5/14/2018 ophthalmoscopy ‘seeing the eye’€¦ · 05/06/2018 · direct ophthalmoscope if observer and patient are emmetropic, lenses can be used to move the point of focus onto

5/14/2018

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ophthalmoscopy

‘seeing the eye’

ophthalmos - the eye

skopeos – to see

from greek:

structure

●principles of fundus examination

–direct ophthalmoscopy

–indirect ophthalmoscopy

●magnification in ophthalmoscopy

●field of view in ophthalmoscopy

classic article

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principles of ophthalmoscopy

● in the emmetropic

patients, light rays

emanating from a point in

the fundus emerge as a

parallel beam

● if this beam enters the

pupil of the observer, the

rays are focussed on his

retina and an image is

formed


● so why do we

generally not see the

fundus when looking

directly into a patient’s

pupil?

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● problem:

● fundus of patient not

illuminated – light

source required

● pupillary axis of

observer must be

aligned with incoming

light rays from light

source to allow

observation of the

fundus

● limiting factor

● patient’s pupil size


● in large animals or

patients with

extremely dilated

pupils, it may be

possible to observe

the fundus by simply

aligning a bright light

source with your

visual axis


● direct ophthalmoscopy

● ‘key-hole’ view

●indirect

ophthalmoscopy

●‘aerial’ image

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direct ophthalmoscope

● first developed by

Charles Babbage

1847

● failed to prove its

function when showing

to eminent ophthalmic

surgeon and

abandoned project

http://www.college-optometrists.org/


● clinical use introduced by

Hermann von Helmholtz

1851

● initially called ‘Augenspiegel’

(eye mirror)

● within 10 years physicians

using it call themselves

‘ophthalmoscopists’

● first fundus photograph

1864

http://www.college-optometrists.org/


● Helmholtz’ principles of

direct ophthalmoscopy

● a source of illumination

● a method of reflecting the

light into the eye

● an optical means of

correcting an unsharp

image of the fundus

● greatest improvements

made were re. source

of illumination

● candle

● oil burning wick

● development of

incandescent bulbs

● halogen

● ….LED light source

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advantages

● ‘real’, upright image

● high magnification

● does not require full dilation

● cheap and mobile

disadvantages

● closeness to patient required

● small field of view

● no stereoscopy

● poor penetration of cloudy media

direct ophthalmoscope head

observer’s view

hole

fast diopter switch

diopter indicator

diopter dial

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direct ophthalmoscope head

shutter

light reflecting

mirror

various sizes

round beam

observers’

viewing

aperture

slit beam setting

red free light

direct ophthalmoscopy – light filters

● green filter (red free

light)

● blood (vessels) appear

black

● grid pattern

● facilitates lesion

documentation

● slit beam

● evaluate surface

topography of retina

and optic nerve

Direct ophthalmoscope head

Shutter

Light

reflecting

mirror

Various

sizes round

beam

Observers’

viewing

aperture

Slit beam setting

Red free light

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Direct ophthalmoscope on a slit

Direct ophthalmoscope on a slit


● if patient and observer are

emmetropic, no lenses

required to focus on retina

● why are lenses used?

● lenses can be used to

correct known refractive

error of observer

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● if observer and patient

are emmetropic,

lenses can be used to

● move the point of

focus onto structures

posterior to the retina

or structures more

anterior within the eye


● negative lenses can

be used to focus onto

● structures posterior to

the retina

●colobomata, areas of

scleral ectasia, cupped

disc


● positive lenses can


● structures more


● lens, iris, cornea,

adnexa

courtesy J Mould

0 8 12 20

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Mirror

Mirror

12 dioptre lens

Mirror

15 dioptre lens

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Mirror

20 dioptre lens


●diopter equivalent

●distance (in mm) the focal moves anterior/posterior within the eye per diopter change

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focus on ONH on + 6 D0 D = focus on retina

Diopter equivalent - clinical

Diopter equivalent - clinical

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● positive lenses can


● structures more


● lens, iris, cornea,

adnexa

courtesy J Mould

0 8 12 20


● how to do it

● align ophthalmoscope with your visual axis

● dim light beam to minimal intensity

● gain control over patient’s head with your free hand

● pick up fundus reflex at arms length

● (compare fundus reflex of both eyes)

● following the fundus reflex, come close to the eye until

the fundus becomes visible

● search fundus in reproducible manner

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Tora

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Fig. 1.12a-c

● Milk bottles

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● apply a replicable,

systematic approach to

the assessment of the

fundus

● keep ‘landmarks’ in mind

● consider that varying

species may require a

varying routine!

indirect ophthalmoscopy

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http://medical-

dictionary.thefreedictionary.com/_/viewer.aspx?path=ElMill&name=F0O-01-

S2958.jpg

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light options

‘all pupil’ setting

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advantages

● larger field of view

● stereoscopic (with head

mounted)

● penetrates opaque

media better

disadvantages

● limited magnification

● inverted & upside down

image

condensing lenses

● acrylic or glass

● glass superior optics

● biconvex

● coated to reduce

reflections

● variety of strenghts

● most commonly used

●2.2 pan-retinal

●20 D

●30D

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high power condensing lenses

image – direct vs indirect

direct: 2.5 mm 20D lens: 13 mm

magnification and field of view

●magnification

● process of enlarging something only in appearance

but not in physical size

● field of view● (angular or linear or areal) extent of the observable

world that is seen at any given moment

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magnification and field of view

of the fundic image depend on two main factors

● optical properties of the eye to be examined

● small eyes = high dioptric power

● large eyes = less dioptric power

● form of ophthalmoscopic technique employed

● direct ophthalmoscopy = high magnification, small field of

view

● indirect ophthalmoscopy = low magnification, larger field of

view

magnification

● optical properties of the eye to be examined

● small eyes = high dioptric power

●high magnification

● large eyes = less dioptric power

● low magnification

optical properties

of the eye to be

examined

●small eyes = high

dioptric power

●high

magnification

●large eyes = less

dioptric power

●low

magnification

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ophthalmoscopy - magnification

●lateral magnification

●magnification of an area

●axial magnification

●magnification in depth

lateral magnification

● magnification across an

axis perpendicular

towards viewing

● e.g. left-right, dorsal-

ventral with regards to

fundus

● exemplified by action of

slide projector throwing a

much magnified image

onto a screen

lateral magnification – direct ophthalmoscopy

● the magnification M of

a direct

ophthalmoscope is

equal to

● M = Fe/4

● where Fe is the total

refractive power of the

eye.

● e.g.

● horse 8

● dog 17

● cat 20

● Rat 77

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lat magnification – indirect ophthalmoscopy

● lat magnification of a

lens =

● total refractive power

of eye/power of lens

dependent on lens

power and eye to

be examined

lateral magnification - indirect ophthalmoscopy

● power of lens inversely related to magnification

● high power = low magnification

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axial magnification

● describes magnification in depth

● precisely, the ratio of the distance along the optical axis

between two points in image space (A’B’) to the

distance along the optical axis between the

corresponding two points in object space (AB)

● this magnification is useful when considering an image

in its three dimensions

axial magnification

● magnification along

the axis of viewing

● e.g. elevation of

optic nerve head,

colobomata

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axial magnification

● axial magnification is approximately the lateral

magnification squared

● e.g. ● lateral magnification = 2 means axial magnification = 4

(axial) = (lateral)2


● in any given species,

the dioptric power of

lens used determines

● magnification

● field of view

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axial magnification in diff. species

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axial magnification in diff species

field of view - direct ophthalmoscopy

● field of view in direct

ophthalmoscopy is

determined by

● pupil size of

●examiner

●patient

● size of light beam

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● in any given species,

the dioptric power of

lens used determines

● magnification

● field of view


● high diopter

● large field of view,

● (low magnification)

● low diopter

● small field of view

● (high magnification)

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hand held ‘PanOptic’

● cross between direct and

indirect ophthalmoscope

● increased field of view

compared to direct

● 25 degree vs. 5 degree

● increased magnification

● (by 26% compared to


with 14 D lens)

● virtual upright image

● greater working distance

● monocular view

hand held ‘PanOptic’

http://www.welchallyn.com

Optics of Direct Ophthalmoscopy

Species Field of View

(using large

aperture)

Lateral

Magnification

Axial

Magnification

Dioptric

Equivalent

(mm/D change)

Horse 5.5mm 8x 84x 1.33mm

(0.75 D = 1mm)

Dog 2.5mm 17x 405x 0.28mm

(3.5 D = 1mm)

Cat 2.2mm 19x 508x 0.22mm

(4.5 D = 1mm)

Rat 0.6mm 77x 7965x 0.014mm

(71 D = 1mm!)

1. Field of view varies directly with axial length

2. Dioptric equivalent varies directly with axial length

3. Lateral and axial mag vary inversely with axial length

4. Axial mag ~= lateral mag2

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Optics of Direct Ophthalmoscopy

Species Field of View

(using large

aperture)

Lateral

Magnification

Axial

Magnification

Dioptric

Equivalent

(mm/D change)

Horse 5.5mm 8x 84x 1.33mm

(0.75 D = 1mm)

Dog 2.5mm 17x 405x 0.28mm

(3.5 D = 1mm)

Cat 2.2mm 19x 508x 0.22mm

(4.5 D = 1mm)

Rat 0.6mm 77x 7965x 0.014mm

(71 D = 1mm!)





Magnification-Indirect Ophthalmoscopy

Species Lateral Magnification Axial Magnification

14D

Lens

20D

Lens

30D

Lens

14D

Lens

20D

Lens

30D

Lens

Horse 1.2x 0.8x 0.6x 1.9x 0.8x 0.4x

Dog 2.6x 1.8x 1.1x 9.0x 4.0x 1.7x

Cat 2.9x 2.0x 1.3x 11.3x 5.1x 2.1x

Rat 11.5x 7.7x 5.0x 177x 89x 18

1. Lateral and axial mag vary directly with axial length

2. Lateral and axial mag vary inversely with dioptric power of indirect lens


*Pan Retinal lens has similar magnification to 20D lens, and similar field of view to 28D

lens

Field of View - Indirect Ophthalmoscopy

Species Field Size (mm)14D

Lens

20D

Lens

30D

Lens

Horse 17.3mm 27.6mm 38.3mm

Dog 7.9mm 12.5mm 17.4mm

Cat 7.0mm 11.2mm 15.5mm

Rat 1.8mm 2.8mm 3.9mm


2. Field of view varies directly with dioptric power of indirect lens

*Pan Retinal lens has similar magnification to 20D lens, and similar field of view to 28D lens

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Optics of Direct vs. Indirect Ophthalmoscopy

Species Direct Ophthalmoscopy Indirect Ophthalmoscopy (20D

Lens)

Field

Size

Lateral

Mag

Axial

Mag

Field

Size

Lateral

Mag

Axial

Mag

Horse 5.5mm 8x 84x 27.6mm 0.8x 0.8x

Dog 2.5mm 17x 405x 12.5mm 1.8x 4.0x

Cat 2.2mm 20x 508x 11.2mm 2.0x 5.0x

Rat 0.6mm 77x 7965x 2.8mm 7.7x 70x





*Direct = small field size, large mag; Indirect = large field size, small mag

Direct PanOptic Indirect (20D)Ophthalmoscope Ophthalmoscope Ophthalmoscope

FOV 9o (2.5 mm) 29o (7 mm) 41 (12.5mm)

LM 17.24x 3.2x 1.74x

AM 405x 7.43x 4.04x

PanOptic field size and magnification falls between direct and indirect

www.volk.com

5/14/2018 ophthalmoscopy ‘seeing the eye’€¦ · 05/06/2018 · direct ophthalmoscope if observer and patient are emmetropic, lenses can be used to move the point of focus onto

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