorus metabolite levels
and lens clarity has been established to date, despite numerous NMR
and classic biochemical studies. This lack of correlation suggests
the importance of biophysical investigations aimed at the
interaction behavior and organization of the constituent lens
proteins in the cytoplasm, the macromolecular entities responsible
for light scattering associated with cataract.72NMR spectroscopy
may be viewed as an important adjunct to the better established
laser light scattering studies of the lens, and has remained mainly
a laboratory, rather than a clinical, method of studying the human
lens.72Wavefront Technology
Image degradation in the macula because of cataract may not only
be caused by light scattering but also by optical aberrations. As
discussed above (see Visual Acuity/Function Tests), several devices
such as the Oqual and resolution test target test have been devised
as simple ways to test for the effect of optical degradation in the
retina.
A new technology using wavefront analysis to study optical
aberrations of the eye and in particular the cornea to enhance the
results of refractive surgery in patients has also been used on the
lens. Kuroda et al.,17 using the Hartmann-Shack (H-S) Aberrometer
(Topcon Corp., Tokyo, Japan), found that ocular total higher order
optical aberration in eyes with a cortical or nuclear cataracts was
significantly higher than in normal subjects. Corneal total
high-order optical aberration in eyes with mild cortical or nuclear
cataracts did not differ from normal subjects. This suggests that
high-order optical aberration increases in eyes with cataract
because of the local refractive change in the lens. Another finding
was that the polarity of spherical aberration was different between
nuclear and cortical cataracts. In nuclear cataract, the polarity
is always negative, suggesting a delay of the light wavefront
occurs when the ray travels inside the hard nucleus with increased
refractive index. In contrast, in cortical cataract, the polarity
was always positive.
These findings suggest that in mild cortical and nuclear
cataracts, not only light scattering but also optical aberrations
in the lens contribute to loss of visual function as measured by
loss of contrast sensitivity.17 Thus, this new technique may be
useful in studying the total effect of early cataracts on visual
function, and explain some patient complaints such as monocular
diplopia, in the presence of mild lens changes.
Raman Spectroscopy
Raman spectroscopy is routinely used as an analytical tool in
chemistry laboratories. It is a light-scattering technique based on
the Raman effect, which was discovered in 1928. The light (or
photons) impinging on a molecule interacts in various ways but the
final outcome always results in the scattering of light. For
example, we do not see light directly. We always see light and
objects as a result of scattered light. Scattering is absorbance of
incident light used in exciting the atom and reradiation of this
light. The Raman scattering is the result of inelastic collisions
in which the scattered photons exchange energy with the vibrational
energy modes of an atom. This frequency shift (or the difference in
frequency of an incident photon and the scattered photon) points to
specific structural information about a constituent molecule
analogous to a certain specific fingerprint that can identify any
species present in the system being investigated. However, the
Raman signal is very weak. Of 1061010 incident photons only one
scattered photon exhibits a Raman shift. Because of this, the Raman
method has remained limited to chemistry research laboratories
since its discovery.
Raman spectroscopy has furthered our knowledge of normal aging
and pathologic processes in the lens73 that would not have been
possible with other currently available methods. The structural
information it provided includes: SH, SS, H2O, Trp, Try, Phe, and
protein secondary structure. Studies can be carried out in the
intact living lens, thus avoiding any protein disruption or
possible autooxidation of sulfhydryl inherent in studying isolated
protein fractions of lens. Using the optical dissection technique,
the Raman scattered light can be analyzed from any portion of the
lens along the visual axis (or along any axis). This technique
monitors aging changes within the lens so that older nuclear
proteins can be easily compared with those newly synthesized in the
cortex. By coupling with an optical microscope, laser Raman
instrumentation has been transformed into a unique imaging device
with excellent spatial resolution.
Raman spectroscopy has also been used to demonstrate regional
swelling of the lens in diabetes. In mildly diabetic rats, the
overall increase in lens hydration is hardly detectable. However,
regional swelling was demonstrated by Mizuno et al.74 with this
noninvasive technique. This method permitted determination of water
content from the periphery of the lens to the center. The advantage
of this type of noninvasive technique, similar to that of NMR
spectroscopy and QELS, is that it permits analysis of discrete
areas of the lens. Thus, these methods may be helpful in
determining the changes that occur in certain regions of the lens
during cataract formation. The Raman spectra of animal and human
lenses has been discussed by Ozaki in a review article.75Clinical
in vivo use of this technology is limited by the need to use high
laser power, and comprehensive spectral data libraries must first
be generated and established. It then can be used as searchable
fingerprints (indices) for ocular and other diseases.
Autofluoresence Spectroscopy
Ocular tissues exhibit natural or auto fluorescence (AF) and it
has been found to increase with age in healthy individuals.76
Accumulation of fluorescent proteins in ocular tissue can result
from long-term exposure to the UV or UVA radiation in sunlight.
This accumulation of fluorophores may also be responsible for lens
opacification and can be considered a risk factor for cataract
formation. These fluorophores can be found during cataract
formation. In initial stages these can be characterized by
exhibiting fluorescence in the near ultraviolet and violet regions
of the spectrum (340 and 411 nm). However, in advanced stages of
cataract development an increase in the intensity of the long-wave
fluorescence of the lipids in the blue-green region (430/480 nm)
occurs.77 AF from transparent (noncataractous) lenses exhibits a
strong correlation as a function of age (Fig. 13). In this figure,
the increased level of fluorescence from the lens can be attributed
to oxidative stress or absorbance of UV light as a function of age.
Because the cornea does not absorb UV, its AF level remains
constant. However, both diabetic lens and cornea show significantly
increased AF levels.
Fig. 13. Natural autofluorescence of the eye with aging.
Studies of the AF properties of the ocular media have shown that
ocular AF can be related to metabolic disorders.78,79 Thus
prepathologic states can easily be studied by measuring AF
intensity from the corneal tissue because it is readily accessible
(no dilation needed) and its intensity is not age-dependent.
Corneal AF is mainly the result of the pyridine nucleotides and
flavoproteins found in the corneal epithelium and the endothelium.
The accumulation of these fluorophores can be related to the
severity or duration of some pathologies and therefore the corneal
AF can be exploited as a diagnostic index of this class of
disorders. In particular, an increase in the corneal AF has been
observed in the presence of early stage diabetic retinopathy (DR)80
by using a novel instrument.81Optical Coherence Tomography
OCT is a near-infrared optical ranging imaging technique. The
images obtained by OCT are of much higher resolution (approximately
115 m) compared to images obtained by low-frequency ultrasound,
pulse-echo imaging (approximately 100 m). The two-dimensional image
of optically reflecting and backscattering from tissue
microstructure in OCT is constructed using low-coherence
interferometry. Photons that have scattered multiple times
(multiple scattering) are rejected by coherent detection because it
takes advantage of short temporal coherence of broadband light
source, e.g., light-emitting diodes (LEDs). The interferometric
system selects photons that have traveled a specific distance in
the tissue. The beam scans turn the one-dimensional depth profile
into a two-dimensional image. The images are similar to that of
histologic sectioning. At present this technique is being used to
obtain retinal images. High-resolution OCT images provide detection
of subsurface retinal changes that are not seen by ophthalmologists
in conventional settings. This is important in monitoring injury to
the optic nerve from glaucoma. Most biologic tissues highly scatter
in the visible and near-infrared range.
In ophthalmology, OCT represents a novel, noninvasive,
noncontact transpupillary tool, which can image the fine anatomic
structures within the eye, structures too fine to be adequately
assessed by conventional techniques. The appearance of a variety of
anterior/posterior segment pathologies can be diagnosed using OCT
including cataract, glaucoma, diabetic retinopathy, macular holes,
epiretinal membranes, cystoid macular edema, central serous
choroidopathy, and optic disk pits.8285 Although OCT has been shown
to image cataracts, it has not yet been used extensively in this
area.
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"Next page in this title" Next >Chapter 117The Funduscopic
Examination
Henry Schneiderman.
Definitions
Funduscopic examination is a routine part of every doctor's
examination of the eye, not just the ophthalmologist's. It consists
exclusively of inspection. One looks through the ophthalmoscope
(Figure 117.1), which is simply a light with various optical
modifications, including lenses. The ophthalmoscope illuminates the
retina through the normal iris defect that is the pupil. Light rays
forming the image of the retina re-emerge through the pupil. The
viewing aperture (window) of the ophthalmoscope contains a lens
that modifies light rays to assist the user. In the procedure, one
looks at structures lying in the innermost aspect of the globe,
collectively known as the eyegrounds: retina, retinal blood
vessels, optic nerve head (disk), and to a limited degree,
subjacent choroid. The pupil is frequently dilated
pharmacologically to render retinal inspection easier, and for
examination of the macula. One paralyzes the pupilloconstrictor
muscle of the iris with nonabsorbable, short-acting topical
parasympatholytic drugs, resulting in a larger pupillary aperture.
In comparison to the ophthalmologist, the internist, neurologist,
or pediatrician concentrates particularly on funduscopic
manifestations of systemic disease and less on local ocular
disease. Synonyms for funduscopic examination include funduscopy,
ophthalmoscopy, and direct ophthalmoscopy. Only ophthalmologists
perform retinoscopy and indirect ophthalmoscopy, which require
other equipment and provide different information.
Figure 117.1The instrument. (A) A portable ophthalmoscope in
front (patient's-eye) view. a. Rheostat runs along circumference of
tube and controls intensity of beam output. In wall-mounted
instruments, this is located instead on the fixed panel, b. Handle
containing (more...)The term temporal is used in describing
ophthalmoscopic landmarks and findings, rather than "lateral"; and
nasal replaces "medial." The optic nerve head or disk is seen when
one looks through the pupil from an angle about 15 degrees temporal
to the optical axis (the patient's line of sight, "straight
ahead"). The disk is a yellow-pink color that stands out from the
redder, browner, or more orange retina proper (see Figure 117.4D).
The disk is sharply demarcated temporally and to a lesser degree
nasally from the background retina, which is all the retina that is
not disk, vessels, or macula. Frequently, a narrow crescent of
stippled pigment adjoins the sides of the disk, especially the
temporal side (house staff have called the author to see "lesions"
that turned out to be this normal feature). The disk is slightly
taller than wide. The central part of the disk is paler, and is
called the optic cup or physiologic excavation; normally this
occupies less than one-third the diameter of the disk. In glaucoma
and in high myopia the cup is enlarged. The transverse diameter of
the disk is a standard yardstick in fundal description, so that,
for example, a lesion may be characterized as "one-half disk
diameter out at two o"clock, and extending two disk diameters
superiorly therefrom." Although some examiners realize that the
disk is 1.5 mm wide, nobody describes a lesion as 3 mm across. Near
mid-disk, the central retinal artery and vein emerge from the optic
nerve, with which they have run forward into the orbit. Each soon
bifurcates into superior and inferior branches, which run "flat,"
that is, parallel with the retinal surface. Beyond one disk
diameter out, they are called arterioles and venules. With all
retinal vessels, the artery/arteriole appears slightly smaller, and
distinctly lighter, more orange-red and less purple than the
vein/venule. The color difference reflects the contained blood
column that is visualized: the vascular walls are transparent, and
deoxygenated venous blood is darker than arterial blood. Before it
crosses the disk edge, each large vessel divides into a nasal and a
temporal branch. Thus the principal arteries, veins, and quadrants
of any retina are the superior temporal, inferior temporal,
inferior nasal, and superior nasal. The avascular, dusky area two
disk diameters due temporal to the disk is the macula. This is the
area of greatest visual acuity. Apart from this zone, the
background retinal color will parallel the patient's skin and hair
pigmentation, from pale in light-skinned blondes to an umber shade
in the darkest black people.
Figure 117.4The value of pupillodilation. (A) External
examination shows miotic pupil at baseline. (B) Scant fundus is
revealed through this pupil. (Peripheral clouding is an artifact of
photography through undilated pupil; actual funduscopy in this case
showed retina (more...)Normally, the largest veins pulsate slightly
and the arteries do not, the reverse of the situation elsewhere in
the body. No hemorrhage is seen in normal fundi. Any yellow,
yellow-white, gray, or black interruptions of the background
retinal color pattern suggest pathologic exudate, edema, or scar.
No wrinkling of the retina should be seen. Chalky whiteness or
erythema of the disk is abnormal, as are indistinct disk margins.
Any sharp change in elevation that renders one area out of focus
with the ophthalmoscope, while the remainder of the retina remains
in focus, is abnormal. Tortuous blood vessels usually bespeak
pathology.
Technique
The Instrument
Spend time becoming accustomed to looking through the
ophthalmoscope in a nonpressured setting. Hold the instrument with
the hand ipsilateral to the examining eye; both are ipsilateral to
the eye being examined: examine every left fundus with your left
eye, holding the ophthalmoscope in your left hand (Figure 117.2);
and every right fundus with your right eye and hand. Students with
strong dominance of one eye may at first experience difficulty and
anguish employing the other eye. They always gain success and
comfort in time. The forefinger turns the horizontal rheostat and
the vertical lens wheel. A portion of this lens wheel, containing a
single lens, overlies the window at any given setting. Lenses have
red numbers for negative diopter values (progressively more distant
focusing with higher numbers); black numbers are positive (i.e.,
higher black numbers mean shorter focal length). Of the several
light beams available, only the two plain white circles are
important to the generalist. The larger illuminates a wider field,
but the smaller decreases corneal glare. Hold the instrument
against your bony orbit, with the bumper ridge against your
forehead. Practice will show how to focus it. The ophthalmoscope
does not magnify images except slightly at high positive diopters.
Rather, the fundus appears magnified at funduscopy because of the
magnification produced by the patient's lens; aphakic fundi look
tiny and far away through the ophthalmoscope. Practice turning your
head and craning your neck every which way, while maintaining a
constant relationship between your eye, your hand, and the
instrument. A viewing aperture moved 1 mm out of your optical axis
can mean loss of half the available field, so make the spatial
relations as constant as though yoked.
Figure 117.2Holding the instrument. (A) Examiner correctly holds
the ophthalmoscope in the left hand and looks through it at
subject's left eye, using her own left eye. All three organs are
ipsilateral! (B) Bungler employs left hand and crosses over. He
uses his (more...)The Procedure
Lower the room lights. Remove your and the patient's eyeglasses,
but not contact lenses. Have the patient hold her glasses or put
them someplace safe. Give the patient a tissue in case of tearing.
Show the patient a spot directly ahead of her, on which to fix her
gaze. An object or picture 1 m ahead is perfect. She will be able
to fixate only with the eye that is not being examined (i.e., that
is not being blocked and spotlighted by the examiner). Do not
interview during ophthalmoscopy; people involuntarily turn their
eyes toward a speaker, and that is the end of visualization. Hold
the ophthalmoscope to your eye so that you can see well through it.
Keep your other eye open, but ignore its input for now.
Begin with your light at two-thirds strength. Stand 15 degrees
temporal to the patient's optical axis, your eye 30 cm from hers.
Set your lens wheel at + 10 diopters. Trans-illuminate the pupil
and observe reflected red light, the red reflex. Place your
contralateral palm on her forehead, with your abducted thumb on her
supraorbital ridge, to prevent accidentally bumping brow or eye
with the instrument. (Some doctors advocate using this thumb to
help hold the eye open, but patients hate and resist it. Have a
colleague try it on you if you need convincing.) Slowly move toward
the patient, slowly decreasing your diopters toward zero
(increasing your focal length). In this way you focus successively
on cornea, lens, vitreous, and finally retina. The appearance of
black spots at any point tells you that opacities are in the path
of the light and will have to be accounted for or circumvented in
inspecting the retina. You should wind up with the instrument only
3 to 5 cm from the patient's eye. When you see the retina, look for
the first distinct structure in the area. Sometimes this will be
the optic disk, more often a vessel. Bring the structure into sharp
focus by rotating (changing) your lens wheel as needed. A zero
reading often works well. Myopic examiners need a negative or red
number (unless the patient's refractive error balances the
examiner's). Aphakic patients require a high positive, often + 10;
by contrast, the patient who has an intraocular lens implant after
cataract surgery has no such special need.
Now move along the vessel in the direction leading to larger
caliber (i.e., toward "junctures" of vessels). Soon you will reach
the optic disk. Study its color, its lateral margins, the size of
the optic cup, the disk's elevation if any, and the pattern of
vessels emerging from it. Record the details in a drawing if you
wish to discuss abnormalities, to follow them over time, or to
enhance your funduscopic technique. Try to appreciate pulsations of
the retinal veins overlying the disk. This is easiest to see where
an artery, or a bend in the vein, causes apparent diagonal
interruption of one edge of the contained venous blood column.
Next, move out along the superior temporal artery, observing its
normally slowly diminishing caliber, its crossovers of veins, any
focal change in color, caliber, or content, and any abnormalities
of background retina that come into view in the same fields. Move
as far peripherally as possible, by directing the light in that
direction. You will have to crane your own head and neck in an
opposite direction, and sometimes it will feel as though you are
about to twist yourself right under the table, not just 2 cm down
and in! If the patient can slowly and steadily move her fixation
point in the same direction that you want to go, that will bring
more peripheral retina into view; a rapid or jerky movement by
either patient or examiner will take the pupil out of the light
path of your instrument. Recall, in asking the patient's
assistance, that the patient's left and right are the opposite of
your own! Now return to the disk by way of the vein, making
parallel observations until the disk is reached. This procedure is
repeated in the three other quadrants, and at the end one studies
any areas of background retina that were not visualized
earlier.
Finally the beam is directed temporally, or the patient is asked
to look at the light. Unless the pupil is pharmacologically
dilated, at best a fleeting glimpse of the macula may be obtained
before pupilloconstriction or involuntary movement takes it out of
view.
Do not let an interesting finding distract you from a fixed
routine. Study the entity, then proceed. If you spot it out of
sequence, such as with the first focus on the retina, leave it
alone until its proper time. Examiners sometimes fear that they
will lose the lesion and never find it again, but this never
happens if one is relaxed, systematic