Tonometry

Tonometry

Sanket Parajuli

Tonometry:

• procedure performed to determine the intraocular pressure (IOP)

• Unit of IOP : millimeters of mercury (mm Hg)

• Normal range: 8 to 21 mmHg

• Mean IOP = 15.5 mmHg

HISTORY1826-William Bowman Digital tonometry(as routine examination)

1863-Albrecht von Graffe Designed first instrument to attempt to measure IOP

1865-Donders

1880-Preistly

Further instruments followed Indentation type(no anaesthetic was used until 1884)

1885-Malkalov 1st Applanation tonometer

1905-Hjalmar Schiotz Indentation tonometry

1948,1955-Friedenwald Coefficient of ocular rigidity

1954-Goldmann Prototype Applanation tonometer

Grant Electronic indentation tonometer

Halberg Hand held tonometer

Albrecht von grafe tonometer Donders tonometer

Ideal tonometer

• Should give accurate and reasonable IOP measurement • Convenient to use • Simple to calibrate • Stable from day to day • Easier to standardize • Free of maintenance problems

Tonometry DirectIndirect

IndentationEg: Schiotz

Manomerty Applanation

ContactNon-contact Eg: Airpuff

Ocular Response Analyzer(ORA) Pascal Dynamic contour tonometer

Fixed weight, variable areaEg: Maklakow

Fixed area variable weight Eg: Goldmann, Perkins, Tonopen

TYPES OF TONOMETRY

Digital

• Newer tonometers :

1. Trans –palpebral Tonometer

2. Dynamic contour tonometer

3. Ocular Response Analyzer

4. Bioresonator: applanation resonance tonometer (ART)

A. High displacement tonometer B. Low displacement tonometer

Displace a large volume of fluid and consequently raise IOP significantly.

IOP is negligibly raised during tonometry (less than 5%).

Eg:

1. Schiotz.2. Maklakov

Eg:

1. Goldmann applanation tonometer.

Accuracy: low High

Direct method

Manometry:• Most accurate method• Requires insertion of a cannula into AC through

a self sealing beveled corneal puncture• Not practical clinically.

Mechanism of action:• Needle connected to a fluid filled tubing• Height of the tubing corresponds with the IOP

• DISADVANTAGES:1. Not practical for human

USES:

1. It is used for continuous measurements of IOP

2. Used in experiment, research work on animal eyes.

Indirect tonometery

• Digital tonometry

• Three basic types of Tonometers :

Indentation

Applanation (flattening)

Non-Contact

Digital tonometry:

Response of eye to pressure applied by finger pulp.

ADVANTAGES DISADVANTAGES

Easiest to perform Reading not proper

No equipment Only depends on examiner

No anesthesia Over-estimation or under-estimation

No staining

Estimation of IOP with irregular corneas, where applanation tonometry not possible.

• ProcedurePatient looks down

Index finger of both hands used

One finger is kept stationary which feels the fluctuation produced by the indentation of globe by the other finger.

Indents easily – low IOPFirm to touch – normal IOP

Hard to touch – high IOP

Applanation tonometer

1. Contact tonometers:

Fixed area variable weight • Eg: Goldmann, Perkins, Tonopen

Fixed weight, variable area• Eg: Maklakow

Ocular Response Analyzer (ORA) Pascal Dynamic contour tonometer

Goldmann applanation tonometer

Gold standardMost commonMounted of a slit lamp

Determines the force necessary to flatten/applanate an area of cornea of 3.06 mm in diameter

GOLDMANN APPLANATION TONOMETER

Biprism (measuring prism)

Feeder arm

Housing

Adjusting knob

Connects to the slit lamp

Control weight insert

PRINCIPLE

• Goldmann -1954.• Imbert Ficks Law:• “Pressure inside an ideal sphere (P) is equal to force (F) necessary to flatten its surface,

divided by the area of the flattening (A)”

• P can be determined if• Force F is fixed or• Area A is fixed

P=F/A

• Ideal sphere here is :• Dry• Thin-walled• Flexible.

• Cornea: not ideal sphere• Two extra forces acting on cornea -

• Capillary attraction of tear meniscus (T), tends to pull tonometer towards cornea

• Corneal rigidity (C) resists flattening

• Modified Imbert Ficks Law• F = PA , becomes• F + T = PA + C , or

P =( F + T - C) / A

• These two forces cancel each other when flattened area has diameter of 3.06 mm

• P=F / 7.35mm2

• Applanation tonometry• Displaces only about 0.5 microlitre of aqueous humor• Raises IOP by about 3%• Because the volume displaced is so small, ocular rigidity, or the ‘stretchability’ of the globe

has little effect on the readings

Contraindications

• Active eye infections• Recurrent corneal erosions• Corneal abrasions that are not healed

ProcedureInstructions and prerequisite

• Patient : must not drink alcoholic beverages (lower IOP) or take large amounts of fluid (e.g., 500 ml or more) 2 hours before the test (raise the IOP), seated position• Inform patient about the purpose of test and brief about the test• The angle between the illumination and the microscope should be approximately 60°• Room illumination: reduced• A fixation light may be placed in front of fellow eye. • The tension knob is set at 1 g

• Instill 1 drop of topical anesthetic: 4% xylocaine on each eye

• Tip of a moistened fluorescence strip is touched to the tear layer

• The patient should blink the eyes once or twice to spread the fluorescein-stained tear film over the cornea, and then should keep the eyes open

• View in cobalt blue light in slit lamp biomicrsoscope

• Do not to place any pressure on the globe because this raises IOP.

Procedure cont..• In some patients, it is may be necessary to hold eyelids open

with thumb and forefinger of one hand against the orbital rim

• Adjust the force by manually rotating a dial calibrated in grams

• The prisms are calibrated in such a fashion that inner margin of semicircles touch when 3.06 mm of the cornea is applanated

• The Intra ocular pressure is then read directly from a scale on the tonometry housing.

The view through the Goldmann tonometer prism

• The fluorescent semicircles are viewed through the biprism and the force against the cornea is adjusted until the inner edges overlap.

The fluorescein rings should be ~ 0.25–0.3 mm in thickness or about one-tenth the diameter of the flattened area.

The position of the semicircles when

(top) insufficient

(middle) correct

(bottom) excessive pressure has been applied by the tonometer arm to the cornea.

The tonometer is not centralThe semicircles are unequal.

Potential Sources of Error – During Measurement

1. Fluorescein rings are too wide:

Reading is higher than actual IOP

An excessively wide fluorescein ring = overestimation IOP

To correct:Blot eyes carefully with a tissuedry out the front surface of prism with lint-free material.

Potential Sources of Error – During Measurement

2. Fluorescein rings are too narrow:Reading is lessing than actual IOP= IOP underestimated

To correct:Ask blink two or three times to replenish the fluorescein Additional fluorescein may be added if necessaryThen repeat the procedure

Potential Sources of Error – During Measurement

3. If the fluorescein ring are of unequal sizes:

Reading is considerably higher than actual IOP

To correct:Lower the slit lamp with a higher adjustment mechanism until Both the semicircles equal in size

Potential Sources of Error – During Measurement

4. The inner border of the fluorescein rings do not touch each other:

To correct:Increase the pressure further by turning the measuring knob

Potential Sources of Error – During Measurement

Correct final position:

The inner age of the semicircle makes contact with each other

POTENTIAL SOURCES OF ERROR

Features FALSELY LOW IOP FALSELY HIGH IOPfluorescein Too little Too muchCorneal thickness Thin Thick Corneal pathology Corneal edema Corneal scarAstigmatism With the rule astigmatism

1mm Hg per 4DAgainst the rule astigmatism 1 m Hg per 3D

Other: After LASIK, photorefractive keratectomy

Wider meniscus

Repeated tonometry Widening the lid fissure excessivelyElevating the eyes more than 150

EFFECT OF CENTRAL CORNEAL THICKNESS

• THINNER cornea less force to applanate Underestimation

THICKER cornea more force to applanate Overestimation

Goldmann applanation tonometer was designed to give accurate readings when the CCT was 520 μm.

it can be estimated that for every 10 μm difference in CCT from the population mean (approximately 542 μm), there is a 0.5 mm Hg difference between actual IOP and the IOP measured with a Goldmann tonometer

Measurement in patients with significant astigmatism

• Corneal astigmatism may result in false pressure readings

• The error has been calculated at 1 mm for every 4 D (underestimated for with-the-rule; overestimated for against-the-rule).

• The biprism should be rotated in its housing so that the axis of least corneal curvature aligns with the red line on the prism holder;

• alternatively, an average of the pressure obtained with semicircles aligned horizontally and then vertically may be used

Calibration

• At least once a month

• If the Goldmann tonometer is not within 0.1g (+1mmHg) of the correct calibration, the instrument should be repaired

• However, calibration errors of up to +2.5 mmHg may still be tolerated clinically.

Sterilization• Centers for Disease Control and Prevention (CDC) suggests a 5-10 min soaking

in 3% hydrogen peroxide or 70% ethanol or isopropanol.

• The tip should then be washed under running water and dried thoroughly before reuse.

• A ‘clinical alert’ issued by the American Academy of Ophthalmology in 1989 states that rubbing the prism tip with a 70% isopropyl alcohol wipe was an acceptable procedure for disinfection

PERKIN’S TONOMETER

• Portable applanation tonometers• Built-in biprisms • Position: supine• Hand-held version of the Goldmann

tonometer with same principal

Draeger Tonometer

• Principal : as in Goldmann/Perkins • Portable, counterbalanced• Different set of prisms • Force: supplied by electric motor.

Mackay Marg tonometer

• Mobile plunger of 1.5mm diameter which Protrudes slightly from surrounding footplate of sleeve• Movement of plunger measured by transducer• When instrument touches cornea, recorded on a paper strip• Transfer of corneal bending force occurs at applanation area of 6

mm diameter• Applanation over this area displaces ~ 8μl aqeuous humour• It rises IOP ~6-7mmHg

IOP tracing with MacKay-Marg tonometer

A. Advancing plunger is opposed by IOP and corneal bending pressure

B. Notch indicates corneal bending pressure has been transferred Height of notch corresponds with IOP

C. With further advancement of plunger, cornea is indented, and IOP rises

Cont…

• Uses: IOP measurement : scarred, irregular, or edematous corneas

• Because End point does not depend on the evaluation of a light reflex sensitive to optical irregularity, as does the Goldmann tonometer

Tonopen • Small, extremely portable• Area of contact with cornea=1.5 mm sq• Uses a strain gauge working on the Mackay–

marg principle• Pressure exerted by applanating plunger on the

cornea is recorded electronically from the strain gauge• A liquid crystal display (LCD) screen displays

the average result of Four reading• Has a disposable rubber tip

• Tono-Pen also is more accurate in corneas with irregular surfaces

Pneumatonometer/pneumatic tonometer

• It is like Mackay-Marg tonometer• Consists: Sensing device with gas chamber covered by a polymeric silicone

diaphragm• Transducer converts gas pressure in chamber into an electrical signal• Recorded on a paper strip

• The gas in the chamber escapes through an exhaust vent between diaphragm and tip of the support nozzle

As the diaphragm touches the cornea

Gas vent is reduced in size

Pressure in the chamber rises.

Applanation

2. Non contact:• Air puff tonometer

Non contact tonometer (NCT)

• Introduced by Grolman

• Applanates cornea with jet of air• Force of jet of air increases rapidly and linearly with time• Instrument also emits a collimated beam of light that is reflected from cornea and the

received by observer

• Use: screening programs

PRINCIPLEA puff of room air

creates a constant force

momentarily flattens cornea (3.6mm)

force of air jet generated by a solenoid activated piston increases linearly over time

As reflected light is at peak intensity

cornea is presumed to be flattened.

• The time elapsed is directly related to the force of jet necessary to flatten the cornea and correspondingly to IOP.

• A puff of air of known area is generated against cornea (B)• At the moment of corneal applanation, a light (T), which is usually reflected from

the normal cornea into space, suddenly is reflected (R) into an optical sensor (A). • When sensor is activated by reflected light, air generator is switched off• Level of force at which the generator stops is recorded, and a computer calculates

and displays the intraocular pressure.

• Accuracy decreases• With increase in IOP • In eyes with abnormal cornea or poor fixation.

• Use: screening programs advantages:• operated by non-medical personnel as well• topical anesthesia not required• Non contact technique

• New NCT, Pulsair is a portable hand held tonometer.

Ocular Response Analyzer• Newer breed of NCT• It directs the air jet against the cornea• Measures two pressures at which applanation

occurs

1) Air jet flattens cornea as the cornea is bent inward

2) air jet lessens in force and the cornea recovers.

• The first reading is the resting IOP• Difference between 1st and 2nd reading of pressure is : corneal hysteresis • Low hysteresis : correlates with progression of glaucoma

Applanation: Fixed weight, variable area: Eg: Maklakov

MAKLAKOV TONOMETER

Principal :• A known force is applied to the eye, and the area of

applanation is measured

• Intraocular pressure is inferred from the weight (W) and the diameter of the area of applanation (d) by using the following formula:

• IOP: gms/cm sq• converted to mm Hg by dividing by 1.36

Pt= W /π(d/2)2

Procedure:

• Instrument : wire holder into which a flat-bottom weight (5 to 15g)1. Patient’s position: supine2. Anaesthesia (+) 3. Surface : painted with dye - mild silver protein (Argyrol) mixed with glycerin4. Weight lowered onto the cornea. 5. Weight is lifted from the cornea6. area of applanation is taken to be the area of missing dye

measured either directly or indirectly from an imprint on test paper.

• Displaces a greater volume of aqueous humor than the other applanation devices (but less than a schiøtz tonometer),

• IOP readings are more influenced by ocular rigidity.

DISADVANTAGE:

Indentation tonometry• Introduced by schiøtz in 1905 • Schiøtz tonometer: inexpensive, portable, autoclavable instrument• For use in non ophthalmologic or non optometric settings

• Foot plate: For placement upon patient’s cornea• Metal plunger: Moves up and down inside a cylinder and rests upon

patient’s cornea in the center of foot plate• Weights: changeable, for indentation force, 5.5g weight is used first• Scale: numbered, shows measurement of indentation with help on

indicator Needle• Needle : moved by lever arm

• Instrument is held at around the holder

• More the plunger indents the cornea, higher the scale reading and lower the IOP

Parts

PRINCIPLE

• The weight of tonometer on the eye increases the actual IOP (Po) to a higher level (Pt)

• The change in pressure from Po to Pt is an expression of the resistance of the eye (scleral rigidity) to the displacement of fluid.

• IOP with tonometer in position Pt= Actual IOP (Po) + scleral rigidity (E)

P(t) = P(o) + E

• Determination of Po from a scale reading Pt requires conversion which is done according to Friedenwald conversion tables

Friedenwald formula

• for linear relationship between the log function of IOP and the ocular distension.Pt = log Po + C ΔV

• This formula has ‘C’ : coefficient of ocular rigidity (0.025)

• ΔV is the change in volume

Friedenwald conversion table

•

  Plunger LoadScale Reading 5.5 g 7.5 g 10 g 15 g3.0 24.4 35.8 50.6 81.83.5 22.4 33.0 46.9 76.24.0 20.6 30.4 43.4 71.04.5 18.9 28.0 40.2 66.25.0 17.3 25.8 37.2 61.85.5 15.9 23.8 34.4 57.66.0 14.6 21.9 31.8 53.66.5 13.4 20.1 29.4 49.97.0 12.2 18.5 27.2 46.57.5 11.2 17.0 25.1 43.28.0 10.2 15.6 23.1 40.28.5 9.4 14.3 21.3 38.19.0 8.5 13.1 19.6 34.69.5 7.8 12.0 18.0 32.010.0 7.1 10.9 16.5 29.6

Performing Schiotz Tonometry• Patient position: reclining, eyes are looking straight up at a target• Instruction : relax and keep both eyes open and positioned straight ahead; a target placed on the

ceiling is helpful• Prerequisite : already cleaned and tested tonometer1. Anesthetic drop instilled (4%lignocaine or 0.5% proparacaine )

2. Scale mount must rotate easily and turned towards examiner3. Gently hold the lids of the patient’s right eye apart with other hand4. Gently lower tonometer onto the eye so as to rest footplate central cornea5. Instrument should be perpendicular6. Lower the cylinder slightly, so that tonometer is resting on the eye7. Note the needle position at the scale8. The tonometer is quickly lifted straight up off

• The 5.5 gm weight is initially used.• If scale reading is 4 or less, additional weight is added to plunger.• Conversion table is used to derive IOP in mm Hg from scale reading and plunger

weight.

SOURCES OF ERROR

• Ocular rigidity varies from eye to eye.• As conversion tables are based on an average coefficient of ocular rigidity; eye that

varies significantly from this value gives erroneous IOP. • Repeated measurements lower IOP • Steeper or thicker cornea - greater displacement of fluid-falsely high IOP • Schiøtz reads lower than Goldmann AT

Factors Affecting Scleral Rigidity

• High Scleral Rigidity• hyperopia• long standing

glaucoma

•Low Scleral Rigidity•increasing age• high myopia•Postoperative after RD surgery (vitrectomy, cryopexy, scleral band) •intravitreal injection of compressible gas.•keratoconus

ADVANTAGES DISADVANTAGES LIMITATIONS

1. Portable 1. Falsely high/low IOP Ocular rigidity

1. Instrumental errors

2. Sturdy 2. Cannot be used in traumatic cases/early post op cases/corneal diseases.

2. Muscular contractions of extraocular muscles raises IOP Accomodation decreases IOP

3. Relatively inexpensive 3. Variations in volume of globe Micropthalmos High myopia Buphthalmos

4. Easy to operate 4. Recorded in supine position only.

5. Easy to clean & maintain 5. Reading also influenced by the size of the footplate hole and the thickness and curvature of the cornea.

6. Does not require slit lamp/power supply.

Calibration• Calibrated before each use • Place it on a polished metal sphere• Check: scale reading is zero.• If the reading is not zero, the instrument must be repaired.

Sterilization• Disassemble between each use• Barrel cleaned with 2 pipe cleaners• first soaked in isopropyl alcohol 70 % or methylated spirit• second dry.

• Foot plate - alcohol swab. • All surfaces must be dried before reassembling• can be sterilized with ultraviolet radiation, steam, ethylene oxide.• Other: hydrogen peroxide and bleach.

Features Indentation Applanation

Example Schiotz tonometer Goldmann tonometer

Displacement of fluid Large volume – 30 μl Small volume – 0.5 μl

Influence by scleral rigidity Greatly influenced Not much influenced

Mechanism Fully mechanical Mechanical + optical

Other instrument Not required Slit lamp required

Position of patient supine Sitting (both in perkin’s)

Size small, light, portable Attached to slit lamp so bulky

Fluorescein and cobalt blue filter

Not required Required

Sterilisation Effective Is difficult

Impact Rebound Tonometer

• New and updated indentation tonometer• also known as “impact” or “dynamic tonometry,” was first introduced by

Obbink about 60 years ago • commercially available as the iCare tonometer in 2003 • The method is based on the use of a moving probe that collides with the eye:

the motion parameters of the probe, which vary according to eye pressure, are monitored and used in the calculation of IOP

• Very light, disposable, sterile probe is propelled forward into cornea by a selenoid

Principal based on• Time taken for probe to return to its resting position• Characteristics of rebound motion

• Longer time taken = lower IOP• Shorter time taken = higher IOP

TRANS-PALPEBRAL TONOMETER

Pascal Dynamic Contour Tonometer

Principle: the contours of cornea and tonometer when matched, then the pressure measured at the surface of the eye equals the pressure inside the eye

• Tip of probe matches contour of cornea.

• A pressure transducer built into the center of probe measures outside pressure, which should equal inside pressure

• IOP is recorded digitally on the liquid crystal display (lCD)

• Accuracy : superior to Goldmann tonometry and Pneumotonometry

• IOP is not affected• Corneal thickness.• Corneal refractive surgery that thins the cornea.

BIORESONATOR APPLANATION RESONANCE TONOMETER (ART)• Based on the resonance technique proposed in 2003 • made up of a sensor and transducer that can measure the contact

force in a continuous manner.• Resonance sensor is made up of a cylindrical piezoelectric element

that has a known resonance frequency• When the sensor is brought in contact with the cornea, the acoustic

impedance of the cornea mechanically loads the sensor and modifies the resonance frequency, with a frequency shift, which is proportional to the contact area between sensor and cornea • IOP is calculated from the slope of the relationship between force

and frequency evaluated in a specific frequency shift interval corresponding to an interval applanation area between 4.9 and 11.0 mm2 • The sensor module of the ART is attached to a standard slitlamp in

a similar position as the GAT probe.

REFERENCES

• BECKER-SHAFFERS GLAUCOMA 8th EDITION• SHIELDS GLAUCOMA 6th EDITION• Yanoff and duker ophthalmology, ed 3rd • Ophthalmology investigations and examination techniques • Internet sources

• Ocular Hypertension Treatment Study (OHTS) found that a thinner central cornea was a strong predictive factor for the development of glaucoma in subjects with ocular hypertension.

• Subjects with a corneal thickness of 555 μm or less had a threefold greater risk of developing POAG compared with participants who had a corneal thickness of more than 588 μm.

• Whether this increased risk of glaucoma is due to underestimating actual IOP in patients with thinner corneas or whether thin corneas are a risk factor independent of IOP measurement has not been completely determined, but the OHTS found CCT to be a risk factor for progression independent of IOP level.

Tonometry for special clinical circumstances

Irregular cornea •Pneumatic tonometer-Preferred•Goldmann,NCT,Tonopen-limited accuracy

Soft contact lenses •Pneumo tonometry,Tonopen

Gas filled eyes •Pneumatic tonometry,Tonopen

Flat AC• Goldmann,pneumotonometer,t

onopen

Keratoprostheses • Tactile assessment

Calibration• Calibration arm fits into a slot on the side of the tonometer

• The rod is positioned, central mark is aligned with mark on its holder

• Measuring drum placed at 0

• Pressure arm should gently rock forwards and backwards

• Moving measuring drum between –0.5 and +0.5 mmhg should likewise cause the pressure arm to rock

• Rod is advanced to the next mark and the process repeated at 20 mmhg and then 60 mmhg

• Tonometer arm should rock between 19.5 mmHg and 20.5 mmHg and 59 mmhg and 61 mmHg respectively.