different types of tonometry

TONOMETRY

PRESENTER: DR.SIDDHARTH GAUTAM

DEFINITIONS Tonometry is the procedure performed to determine the intraocular

pressure (IOP).

“Normal” intraocular pressure (IOP) may be defined as that pressure which does not lead to glaucomatous damage of the optic nerve head.

Such a definition cannot be expressed in precise numerical terms as-

1) Individuals show different susceptibility to optic nerve damage at given pressure levels.

2) It also depends on the underlying form of glaucoma.

The best we can do is to describe the distribution of IOP in general populations to establish levels of risk for glaucoma within different pressure ranges

HISTORY1826-William Bowman Digital tonometry(as routine

examination)1863-Albrecht von Grafe Designed first instrument to

attempt to measure IOP1865-Donders1880-Preistly

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

1885-Malkalov 1st Applanation tonometer1905-Hjalmar Schiotz Indentation tonometry1948,1955-Friedenwald Coefficient of ocular rigidity1954-Goldmann Prototype Applanation

tonometer(constant area)1972-Grolmann NCTGrant Electronic indentation tonometerHalberg Hand held 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

FACTORS INFLUENCING IOP

Rate of aqueous formation

Resistance of aqueous outflow ( drainage)

Increased episcleral venous pressure

Dilatation of pupil

Heredity

Age

Sex

Diurnal variation

Postural variation

Blood pressure

Osmotic pressure of blood

Tonometry Direct

ManometryIndirect

TYPES OF TONOMETRY

INDENTATION APPLANATIONSchiotz Variable force

Examples- GAT, Perkins, MMT, Tonopen, Pneum. Tonometer

Mercurial Constant forceExamples- Maklakov, Glucotest, Applanometer

ElectronicScleral Tonometry

Vogelsang 1927 – Ballistic Tonometer .The rebound of a small metal ball from the eye is measured and this depends to a large extent on the physical properties of the coats of the eye.

Roth & Blake 1963- Vibration tonometer cause minimal deformation by oscillating force by a probe, which also functions as a sensor and measures the resonant frequency of the eye.

Newer tonometers1. Trans –palpebral Tonometer2. Disposable tonometer

Tonosafe – acrylic biprismTonoshield- silicone shield

3. Dynamic contour tonometer4. Ocular Response Analyzer

A.HIGH DISPLACEMENT TONOMETERS

B.LOW DISPLACEMENT TONOMETERS

Tonometers that displace a large volume of fluid and consequently raise IOP significantly.

Tonometers in which the IOP is negligibly raised during tonometry (less than 5%).

Examples1. Schiotz.2. Maklakov.

Examples1. Goldmann Applanation

Tonometer.2. Mackay-Marg tonometer.

The Goldmann tonometer displaces only 0.5 μl of aqueous humor and raises IOP by only 3%.

Less accurate More accurate

MANOMETRYNeedle inserted into the AC through a self-sealing,

beveled corneal puncture

Movement of membrane, recorded optically or electronically, is a measure of IOP.

Tubing can also be connected to a fluid-filled reservoir that has a pressure-sensitive membrane.

Height of the fluid in the tubing corresponds to IOP.

Needle is connected to a fluid-filled tubing

DISADVANTAGES:

1. Not practical method for human beings.

2. Needs general anesthesia.

3. Introduction of needle produces breakdown of blood aqueous barrier and release of prostaglandins which alter IOP.

USES:

1. It is used for continuous measurements of IOP.

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

TONOMETRY It is an indirect method of measuring the IOP.

It includes digital tonometry.

Three basic types of Tonometers:o Indentationo Applanation(flattening)o Non-Contact

DIGITAL TONOMETRY/PALPAT

ION METHOD Intraocular pressure (IOP) is estimated by response of

eye to pressure applied by finger pulp.

PROCEDURE: Patient 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.If IOP is raisedfluctuation produced is feeble or absent and the eyeball feels firm to hard.When the IOP is very loweye feels soft like a partially filled balloon.

`

ADVANTAGES DISADVANTAGESEasiest to perform Reading not properNo equipment Only depends on examinerNo anaesthesia Over-estimation or under-

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

INDENTATION TONOMETRY

All clinical tonometers measure the IOP by relating a deformation of the globe to the force responsible for the deformation.

In indentation tonometry, a known weight is placed on the cornea, and the IOP is estimated by measuring the deformation or indentation of the globe.

Eg-Schiotz tonometer

Parts of schiotz tonometer scale

needle

Weight 5.5g plungerholder

Foot plate

lever

3mm diameter

ROC 15mm

Tonometer weight = 11g

Additional weights 7.5,10,15g

The extent to which cornea is indented by plunger is measured as the distance from the foot plate curve to the plunger base and a lever system moves a needle on calibrated scale.

The indicated scale reading and the plunger weight are converted to an IOP measurement.

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

Each scale unit represents 0.05 mm protrusion of the plunger.

PRINCIPLE When the plunger indents the cornea, the baseline or

resting pressure(Po) is artificially raised to a new value(Pt).

Change in pressure from Po to Pt Expression of resistance an eye offers to displacement of a volume of fluid (Vc).

Because the tonometer actually measures Pt , it is necessary to estimate Po for each scale reading & weight.

Pt=P0 + E(Scleral Rigidity)

Friedenwald’s gave a mathematical formula.

The formula has a single numerical constant, the coefficient of ocular rigidity (K), which is roughly an expression of the distensibility of the eye.Its average value is 0.025.

He developed a nomogram for estimating K on the basis of two tonometric readings with different weights

in which Pt1 , and V1 , represent the tonometric pressure and the volume of the indentation caused by the bar in the determination made with the first weight, whereas Pt2 and V2 represent the respective values as obtained with the second weight.

Log Pt2/Pt1 = K (V2-V1)

On the basis of this formula, he developed a set of conversion tables for IOP.

  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

PROCEDURE Patient should be anasthetised with 4% lignocaine or 0.5% proparacaine

With the patient in supine position, looking up at a fixation target while examiner separates the lids and lowers the tonometer plate to rest on the anesthetized cornea so that plunger is free to move vertically .

Scale reading is measured.

The 5.5 gm weight is initially used.

If scale reading is 4 or less, additional weight is added to plunger.

IOP measurement is repeated until 3 consecutive readings agree within 0.5 scale units.

Conversion table is used to derive IOP in mm Hg from scale reading and plunger weight.

OCULAR RIGIDITY Measure of distensibility or resistance to deformation of ocular coats. Important in indentation tonometer Increase in ocular rigidity increase IOP

Long standing glaucoma ARMD High Hyperopia Vasoconstrictors

Decrease in ocular rigidity decrease in IOP Increasing age Strong Miotic therapy Vasodilator therapy Post operative after RD surgery(Vitrectomy, cryopexy,scleral

band) High Myopia Compressible gas

ADVANTAGES DISADVANTAGES LIMITATIONS1. Portable 1. Falsely high/low IOP

Ocular rigidity1. 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 IOP3. 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 CHECK PROCEDURE

A calibration check should be done at the start of every day. Place the footplate of the instrument on the rounded test block (the dummy cornea) provided with the tonometer’s storage case.

With the footplate resting on the test block, a correctly calibrated instrument will have a scale reading of zero.

If not, you can calibrate it to zero.

If the needle is to the left of zero, rotate the footplate in a clockwise direction and check again.

If the needle is to the right of the zero position, rotate the footplate in an anti-clockwise direction.

STERILIZATION The tonometer is disassembled between each use and the barrel is

cleaned with 2 pipe cleaners, the first soaked in isopropyl alcohol 70 % or methylated spirit and the second dry.

The foot plate is cleaned with alcohol swab.

All surfaces must be dried before reassembling.

The instrument can be sterilized with ultraviolet radiation, steam, ethylene oxide.

In between patients, the Schiotz tonometer should be disinfected by soaking it in sodium hypochlorite.

As with other tonometer tips, the Schiotz can be damaged by some disinfecting solutions such as hydrogen peroxide and bleach.

DIFFERENTIAL TONOMETRY

It is done to get rid from ocular rigidity.

A reading is taken with one weight on the Plunger and then a second reading' is taken with a different weight.

Making a diagnosis of glaucoma in a pt. with myopia presents unusual difficulties. The low ocular rigidity in these eyes result in Schiotz readings within normal limits.

5.5g 10g Ocular rigidity

IOP

18 mm Hg 15 mm Hg lower >1818 mm Hg 21 mm Hg higher <18

18 mm Hg 18 mm Hg equal 18

ELECTRONIC SCHIOTZ

TONOMETER Has a continuous recording of IOP that is used for

tonography.

ADVANTAGE:

Scale is magnified, which makes it easier to detect small changes in IOP.

IMPACT-REBOUND TONOMETER

New and updated version of an indentation tonometer.

PROCEDURE ADVANTAGES DISADVANTAGES

Sterile probe is propelled forward into the cornea by a solenoid; the time taken for the probe to return to its resting position and the characteristics of the rebound motion are indicative of the IOP (and also the biomechanical properties of the cornea)

1. Very light2. Disposable3. Probe is

extremely light and its contact with the cornea is very short , used without first anesthetizing the eye.

1. Tend to read slightly higher than the Goldmann.

2. Accuracy falls off in scarred corneas.

Time taken for the probe to return to its resting position is longer in eyes with lower IOP and faster in eyes with higher IOP.

4.Comparable to the Goldmann in both normal and post-keratoplasty human eyes.

Rebound tonometer does correlate, like the Goldmann, with central corneal thickness.

5.Used in screening situations when patients are unable to be seated or measured at the slit lamp.

APPLANATION TONOMETRY(Haag Streit, Koeniz, Switzerland)

PARTS OF GOLDMANN APPLANATION TONOMETER

Biprism (measuring prism)

Feeder arm

Housing

Adjusting knob

Connects to the slit lamp

Control weight insert

PRINCIPLE The concept was introduced by Goldmann is 1954.

It is based on IMBERT FICKS LAW.

It states that the 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

The ideal sphere is dry, thin-walled and flexible.

The cornea is 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

Thus, 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.

Applanation tonometry displaces only about 0.5 microlitre of aqueous humor, which 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 pressure readings.

PROCEDUREPatient is asked not to drink alcoholic beverages or large amounts of fluid (e.g.,

500 ml or more) for 2 hours before the test, as the former will lower IOP and

the latter may raise it.

Patient is told the purpose of the test and is reassured that the measurement is

not painful. The patient is instructed to relax, maintain position, and hold the

eyes open wide.

One drop of a topical anesthetic, such as 0.5% proparacaine, is placed in each

eye, and the tip of a moistened fluorescein strip is touched to the tear layer on

the inner surface of each lower lid.

Tonometer tip is cleaned with a sterilizing solution, and the tip and prism are set

in correct position on the slit lamp.

Tension knob is set at 1g. If the knob is set at 0, the prism head may vibrate when it

touches the eye and damage the corneal epithelium. The 1 g position is used before each

measurement. As a rule, it is more accurate to measure IOP by increasing rather than

decreasing the force of applanation.

0 graduation mark of the prism is set at the white line on the prism holder.

Cobalt blue filter is used with the slit beam opened maximally. The angle

between the illumination and the microscope should be approximately

60°.The room illumination is reduced.

Heights of the slit lamp, chair, and chin rest are adjusted until the patient is

comfortable and in the correct position for the measurement.

Palpebral fissure is a little wider if the patient looks up. However, the gaze should be no more than 15° above the horizontal to prevent an elevation of IOP that is especially marked in the presence of restrictive neuromuscular disease such as dysthyroid ophthalmopathy.

Operator sits opposite the patient, the assembly is advanced towards the patient

with the tester observing from the side until the limbal zone has a bluish hue.

If the tonometer tip touches the lids, the fluorescein rings will thicken, which

may cause an overestimation of IOP.

Clinician observes the applanation through the biprism at low power. A

monocular view is obtained of the central applanated zone and the surrounding

fluorescein-stained tear film.

Using the control stick, the observer raises, lowers, and centers the assembly

until two equal semicircles are seen in the center of the field of view.

If the two semicircles are not equal in size, IOP is overestimated.The clinician

turns the tension knob in both directions to ensure that the instrument is in good

position.

If the semicircles cannot be made ‘too small,’ the instrument is too far

forward. If the semicircles cannot be made ‘too large,’ the instrument is too

far from the eye.

Fluorescein rings should be approximately 0.25–0.3mm in thickness – or

about one-tenth the diameter of the flattened area. If the rings are too narrow,

the patient should blink two or three times to replenish the fluorescein.

If the fluorescein rings are too narrow, IOP is underestimated.

The tension knob is rotated until the inner borders of the fluorescein rings

touch each other at the midpoint of their pulsations.

Intraocular pressure is measured in the right eye until three successive readings

are within 1 mmHg. Intraocular pressure is then measured in the left eye.

Reading obtained in grams is multiplied by 10 to give the IOP in millimeters

of mercury.

cont….

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 approximately 0.25–0.3 mm in thickness – or about one-tenth the diameter of the flattened area.

POTENTIAL SOURCES OF ERROR

FALSELY LOW IOP FALSELY HIGH IOPToo little fluoroscein Too much fluorosceinThin cornea Thick corneaCorneal edema Steep corneaWith the rule astigmatism 1mm Hg per 4D

Against the rule astigmatism 1 m Hg per 3D

Prolonged Cataract Wider meniscusRepeated tonometry Widening the lid fissure

excessivelyElevating the eyes more than 150

Wider meniscus or improper vertical alignment gives higher IOP readings

If the two semicircles are not equal in size, IOP is overestimated.

For every 3D increase in corneal curvature, IOP raises about 1 mm Hg as more fluid is displaced under steeper corneas causing increase in ocular rigidity

More than 6 D astigmatism produces an elliptical area on applanation that gives erroneous IOP. 4D with-the-rule astigmatism underestimate IOP and 4D against-the-rule astigmatism overestimate IOP.

Mires may be distorted on applanating on irregular corneas .

Elevating the eyes more than 15° above the horizontal causes an overestimation of IOP.

Widening the lid fissure excessively causes an overestimation of IOP

Repeated tonometry reduces IOP, causing an underestimation of the true level.This effect is greatest between the first and second readings, but the trend continues through a number of repetitions.

A natural bias for even numbers may cause slight errors in readings.

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 550 μm.

The deviation of CCT from 550 μm yields a change in applanation readings of 0.7 mm Hg per 10 μm.

IOP measurements are also modified after PRK and LASIK.

Thinning of the central cornea is gives lower readings on applanation.

Potential Sources of Error – During Measurement

Potential Sources of Error – During Measurement

Potential Sources of Error – During Measurement

Potential Sources of Error – During Measurement

Potential Sources of Error – During Measurement

Potential Sources of Error – During Measurement

Potential Sources of Error – During Measurement

Potential Sources of Error – During Measurement

Potential Sources of Error – During Measurement

Potential Sources of Error – During Measurement

ADVANTAGESMost accurate

No indentation, so not much force is applied on cornea

Does not get affected by corneo-scleral rigidity

Readings are directly from knob

Can be done on post op cases/injury cases

CALIBRATION Goldmann tonometer should be calibrated 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 Biprism should be rinsed and dried immediately after

use.

Between uses, the prism head should be soaked in a solution such as diluted bleach or 3% hydrogen peroxide.

70% ethanol and 70% isopropanol are effective as sterilizing solutions but were shown in one study to cause mild damage to the tonometer tip after one month of immersion.

Other methods of sterilization include: 10 min of rinsing in running tap water, wash with soap and water, cover the tip with a disposable film, and exposure to UV light.

It is possible to transfer bacteria, viruses, and other infectious agents with the tonometer head, including such potentially serious infections as epidemic keratoconjunctivitis, hepatitis B, Jacob-Kreutzfeld and, theoretically, acquired immunodeficiency syndrome.

Care must be taken to be sure any sterilizing solution has been completely rinsed off the tonometer tip, as some of these solutions may be toxic to the corneal epithelium, especially after LASIK or other corneal procedures.

If the tonometer tip is not mechanically wiped after each use, epithelial cells may stick to the tip with the small but serious risk of transmitting Jacob-Kreutzfeld virus.

Disposable tonometer tips may be an acceptable alternative to soaking in, and wiping with, antiseptic solutions.

PERKIN’S TONOMETER

Similar to the Goldmann tonometer.

ADVANTAGES OVER GOLDMANN TONOMETER:

Portable and counterbalanced, so it can be used in any position.

Therefore,useful in a number of situations, including in the operating room, at the bedside, and with patients who are obese or for other reasons cannot be examined at the slit lamp.

DRAEGER TONOMETER

Similar to the Goldmann and Perkins tonometers.

Except :

Uses a different biprism.

Force for applanation is supplied by an electric motor.

MacKay-Marg Tonometer

1.5 mm diameter plunger

Rigid spring

Rubber sleeve.

Movement of plunger is electronically monitored by a transducer and recorded on a moving paper strip.

USEFUL FOR MEASURING IOP IN EYES WITH:

1. Scarred, irregular, or edematous corneas

2. Accurate when used over therapeutic soft contact lenses.

TONOPEN Same principle as that of Mackay-Marg tonometer.

Battery operated, portable.

USES:

1. Community health fairs

2. Ward rounds

3. Children

4. Irregular surfaces

5. Measuring through an amniotic membrane patch graft.

A disposable latex cover which is discarded after each use provides infection control.

TONOPEN

• For pressures from 6 to 24 mmHg, it measured an average of 1.7 mm higher than the Goldmann tonometer.

• Above 24 mmHg, the readings were similar.

DISADVANTAGESOverestimate the IOP in infants so its usefulness in congenital

glaucoma screening and monitoring is somewhat limited.

In band keratopathy where the surface of the pathology is harder than normal cornea, the Tono-Pen tends to overestimate the IOP.

PNEUMATIC TONOMETER

Principle is similar to the MacKay-Marg tonometer.

Cornea is applanated by touching apex by silastic diaphragm covering sensing nozzle.

It is connected to central chamber containing pressurized air.

There is pneumatic to electronic transducer.

It converts the air pressure to recording on paper strip and IOP is read.

MAKLAKOV TONOMETER

Differs from the other applanation instruments

A known force is applied to the eye, and the area of applanation is measured – a technique known as constant-force rather than constant-area applanation.

The instrument consists of a wire holder into which a flat-bottom weight, ranging from 5 to 15g, is inserted.

Surface of the weight is painted with a dye, such as mild silver protein (Argyrol) mixed with glycerin, and then the weight is lowered onto the cornea.

During the procedure the patient is supine, and the cornea is anesthetized.

Weight is lifted from the cornea, and the area of applanation is taken to be the area of missing dye, which is measured either directly or indirectly from an imprint on test paper.

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

Intraocular pressure is measured in grams per square centimeter and is converted to millimeters of mercury by dividing by 1.36.

Pt= W /π(d/2)2

Displaces a greater volume of aqueous humor than the other applanation devices (but less than a schiøtz tonometer), which means that the IOP readings are more influenced by ocular rigidity.

DISADVANTAGE:

OCUTON TONOMETER

Hand-held tonometer.

Works on the applanation principle.

Probe is so light that it is barely felt and, therefore, needs no anesthetic in most patients.

Device is comparable to Goldmann tonometry but tends to read higher than the Goldmann tonometer when the cornea is thicker, and its accuracy may be compromised by diurnal changes in corneal thickness.

NON-CONTACT TONOMETER

Introduced by Grolman.

Original NCT has 3 subsystems:

1. Alignment system: It aligns patient’s eye in 3 dimensions.

2. Optoelectronic applanation monitoring system:

It comprises transmitter, receiver and detector, and timer.

a. Transmitter directs a collimated beam of light at corneal apex.

b. Receiver and detector accept only parallel coaxial rays of light reflected from cornea.

c. Timer measures from an internal reference to the point of peak light intensity.

3. Pneumatic system: It generates a puff of room air directed against cornea.

PRINCIPLE A puff of room air creates a constant force that momentarily

flattens the cornea. The corneal apex is deformed by a jet of air.

The force of air jet which is generated by a solenoid activated piston increases linearly over time.

When the reflected light is at peak intensity, the 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.

The time from an internal reference point to the moment of flattening is measured and converted 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 the sensor is activated by the reflected light, the air generator is switched off. The level of force at which the generator stops is recorded, and a computer calculates and displays the intraocular pressure.

ADVANTAGES LIMITATIONS Screening procedure IOP is near normal,accuracy

decreases with increase in IOP & in eyes with abnormal cornea/poor fixation.

Can be operated by non-medical porsennel

No anaesthesia required No direct contact between

instrument & eye

New NCT, Pulsair is a portable hand held tonometer.

I-CARE TONOMETER Handheld,portable & rebound tonometer.

Turn on the unit by pressing the measurement button. It will beep and then display LOAD.

Place the single use probe into the collar and push the measurement button again.

When the I-care tonometer is ready to use, it is brought to the patient’s eye with the central grove in the horizontal position.

The distance of the eye to the tip of the probe is from 4-8 millimeters.

Press the measurement button and take no less than six measurements.

After the six measurements the IOP will be displayed.

TRANS-PALPEBRAL TONOMETER

PRINCIPLE USES LIMITATIONS

Pressure on the eyelid in most eyes produces retinal phosphenes.

Young children Scleral rigidity

Pressure on the eyelid required to induce these phosphenes is proportional to the intraocular pressure.

Demented patients and severely developmentally-challenged patients.

Thickness of the eyelids

Portable patients can measure their own IOP at home

Orbicularis muscle tone

Potential intra palpebral scarring

DYNAMIC CONTOUR TONOMETRY

Introduced by Kanngiesser

Based on Pascal’s Law of Pressure, which states that pressure applied to a confined fluid is transmitted undiminished throughout the confining vessel of the system.

Concave shape of the tip generates minimal corneal distortion and theoretically eliminates errors in measuring IOP induced by ocular rigidity when the cornea is applanated with a flat tipped tonometer.

Operated in a fashion similar to a goldmann applanation tonometer.

Pascal Dynamic Contour Tonometer (A) utilizes the principle that when the contours of the cornea and tonometer match, then the pressure measured at the surface of the eye equals the pressure inside the eye (B).

Microchip-enabled, solid-state sensor embedded within the tip records 100 IOP measurements per second and averages them over fluctuations in ocular pulse amplitude.

Digital display shows the final averaged IOP as well as a Q-value that can be used objectively to judge the quality of the final measurement.

OCULAR RESPONSE ANALYZER

Ocular response analyser (ORA) is a non-contact (air puff) tonometer that does not require topical anaesthesia and provides additional information on the biomechanical properties of the cornea.

Uses an air pulse to deform the cornea into a slight concavity.

Measures not one but two pressures at which applanation occurs:

1. When the air jet flattens the cornea as the cornea is bent inward .

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

The difference between the pressures at which the cornea flattens inward and outward is measured by the machine and termed corneal hysteresis (CH).

The machine uses this value to correct for the effects of the cornea on measurement.

IOP correlate well with Goldmann tonometry but, on average, measure a few millimeters higher.

Congdon et al found that a ‘low’ hysteresis reading with the ORA correlates with progression of glaucoma, whereas thin central corneal thickness correlates with glaucoma damage.

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,tonopen do not correlate well with manometrically determined pressures.

Keratoprostheses • Tactile assessment

REFERENCES BECKER-SHAFFERS GLAUCOMA 8th EDITION

SHIELDS GLAUCOMA 6th EDITION

ANATOMY & PHYSIOLOGY OF EYE A.K.KHURANA

DIAGNOSTIC PROCEDURES IN OPHTHALMOLOGY -BY HV NEMA, NITIN NEMA

THANK YOU

PG ACTIVITY ON TUESDAY(09/02/16) CASE-PROPTOSIS PRESENTER-DR.SURABHI SHARMA MODERATOR-DR.MALA KAMBLE CONDUCTOR-DR.SACHIN DAIGAVANE