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method available, it is not feasible in human being because of its invasive nature.

Indirect Method

It is based on eyes response to an applied force.

Palpation Method

Intraocular pressure (IOP) is estimated by response of eye to pressure applied by finger pulp (indents easily/firm to touch).

The indirect methods can be broadly divided into contact and non-contact methods. Basic types of contact tonometers differ according to shape and magnitude of deformation.

Contact Tonometers

IOP measurement is performed by deforming the globe and correlating the force responsible for deformation to the pressure within the eye. Both indentation and applanation tonometers effect a deformation of globe but the magnitude varies (Fig. 6.1).

Fig. 6.1: A Deformation of globe during indentation tonometry, B Deformation of globe during applanation tonometry

Indentation Tonometer

Indentation tonometer is used to measure the amount of deformation or indentation of the globe in response to a standard weight applied to the cornea or the area flattened by a standard force.

The shape of corneal deformation is truncated cone. It displaces large intraocular volume so conversion tables based on empirical data is used to estimate IOP. The prototype is Schiøtz tonometer.

Applanation Tonometers

Applanation tonometers are used to measure force necessary to flatten a small, standard area of cornea. The shape of corneal deformation is simple flattening. The shape is constant so IOP is derived from a mathematical calculation. They are of 2 types on the basis of variable that is measured.

Variable force: Area of cornea on applanation held constant, force varies. Prototype is Goldmann tonometer.

Variable area: Force applied to cornea held constant, area varies. Prototype is Maklakov tonometer. The volume displacement is sufficiently large to require a conversion table.

Noncontact Tonometer

Noncontact tonometer measures time required to deform a standard area of corneal surface in response to a jet of air.

Schiøtz Tonometer

Schiøtz tonometer (Fig. 6.2) consists of metal plunger that slides through a hole in a concave metal plate. The plunger supports a hammer device connected to needle that crosses a scale. 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

Fig. 6.2: Schiøtz tonometer

The standard instrument has following characteristics:

Foot plate has concavity of 15 mm radius of curvature. The tonometer weighs 11 gm.

Plunger has 3 mm diameter, a weight of 5.5 gm including the force of the lever rests on top of the plunger. Additional weights are added to plungertoincreaseitto7.5,10,or15gm.Thescale reading is zero when plunger extends 0.05 mm beyond foot plate curve. Each scale unit represents 0.05 mm protrusion of the plunger.

Basic concept: 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. Determination of Po from a scale reading Pt requires conversion which is done according to Friedenwald conversion tables. Friedenwald

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generated an empirical formula for linear relationship between the log function of IOP and the ocular distension. This formula has ‘C’ a numerical constant, the coefficient of ocular rigidity which is an expression of distensibility of eye. Its average value is 0.025.

Technique: Patient should be 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 (Fig. 6.3). A fine movement of needle on scale is in response to ocular pulsations. Scale reading is an average of the extremes of these excursions. 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. The instrument is calibrated before each use to check scale (reading is zero).

Fig. 6.3: Technique of tonometry

Sources of error: Accuracy is limited as 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. High ocular rigidity is seen in high hyperopia, long-standing glaucoma, age-related macular degeneration, andvasoconstrictortherapy.Lowocularrigidity

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is found in high myopia, advanced age, miotics, useofvasodilators,afterRDsurgery(vitrectomy, cryopexy, scleral band) and intravitrealinjection of compressible gas. The variable expulsion of intraocularbloodduringSchiøtztonometrymay influenceIOPmeasurement.Repeatedmeasurements lower IOP. Either a steeper or a thicker cornea causes greater displacement of fluid during tonometry and gives a falsely high IOP measurement.

Variable Force Applanation Tonometers

Goldmann Applanation Tonometer (GAT)

Basic concept: Based on Imbert-Fick law, an external force (W) against a sphere equals the pressure in the sphere (P) times the area flattened (applanated) by external force (A)

W = P × A

Cornea being aspherical, wet, and slightly inflexible fails to follow the law. Moisture creates surface tension (S) or capillary attraction of tear film for tonometry head. Lack of flexibility requires force to bend the cornea (B) which is independent of internal pressure. The central thickness of cornea is about 0.55 mm and the outer area of corneal flattening differs from the inner area of flattening (A1). It is this inner area which is of importance.

Modified Imbert-Fick Law is W + S = PA1 + B

When A1 = 7.35 mm2, S balances B and W =P. This internal area of applanation is achieved when the diameter of the external area of corneal applanation is around 3.06 mm. Grams of force applied to flatten 3.06 diameter of the cornea multiplied by 10 is directly converted to mmHg.

Instrument: Instrument is mounted on the end of a lever hinged on the slit-lamp (Fig. 6.4). Examiner views through the center of plastic

Fig. 6.4: Goldmann applanation tonometry

Fig. 6.5: Biprism in the Goldmann tonometer

biprism (Fig. 6.5) which is used to applanate cornea. Two beam splitting prisms within applanating unit optically convert circular area of corneal contact in 2 semicircles. Edge of corneal contact is made apparent by instilling fluorescein while viewing in cobalt blue light. By manually rotating a dial calibrated in grams, the force is adjusted by changing the length of a spring within the device. The prisms are calibrated in such a fashion that inner margin of semicircles touch when 3.06 mm of the cornea is applanated. Biprism is attached by a rod to a housing which contains a coil spring and series of levers that are used to adjust the force of the biprism against the cornea.

Technique: Cornea is anesthetized, tear film is stained with sodium fluorescein. Cornea and biprism is illuminated by a cobalt blue light.

Fluorescein facilitates visualization of tear meniscus at margin of contact. Fluorescent semicirclesareviewedthroughthebiprism.Force against the cornea is adjusted until the inner edges overlap. Ocular pulsations create excursions of semicircular tear meniscus and IOP is read as the median over which arc glides. This is the end point (Fig. 6.6) at which a reading can be taken from a graduated dial which indicates grams of force applied to tonometer and so this number is multiplied by 10 to obtain IOP in mm Hg.

Fig. 6.6: End point recording of IOP

Sources of error in applanation tonometry

1.Inadequate concentration of fluorescein in precorneal tear film gives hypofluorescence.

2.Fluorescein may lose fluorescence in acidic solution (quenching of fluorescence) causing underestimation of IOP.

3.Wider meniscus or improper vertical alignment gives higher IOP readings (Figs 6.7A and B).

4.Thin corneas underestimate and thick corneas overestimate IOP.

5.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.

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Figs 6.7A and B: Vertical misalignment. To minimize this, tonometer biprism should be rotated so that axis of least corneal curvature is opposite the red line on the prism holder. Other method is to obtain measurements with mires oriented horizontally and vertically and to average these readings

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

7.Mires may be distorted on applanating on irregular corneas.

Effect of central corneal thickness (CCT): Variations in corneal thickness change the resistance of the cornea to indentation so that this is no longer balanced entirely by the tear film surface tension thus affecting the accuracy of IOP measurement. A thinner cornea may require less force to applanate it, leading to underestimation of true IOP while a thicker cornea would need more force to applanate it, giving an artificially higher IOP. The Goldmann applanation tonometer was designed to give accurate readings when the CCT was 520 μm. As shown by Ehlers et al, there can be under estimation or overestimation of IOP when the corneal thickness is less or more than 520 micron, respectively. They interpolated that deviation of CCT from 520 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 believed to give lower readings on applanation.

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Other Variable Force Applanation Tonometers

Hand-Held Goldmann-Type Tonometers

Perkins Tonometer

Perkins tonometer (Fig. 6.8) uses same prisms as Goldmann but is counterbalanced so that tonometry is performed in any position (Fig. 6.9). The prism is illuminated by battery powered bulbs. The force on the prisms is adjusted manually. Being portable it is practical when measuring IOP in infants / children and for use in operating rooms.

Fig. 6.8: Perkins tonometer

Draeger Tonometer

Draeger tonometer is similar to Perkins but uses different set of prisms and operates with a motor adjusting the force on these prisms.

Fig. 6.9: Tonometry with Perkins tonometer

Mackay-Marg Tonometer

Basic concept: Force is required to keep the flat plate of a plunger flush with a surrounding sleeve against the pressure of corneal deformation. Tonometer incorporates a 1.5 mm diameter plunger affixed to a rigid spring that extends 10 μm beyond the plane of surrounding rubber sleeve. Movement of plunger is electronically monitored by a transducer and recorded on a moving paper strip. When the tonometer is placed against cornea, the tracing that represents the force applied to the plunger begins to rise. At 1.5 mm of corneal area applanation, tracing reaches a peak and the force applied = IOP + force required to deform the cornea. At 3 mm flattening, force required to deform cornea is transferred from plunger to surrounding sleeve, creating a dip in tracing corresponding to IOP. Flattening of >3 mm of area gives artificial elevation of IOP. It is accurate in eyes with scarred, edematous and irregular corneas.

Other Mackay-Marg-type Tonometers: CAT 100 Applanation and Biotronic Tonometers

They have an internal logic program which automatically selects the acceptable measurement and 3 or more good IOP readings are averaged and displayed on screen.

Tonopen

Tonopen (Fig. 6.10) is a portable and battery operated tonometer. It has the same principle as that of Mackay-Marg tonometer. The tip has a strain gauge that is activated when in contact with cornea. The built-in microprocessor logic circuit senses a trough force and records until an acceptable measurement is achieved. Four to ten such measurements are averaged to give a final IOP which is displayed.

Fig. 6.10: Tonometry with tonopen

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The probe tip is applied perpendicularly to cornea until it is just indented. An audible click indicates that the measurement is acceptable. The process is repeated 4-10 times until a beep indicates a statistically valid average reading.

Pneumatonometer

Pneumatonometer or pneumatic tonometer is like Mackay-Marg tonometer. It has a core sensing mechanism for measuring IOP while force required to bend the cornea is transferred to surrounding structure. The sensor is a air pressure like electronically controlled plunger in Mackay-Marg tonometer. It can also be used for continuous monitoring of IOP. It gives significantly higher IOP estimates.

Constant Force Applanation Tonometry

Maklakov Applanation Tonometer

With Maklakov applanation tonometer IOP is estimated by measuring the area of cornea flattened by a known weight. It consists of a dumb-bell-shaped metal cylinder with flat end plates of polished glass on either end with a diameter of 10 mm. Tonometers weighing 5, 7.5, 10, and 15 gm are used to measure the IOP. Crossaction wire handle to support instrument on the cornea is used. A thin layer of dye is spread onto the bottom of either end plate and the instrument is brought in contact with anesthetized cornea in supine position for 1 second. A circular white imprint on end plate corresponds to the area of corneal flattening. Area is measured and IOP is read from conversion table in the column corresponding to the weight used.

Noncontact Tonometer

Noncontact tonometer (NCT) was introduced by Grolman. A puff of room air creates a

102Diagnostic Procedures in Ophthalmology

constant force that momentarily flattens the cornea. The time from an internal reference point to the moment of flattening is measured and converted to IOP. 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.

Fig. 6.11: Tonometry with noncontact tonometer

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.

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. NCT is accurate if IOP is nearly

normal, accuracy decreases with increase in IOP and in eyes with abnormal cornea or poor fixation. New NCT, Pulsair is a portable hand held tonometer.

Devices under Investigation

Flushfittingsilasticgelcontactlensinstrumented with strain gauges that measures changes in meridionalangleofcorneoscleraljunctioncaused by variations in IOP. A similar device using a pressure transducer is made in form of a cylindrical guard ring applanation tonometer.

A scleral gauge is embedded in an encircling scleral band to measure the distension of globe.

An instrument using suction cups for recording IOP up to 1 hour in supine position is under investigation.

Comparison, Calibration and

Sterilization of Different Tonometers

Comparison

Goldmann Applanation Tonometer (GAT)

In eyes with regular corneas, GAT is generally accepted as the standard against which other tonometers must be compared. Even with GAT, inherent variability must be taken in account.

Schiøtz Tonometer

Studies indicate that Schiøtz reads lower than GAT even when the postural influence on IOP is eliminated by performing measurements in supine position. The magnitude of difference between the two tonometers and the influence of ocular rigidity are such that Schiøtz indicates only that the IOP is within a certain range and is of limited value even for screening purposes.

Perkins Applanation Tonometer

Perkins applanation tonometer compares favorably against GAT. In one study, difference between readings with the two instruments was 1.4 mmHg. It is subject to the same influence of corneal thickness as the GAT. It is useful in infants and children and is accurate in horizontal as well as vertical position.

Draeger Applanation Tonometer

Comparative studies of Draeger applanation tonometer with GAT have given inconsistent results because of its more complex design. Draeger tonometer is more difficult to use than the Perkins. Patient’s acceptance to Draeger tonometer is poor.

Mackay-Marg Tonometer (MMT)

Highly significant correlation is found between MMT and GAT readings. The average mean MMT values are often higher than GAT.

Mackay-Marg Type Tonometers

Tonopen has compared favorably against manometric readings in human autopsy eyes but it may cause a significant increase in IOP during measurements. It has good correlation with GAT readings within normal IOP ranges. But most studies indicate that tonopen under estimates IOP in the higher ranges and over estimates in the lower range.

Pneumatic Tonometer

Pneumatic tonometer correlates well with GAT readings. However, it gives significantly higher IOP estimates.

Noncontact Tonometer

Noncontact tonometer is reliable within the normal IOP range, although its reliability is

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reduced in higher IOP ranges and is limited by abnormal corneas or poor fixation. Corneal thickness has greater influence on NCT than on GAT. The hand-held pulsair NCT has compared favorably with Goldmann applanation readings in normal and glaucomatous eyes. It tends to read lower IOP above the normal range.

Tonometry on Irregular Corneas

Accuracy of GAT and Maklakov-type applanation tonometers and NCT is limited in eyes with irregular corneas. MMT is considered to be accurate in scarred or edematous corneas. As MMT applanates a small surface area, the effects of corneal resistance to deformation and surface tension of tears are less than that with GAT. Pneumotonometer has also been shown to be useful in eyes with diseased cornea. Tonopen compared favorably with MMT on irregular corneas in a study.

Tonometry over Soft Contact Lens

MMT, pneumtonometer and tonopen can measure the IOP through bandage contact lens with reasonable accuracy although soft contact lenses of different powers create a bias with tonopen. Applanation tonometers are affected by the power of the contact lens with high water content and correction tables are developed to compensate it. The power of soft contact lenses influences the difference in IOP between the paired readings by NCT.

Tonometry over Gas Filled Eyes

Intraocular gas significantly influences scleral rigidity rendering indentation tonometry unsatisfactory.

Pneumatonometer underestimates GAT readings in gas filled eyes while Tonopen compared favorably with GAT readings.

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Calibration of Goldmann Applanation Tonometer

It is essential that Goldmann applanation tonometer (GAT) should be calibrated periodically, at least monthly. Following checks are necessary:

•Check position 0: Turn the zero calibration on the measuring drum downwards by the width of one calibration marking, against the index marker. When the feeler arm is in the free movement zone, it should then move itself against the stop piece in the direction of the examiner.

•Check position 0.05: Turn the zero calibration on the measuring drum upwards by the width of one calibration marking, against the index marker. When the feeler arm is in the free movement zone, it should then move itself against the stop piece in the direction of the patient.

•Check position at drum setting 2: For checking this position, check weight is used. Five circles are engraved on the weight bar. The middle one corresponds to drum position 0, the two immediately to the left and right to position 2 and the outer ones to position 6. One of the marks on the weight corresponding to drum position 2 is set precisely on the index mark of the weight holder. Holder and weight are then fitted over the axis of the tonometer so that the longer part of the weight points towards the examiner.

•Check position 1.95: The feeler arm should move towards the examiner. Check position 2.05.The feeler arm should move in the direction of the patient.

•Check at measuring drum setting 6: Turn the weight bar to scale calibration 6, the longer part shows in the direction of the examiner.

•Check position 5.9/6.1 as performed for drum setting 2.

Sterilization

Schiøtz Tonometer

The tonometer is disassembled between each use and the barrel is cleaned with 2 pipe cleaners, the first soaked in alcohol and the second dry. The foot plate is cleaned with alcohol swab. All surfaces must be dried before reassembling.

Goldmann Applanation Tonometer

A variety of techniques are described for disinfecting the tonometer. Applanation tip should be soaked for 5-15 min in diluted sodium hypochlorite, 3% H2O2 or 70% isopropyl alcohol or by wiping with alcohol, H2O2, povidone iodine or 1: 1000 merthiolate. 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.

Tonopen

Tip is protected by a disposable latex cover.

Pneumatonometer

Tip should be cleaned with an alcohol sponge, taking care to dry the surface before use. Alternative is the use of disposable latex cover over the tip.

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