Tonometry by arun

TONOMETRY

BY

Dr.Arun Kumar Reddy

Tonometry• Tonometry is the procedure performed to

determine the intraocular pressure (IOP)

history• 1826: William Bowman used digital tonometry as a routine examination

test.

• 1863: Albrecht von Grafe designed the first instrument to attempt to measure intraocular pressure.

• Further instruments followed, notably by Donders in 1865 and Preistly-Smith in 1880.These instruments were all of the indentation type and rested on the sclera (no anaesthetic was used until 1884).

• 1885: Maklakov designed an applanation tonometer. This was refined in 1892. Used for a number of years in Russia and Eastern Europe. This was used till 1959.

•1905: Hjalmar Schiotz produced his indentation tonometer. This made tonometry a simple and routine clinical test.

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 standardise

• Free of maintenance problems

Types of tonometry• Tonometry (IOP)

• Direct – manometry

• Indirect –

• A) Static tonometry – a)contact, b)noncontact

• a) contact tonometers

• 1)Indentation

• Schiotz ,Mercurial ,Electronic ,Scleral tonometry

• 2)Applanation

• Variable force • GAT, Perkins, MMT, Tonopen, Pneum.Tonometer

• Constant force• Maklakov, Glucotest, Applanometer

• b)Non Contact tonometer – Pulsair

B) Dynamic instruments

• Ballistic tonometers

• Impact accelerationImpact durationRebound velocity

• Vibration tonometers (Krakau)

• 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.

Types of tonometers• In a normal eye IOP becomes more during Schiøtz

tonometry.

• low-displacement tonometers.

• Tonometers in which the IOP is negligibly raised during tonometry (less than 5%) are termed as low-displacement tonometers.

• Eg. Goldmann Applanation Tonometer.Mackay-Marg tonometer.

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

• high-displacement tonometers

• Tonometers that displace a large volume of fluid and consequently raise IOP significantly are termed as high-displacement tonometers.

• Eg.Schiotz , Maklakov.

High displacement tonometers are mostly less accurate than low-displacement tonometers.

Direct method• Manometry

• Canula inserted into eye.

• Not practical clinically.

• Intraocular pressure is higher than atmospheric pressure; therefore, if a small hollow needle is inserted into the anterior chamber, aqueous humor flows out through the needle.

• If the needle is attached to a

reservoir of fluid that is raised just

high enough to prevent any loss of

aqueous, the height of the column

of fluid, usually calibrated in cm of

water or mm of mercury, reflects

the intraocular pressure

Indirect method• Palpation Method/ digital tonometry

• Intraocular pressure (IOP) is estimated by response of eye to pressure applied by finger pulp.

• indents easily – low IOP

• Firm to touch – normal IOP

• Hard to touch – high IOP

Schiotz tonometry

• Schiotz (1905, Modified 1924/1926)

Parts of schiotz tonometer

scale

needle

Weight 5.5g plunger

holder

Foot plate

lever

3mm diameter

ROC 15mm

Tonometer weight = 11g

Additional weights 7.5,10,15g

Schiotz tonometry - characteristics• 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• 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.

• P(t) = P(o) + E

• IOP with Tonometer in position Pt =

Actual IOP Po + Scleral Rigidity E

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

Friedenwald formula

• Friedenwald generated formula for linear relationship between the log function of IOP and the ocular distension.

• Pt = log Po + C ΔV

• 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

• ΔV is the change in volume

Friedenwald conversion table•

Plunger Load

Scale 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

TECHNIQUE• 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.

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

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.

• Repeated measurements lower IOP.

• steeper or a thicker cornea causes greater displacement of fluid during tonometry and gives a falsely high IOP measurement.

• Schiøtz reads lower than GAT

Factors Affecting Scleral Rigidity

• High Scleral Rigidity• hyperopia

• long standing glaucoma

• ARMD

• vasoconstrictors

Factors Affecting Scleral Rigidity

•Low Scleral Rigidity•increasing age

• high myopia

•miotics

•vasodilators

•Postoperative after RD surgery (vitrectomy, cryopexy, scleral band)

•intravitreal injection of compressible gas.

•keratoconus (?).

•Low ocular rigidity ----- falsely high scale reading ----- falsely low IOP.

LIMITATIONS• Instrumental errors

• Standardisation - testing labs for certification

• Mechanical obstruction to plunger etc.

• Muscular contractions• Of extra ocular muscles increase IOP

• Accomodation decreases IOP

• Variations in volume of globe• Microphthalmos

• High Myopia

• Buphthalmos

• It can be recorded in supine position only

Advantages of schiotz tonometer• Simple technique

• Elegant design

• Portable

• No need for SlitLamp or power supply

• Reasonably priced

• Anodized scale mount which is highly resistant to sterilizing water.

• Schiotz tonometer is still most widely tonometer.

calibration• The instrument should be calibrated before

each use by placing it on a polished metal sphere and checking to be sure that the scale reading is zero.

• If the reading is not zero, the instrument must be repaired.

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.• 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' in 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 >18

18 mm Hg 21 mm Hg higher <18

18 mm Hg 18 mm Hg equal 18

APPLANATION TONOMETER

Biprism (measuring prism)

Feeder arm

Housing

Adjusting knob

Connects to the slit lamp

Control weight insert

PRINCIPLE

• Applanation tonometry is based on the Imbert-Fick principle, which states that the pressure (p) inside an ideal dry, thin-walled sphere equals the force (F) necessary to flatten its surface divided by the area of the flattening (A).

F

• 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.

IMBERT FICKS LAW & MODIFIED IMBERT FICKS LAW

W=PA W+S=PA1+B

• 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.

cont…..

The two beam-splitting prism within the applanating unit optically convert the circular area of corneal contact in to semicircles

cont….

The instrument is mounted on a standard slit lamp in such a way that the examiners view is directed through the centre of a plastic Biprism. 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.Two beam splitting prisms within applanating unit optically convert circular area of corneal contact in 2 semicircles.

procedure• The patient is asked not to drink alcoholic beverages as it

will lower IOP and not to take large amounts of fluid (e.g., 500 ml or more) for 2 hours before the test, as it may raise the IOP.

• The angle between the illumination and the microscope should be approximately 60°.

• The room illumination is reduced.

• A fixation light may be placed in front of the fellow eye.

• The tension knob is set at 1 g. 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.

Procedure cont..• The 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.

• After instilling topical anaestheia, Edge of corneal contact is made apparent by instilling fluorescein while viewing in cobalt blue light.

• The biprism should not touch the lids or lashes because this stimulates blinking and squeezing.

• 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 wide.

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

IOP.

Procedure cont..• In some patients, it is necessary for the examiner to hold

the eyelids open with

the thumb and forefinger

of one hand against the

orbital rim.

• 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.

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

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 – During Measurement

Effect of central corneal thickness (CCT): • 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.

• The 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 gives lower readings

on applanation.

• 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.

Applanation - Possible Errors• Falsely low IOP

• too little flouroscein• thin cornea • corneal edema• with the rule astigmatism

• 1mm Hg per 4 D

• prolonged contact • Repeated tonometry

• Falsely high IOP

• too much flouroscein

• thick cornea

• steep cornea

• against the rule astigmatism

1mm Hg per 3D

• wider meniscus• Widening the lid fissure

excessively

• Elevating the eyes more than 15°

Potential Sources of Error – During MeasurementIf the fluorescein rings are too wide, the patient’s eyelids should be blotted carefully with a tissue, and the front surface of the prism should be dried with lint-free material. An excessively wide fluorescein ring can cause IOP to be overestimated


If the rings are too narrow, the patient should blink two or three times to replenish the fluorescein; additional fluorescein may be added if necessary. If the fluorescein rings are too narrow,IOP is underestimated.









CALIBRATION• GAT should be calibrated periodically, at least monthly. If

the GAT is not within 0.1 g (1 mmHg) of the correct calibration, the instrument should be repaired; however, calibration errors of up to 2.5 mmHg may still be tolerated clinically.

• 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• 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.

• Disposable tonometer tips may also be used

When using disposable tips, they have a smooth applanating surface. The acrylic disposable tips seem to be somewhat more accurate than the silicone ones. While disposable shields or tips may be safer than disinfection solutions, they are not 100% protective against prion disease.

• 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.

SAFETY REGULATIONS• No examination should be undertaken in case of eye

infections (or) injured corneas.

• Only clean and disinfected measuring prism should be used.

• No damaged prisms should be used.

• If the measuring prism come in to contact with the cornea without the drum having previously been correctly set, vibration can occur in the feeler arm, which will produce unpleasant feeling for the patient.

• The tonometer tips should be examined periodically under magnification as the antiseptic solutions and mechanical wiping may cause irregularities in the surface of the tip that can, in turn, injure the cornea.

Perkins tonometer• It uses same prisms as Goldmann

• It is counterbalanced so that tonometry is performed in any position

• The prism is illuminated by battery powered bulbs.

• Being portable it is practical when measuring IOP in infants / children, bed ridden patients and for use in operating rooms.

Draeger Tonometer• Draeger tonometer is similar to Perkins

• It has a different set of prisms

• It operates with a motor.

Mackay marg tonometer

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.

• This instrument is useful for measuring IOP in eyes with scarred, irregular, or edematous corneas because the end point does not depend on the evaluation of a light reflex sensitive to optical irregularity, as does the Goldmann tonometer.

• It is accurate when used over therapeutic soft contact lenses.

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.

Tonopen

Tono pen• Portable

• battery operated .

• same principle as that of Mackay-Marg tonometer.

• It is particularly useful in community health fairs, on ward rounds ,children, irregular surfaces, measuring through an amniotic membrance patch graft, to read from the sclera .

• Tono-Pen tends to overestimate 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

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

Pneumatonometer or pneumatic tonometer

• It is like Mackay-Marg tonometer.

• 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.

• It has a sensing device that consists of a gas chamber covered by a polymeric silicone diaphragm.

• A transducer converts the gas pressure in the chamber into an electrical signal that is recorded on a paper strip.

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

• As the diaphragm touches the cornea, the gas vent is reduced in size, and the pressure in the chamber rises.

Maklakov tonometer•Indentation•Pt supine•wire holder Dumb-bell-shaped metal cylinders with flat end plates of polished glass Diameter of 10 mmThe surface of the weight is painted with a dye, such as mild silver protein (Argyrol) mixed with glycerin.1 sec contactimprint on end plate

• IOP = W / π (d/2) 2

• weight (W) diameter of the area of applanation (d)

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

• widely in Russia and China

• This instrument displaces a greater volume of aqueous humor and thus IOP readings are more influenced by ocular rigidity.

• It does not correct for corneal bending, capillary attraction, or tear encroachment on the layer of dye.

• Many instruments similar to the Maklakow device have been described,like the Applanometer, Tonomat, Halberg tonometer, and GlaucoTest.

The Ocuton tonometer• The Ocuton™ tonometer

• hand-held tonometer

• works on the applanation principle

• probe is so light that it is barely felt

• needs no anesthetic in most patients.

• It has been marketed in Europe for home tonometry

• useful to get some idea of the relative diurnal variation in IOP if the patient or spouse (etc.) can learn to use it.

Rebound tonometer

• It is a new and updated version of an indentation tonometer

• Portable

• can be used without anesthetizing the eye.

• A very light, disposable, sterile probe is propelled forward into the cornea .

• The time taken for the probe to return to its resting position and the characteristics of the rebound motion are indicative of the IOP.

• The 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.

• It is comparable to the GAT.

• It correlates with central corneal thickness like the Goldmann, .

• used in screening situations, when patients are unable to be seated or measured at the slit lamp, or when topical anesthetics are not feasible or usable.

• Not useful in scarred corneas (as does the Goldmann).

Trans palpebral tonometry

used in situations where other, more accurate, devices are not practical, such as in young children, demented patients and severely developmentally-challenged patients.In addition to all the problems facing indentation tonometry, such as scleral rigidity, transpalpebral tonometry adds variables such as the thickness of the eyelids, orbicularis muscle tone and potential Intra palpebral scarring.

• Portable. patients can measure their own IOP at home, DVT

• pressure on the eyelid in most eyes produces retinal phosphenes.

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

• It is not accurate always. inter observer and intra observer variability was large.subsequent studies failed to confirm the accuracy of this device.

Non contact tonometer• Noncontact tonometer (NCT) was 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.

• NCT is accurate if IOP is nearly normal, accuracy decreases with increase in IOP and in eyes with abnormal cornea or poor fixation.

• It is useful for screening programs because it can be operated by non-medical personnel

• It does not absolutely require topical anesthesia .

• There is no direct contact between instrument and the eye.

• The patient should be warned that the air puff can be startling.

• The non-contact tonometer measures IOP over very short intervals, so it is important to average a series of readings.

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

Ocular Response Analyzer• It is an adaptation of the non-contact tonometer.

• It directs the air jet against the cornea and measures not one but two pressures at which applanation occurs

• 1) when the air jet flattens the cornea as the cornea is bent inward and 2) as the air jet lessens in force and the cornea recovers.

Ocular response analyser• The first is the resting intraocular pressure.

• The difference between the first and the second applanation pressure is called corneal hysteresis

• corneal hysteresis is a measure of the viscous dampening and, hence, the biomechanical properties of the cornea.

• The biomechanical properties of the cornea are related to corneal thickness and include elastic and viscous dampening attributes.

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

• Further , while IOP varies over the 24-hour day, hysteresis seems to be stable.

• 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.

• It has practical value in the management of glaucoma.

Dynamic contour tonometer

• Introduced by Kanngiesser

• It is based on a totally different concept other than indentation or applanation tonometry.

• Principle : By surrounding and matching the contour of a sphere (or a portion thereof ), the pressure on the outside equals the pressure on the inside.

• The tip of the probe matches the contour of the cornea.

• A pressure transducer built into the center of the probe measures the outside pressure, which should equal the inside pressure, and the IOP is recorded digitally on the liquid crystal display (LCD).

• The concept developed from a previous contact lens tonometer called the ‘Smart Lens”.

• It superior in accuracy to Goldmann tonometry and pneumotonometry .

• IOP is not affected by corneal thickness.

• IOP is not altered by corneal refractive surgery that thins the cornea.

• Because the DCT measures IOP in real time, the actual measurement, like the IOP, is pulsed. The internal electronics ‘call’ the IOP as the bottom of the pulsed curve and indicate it digitally on the LCD ..

• IOP readings with the DCT are generally lower than GAT because, when properly done, indicates the average difference between the maximum and minimum pressures whereas the DCT reads the minimum.

Ocular pulse amplitude• The DCT indicates the magnitude of the difference

between maximum and minimum IOP as the ocular pulse amplitude.

• OPA may be indicative of the status of ocular blood flow and be differentially affected in different types of glaucoma.

• ocular pulse amplitude is

increased over normals in

most forms of glaucoma and

may be related to the level

of IOP.

Continuous monitoring of intraocular pressure• Applanation instruments inside contact lenses or suction

cups or strain gauges in encircling bands that resemble scleral buckling elements.

• None of these instruments has achieved widespread use.

• resonance applanation tonometry measuring the sonic resonance of the eye when a continuous force over a fixed area is applied.

• use of infrared spectroscopy to measure IOP.

• To build a miniature pressure sensor that can reside inside the eye; one such device is part of an intraocular lens.

Tonometry for Special Clinical Circumstances• Tonometry on Irregular Corneas

• The accuracy of Goldmann and Tono-Pen tonometers and the noncontact tonometers is limited in eyes with irregular corneas.

• The pneumatic tonometer has been shown to be useful in eyes with diseased or irregular corneas .

• Tonometry over Soft Contact Lenses

• Pneumo tonometry and the Tono-Pen can measure with reasonable accuracy the IOP through bandage contact lenses .

• pneumotonometer correlates well with manometrically determined IOP, whereas the Tono-Pen consistently underestimates the pressure.

• Tonometry with Gas-Filled Eyes

• Intraocular gas affects scleral rigidity, rendering indentation tonometry unsatisfactory.

• pneumatic tonometer and Tono-Pen used.

• A pneumatic tonometer underestimates Goldmann IOP measurements in eyes with intravitreal gas

• Tono-Pen compares favorably with Goldmann readings.

• Both instruments significantly underestimated the IOP at pressures greater than 30 mm Hg .

• Tonometry with Flat Anterior Chamber

• IOP readings from the Goldmann applanation tonometer, pneumotonometer, and Tono-Pen do not correlate well with manometrically determined pressures.

• Tonometry in Eyes with Keratoprostheses

• In patients at high risk for corneal transplant rejection, implantation of a keratoprosthesis is now a viable option for vision rehabilitation .

• Most keratoprostheses have a rigid, clear surface, it is impossible to measure IOP by using applanation or indentation instruments.

• In such eyes, tactile assessment appears to be the most widely used method to estimate IOP.

Thank

you

references• 1)anatomy & physiology of eye

A.K.Khurana

• 2) Shields glaucoma

• 3)Becker-Shaffers glaucoma

• 4) Diagnostic Procedures in ophthalmology Nema

• 5) Duane's Clinical Ophthalmology

• 6) kerala journal of ophthalmology Vol. XXII, No.4, Dec. 2010

• 7) Internet

Education

Tonometry by arun