Tonometry

By Dr. RahulModerator Dr.Vijay Shetty

Intra ocular pressure can be measured by

1)Manometry 2)Tonometry

ManometryManometry is only direct measure of IOP

In this method, needle is introduced in AC or in vitreous

It is then connected to mercury or water manometer

Disadvantages

Not practical method for human beings

Needs general anesthesia

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

Uses

It is used for continuous measurements of IOP

Used in experiment, research work on animal eyes

TonometryIt is an indirect method of measuring the IOP

Three basic types of Tonometers:o Indentationo Applanationo Noncontact

HistoryMalkalov 1885 1st Appl tonometerSchiotz 1905 Indentation tonometryFriedenwald 1948 &1955 Coefficient of ocular

rigidityGoldmann 1954 prototype Appl. T

(constant area)Grolmann 1972 N. C. T.Grant Electronic indentation

tonometerHalberg Hand held tonometer

Ocular rigidityMeasure of distensibility or resistance to

deformation of ocular coats.Important in indentation tonometerIncrease in ocular rigidity– increase IOP

Long standing glaucoma ARMD Hyperopic eyes

Decrease in ocular rigidity- decrease in IOP Acutely elevated IOP Osteogenesis imperfecta Miotic therapy Vasodilator therapy Vitrectomy Myopic eyes

Corneal rigidityAbility of the corneal tissue to resist deformation

Important in applanation tonometers

Provided by collagen lamellae – 90% of corneal thickness

Increased corneal thickness– increased rigidity– increase in IOP

Classification

APPLANATION TONOMETER

1. GOLDMANN APPLANATION TONOMETER PROTOTYPE

I. PERKINS APPLANATION TONOMETER

2. MACKAY-MARG TONOMETER PROTOTYPE

I. TONOPENII. PNUEMATIC

TONOMETER

1. Maklakov –tonometer

VARIABLE FORCEVARIABLE FORCE VARIABLE AREAVARIABLE AREA

ClassificationDirect

IndirectIndentation T - Schiotz

Applanation T

Goldmann (prototype)

Goldmann- type- Perkins & Draeger

Mackay- Marg – type T - Tono Pen (hand held)

Continued ..Applanation T

Maklakov tonometer Maklakov type

Planometer Tonomat Halberg Barraquero Pneumatic tonometer

Non contact tonometer (NCT) X –pert T Grolman airblast T Keeler pulsair T (hand held)

Miscellaneous T Continuous IOP monitoring devices Self tonometer Impact tonometer Vibrational tonometer

Newer tonometers Trans –palpebral T Disposable tonometer

Tonosafe – acrylic biprism Tonoshield- silicone shield

Dynamic contour tonometer

Indentation Tonometry

• It is based on fundamental fact that plunger will indent a soft eye more than hard eye

• The indentation tonometer in current use is that of Schiotz

• It was devised in 1905 and continued to refine it through 1927

Basic concept and theory of indentation

As soon as tonometer is placed on cornea different forces come into play

W - weight of tonometer A -Area Vc –volume displaced after indentationT- tensile force, set up in outer coats of eye at

everywhere tangentially to corneal surface

So additional force T to original base line IOP

Resting intraocular pressure (P0) which is artificially raised to a new value (P1)

Thus the scale reading of tonometer actually measures the artificially raised IOP

Conversion of scale reading to baseline IOP

The conversion of P1 to P0 is obtained from conversion tables developed by Friedenwald

The calibration was carried by experiments in cadaveric eyes connected with manometer through cannula

The observation were plotted on semilog scale ,which serve as Friedenwald nomogram

The original conversion tables referred to as 1948 tables, calculated using average K 0.0245 (coefficient of ocular rigidity)

The Friedenwald later revised average K to 0.0215 known as 1955 tables

Subsequent studies indicate 1948 tables agree more closely with measurement by goldmann AT

Parts of schiotzHandle for holding the instrument

in vertical position on cornea

Footplate which rests on cornea

Plunger which moves freely within a shaft in footplate

A bent lever whose short arm rests on upper end of plunger

Long arm which acts as pointer needle

Weights - a 5.5 gm weight is permanently fixed to plunger, can be increased to 7.5 and 10 gm

Techinque of schiotz tonometry

A metal sphere used as dummy cornea. radius of curvature - 15mm

Use - for testing the tonometer & calibration

When the tonometer is placed on the metal sphere, there is no indenting movement of the plunger

The pointer logically should be at 0 marked on the scale because there is no downward movement of the plunger

Techinque of schiotz tonometry

It is customary to start with 5.5 gm Greatest accuracy is attained if deflection of

lever is between 3 to 4if the scale reading is < 3, additional weight

is added to plunger to make it 7.5 gm or 10 gm

Sterilisation - by dipping in ether, absolute alcohol or acetone

Advantages-easy to use, simplicity, low price

Disadvantage

Gives false reading when used in eyes with abnormal scleral rigidity

False low levels of lOP with low scleral rigidity seen in high myopes n following ocular surgery

Errors of indentation tonometry

1)Errors inherent in the instrumentThese may be due to difference in weight,

size ,shape and curvature of footplate

2)Errors due to contraction of extra ocular muscles

- tend to increase IOP

3) errors due to accommdationpatient look at the tonometer and thus

accommodation comes into playContraction of ciliary muscle increases the

facility of aqueous outflow by pulling on trabecule

Thus causes some lowering of IOP

4)Errors due to ocular rigidity

5) Errors due to variation in corneal curvature -Steeper or thicker cornea will cause

greater displacement of fluid -Causes falsely high IOP readingsErrors may arise in cases of – -Microphthalmos -Buphthalmos -High myopia -Corneal scars

6)Moses effect - At low scale reading the cornea may mould

into space between Plunger and hole - Pushing the plunger up and leading to

falsely high pressure reading

Applanation tonometry The concept was introduced by goldmann is

1954

It is based on IMBERT FICK LAW

It states that the pressure inside an ideal sphere (P) is equal to force (W) reqired to flatten(A)

P=W/A

P can be determined if Force F is fixed or

Area A is fixed

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 corneaCorneal rigidity (C) resists flattening

Thus,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 tonometers

1) Goldman tonometer 2)Perkins AT3)Pneumatic tonometer4)Pulse air tonometer5)Tono pen

GOLDMANN TONOMETERMost popular and accurate tonometer

It consists of double prism mounted on slit lamp

The prism applanates the cornea in an area of 3.06 mm diameter

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Goldmann tonometer

Measures the force required to applanate the cornea over a circular area of diameter 3.06mm..

Applanates an area of diameter 3.06 mm for 3 reasons.Amount of fluid displacement is negligible

(approx. 0.5l).Surface tension force and the force required

to counteract the corneal rigidity act opposite to each other.

Tonometer force becomes equal to the force in mmHg.

Area applanated on the cornea is 7.35mm.

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CALIBRATION OF APPLANATRION TONOMETER

Appl, tonometer is supplied with calibration bar.

it should be done once in a weekThe appl. Pressure spring is calibrated

with calibration bar .In ZEISS model rod is placed at the

junction of balance arm.The rod is moved towards the patients .The center is at ring (o) and is set for tension (o) + or –

0.50.The next mark is at 2 gm this represent tension of 19.50

and the when the rod is moved to position 3 that is at 6 gm the tension is between 59 to 61.

TechniqueTopical anesthesia Staining tear film with fluorescein The cornea and biprisms are illuminated with

cobalt blue light Biprism is the advancd until it just touches

the apex of corneaAt this point two fluorescent semicircles are

viewed through prism

Applanation force against cornea is adjusted until inner edges of two semicircles just touch

Intraocular pressure is determined by multiplying dial reading with ten

Potential errors

Patient related

Thin corneaThick corneaAstigmatismIrregular cornea

Technical

Tonometer out of calibrationRepeated tonometryPressing on the eyelids or globeSqueezing of the eyelidsObserver bias (expectations and even

numbers)

Potential errors

Perkins TonometerIt uses the same biprism as the Goldmann

applanator.

The light source is powered by battery.

The readings are consistent and compare quite well with the Goldmann applanator.

Perkins TonometerPerkins –

HandheldHorizontal as well as verticalInfants, children, O T, recumbent patients

Mackay Marg TonometerPlunger plate has diameter of 1.5mm

Surrounding Sleeve has 3 mm

Force required to keep the plate flush with the sleeve is electronically monitored – recorded on a paper strip

Source of error- >3 mm flattening – high IOP Multiple readings to compensate ocular pulsation

Specific utility- irregular and edematous cornea

Pneumatic tonometer

Pneumatic tonometerCornea is applanated by touching apex by

silastic diaphragm covering sensing nozzleIt is connected to central chamber containing

pressurized airThere is pneumatic to electronic transducer It converts the air pressure to recording on

paper strip and IOP is read

Principle The principle is similar to the MacKay-Marg

tonometer. Corneal contact of the pencil-like tip records

both the IOP and the force required to bend the cornea.

advancement of the tip transfers the latter force to the surrounding “collar.”

The “plunger” is replaced by a column of air and the contact surface is a Silastic membrane

PrincipleThe air column is continually vented via a

port.

Changes in pressure in the column records via a transducer on a moving strip of paper.

instrument is useful with edematous and irregular corneas

Noncontact tonometer

It is an applanation tonometer and works on the principle of a time interval.

Measuring the time from initial generation of the puff of air to cornea gets flattened (in milliseconds) to the point where the timing device stops.

It takes less time for the puff of air to flatten a soft eye than it does a hard eye.

• Three subsystems:

• Alignment system

• Optoelectric applanation monitoring system

• Transmitter

• Receiver and detector

• Pneumatic system

• Time for max light detection= time to applanate the cornea = corelated with IOP

• Limitations

• Ocular pulse

• Glaucomatous eyes

Average of 3 readings

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CALIBRATION NON-CONTACT TONOMETER• The use of logic circuit in the instruments,

which are necessary to measure and record IOP , enables the operator to check the calibration of pneumatic electronic network as under

• Turn instrument to on [red dot ]• Remove the objective cap for 30 sec for warm

up.• Depress the trigger switch –display at –68.

Noncontact tonometer

Tono penThis is handheld Mackay Marg type

tonometer

It is a computerised pocket tonometer

It converts IOP into electric waves

Tono penThe wave form is internally analyzed by a

microprocessor.

Three to six estimations of the pressure are then averaged.

The instrument is 18 cm in length and weighs 60 g.

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.

Dynamic Contour Tonometer The PASCAL (DCT) is a slit lamp–mounted device

It measures IOP independent of corneal rigidity or thickness.

It was commercially launched in August 2004.

PrincipleDCT uses the principle of contour

matching instead of applanation.

The tip contains a hollow miniature pressure sensor in its centre.

 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).

PrincipleThe probe is placed on the pre-corneal

tear film on the central cornea

The integrated piezoelectrical ( electricity resulting from pressure)  pressure sensor records data, measuring IOP 100 times per second.

The tonometer tip rests on the cornea with a constant appositional force of one gram.

When the sensor is subjected to a change in pressure, the electrical resistance is altered

The PASCAL's computer calculates a change in pressure according to the change in resistance.

A complete measurement cycle requires about 8 seconds of contact time.

It is less influenced by corneal thickness than other methods

As the tip shape is designed for the shape of a normal cornea, it is more influenced by corneal curvature.

Ocular Response Analyzer

It is similar to Reichert’s current generation NCT and provides a Goldmann-equivalent IOP reading.

It analyzes the signal obtained from the corneal response to measure the biomechanical properties of the corneal tissue.

Principle

It utilizes a dynamic bi-directional applanation process to measure pressure of the eye.

During measurement, a precisely metered collimated-air-pulse applies force to the cornea.

PrincipleUnder the force of the air pulse, the

cornea moves inwards, past applanation, and into a slight concavity

As the air pulse pressure decreases, the cornea return to its normal configuration.

In the process, it once again passes through an applanation state.

Principle

An advanced electro- optical system monitors the changes in curvature of the cornea

Two independent pressure values are derived the inward and outward applanation events.

Due viscous damping in the cornea causes delays, resulting in the different pressure values.

The average of these two pressure values provides  Goldman-Correlated IOP value (IOPG).

The difference between these two pressure values is Corneal Hystersis.

How it works The ORA produces a rapid air impulse and

uses an electro-optical system to monitor the deformation.

The device records two applanation events: inward movement ; the other as it returns.

The difference between the “in” and “out” pressure values is known as corneal hysteresis

Ocular Response AnalyzerThe Ocular Response Analyzer (A) utilizes a

collimate air pulse to applanate the cornea, along with an infrared electro-optical detection system (B).

Hysteresis

• The phenomenon was identified, and the term coined, by Sir James Alfred Ewing in 1890.

• Hysteresis is a property of physical systems that do not instantly follow the forces applied to them, but react slowly, or do not return completely to their original state. 

Corneal Hysteresis

 It is the "energy absorption capability" of the cornea

This because of the speed at which the cornea is deformed during the dynamic bi-directional applanation process in ORA

average value of (CH) in normal subjects is approximately 11 mmHg. 

However, it is very likely that CH values will vary depending on age and race. 

CRFCRF is a measurement of the cumulative

effects of both the viscous and elastic resistance encountered by the air jet while deforming the corneal surface.

CRF exhibits the expected property of increasing at significantly elevated pressures.

Rebound tonometry

It determines IOP by bouncing a small plastic tipped metal probe against the cornea.

The device uses an induction coil to magnetize the probe and fire it against the cornea.

As the probe bounces against the cornea and back in to the device, it creates an induction current from which the intraocular pressure is calculated.

It is portable no eye drops

suitable for children and non-cooperative patients

OCT TonometryNon-contact tonometry using optical

coherence tomography (OCT) is currently under development.

It works as a force being applied to the cornea and simultaneous measurement of the corneal reaction.

In the case of OCT tonometry, the force applied to the cornea can be

-air pressure in the form of a high pressure jet

-a shock or acoustic wave

- low pressure air using air pumped into a sealed chamber around the eye (like scuba mask).

Transpalpebral tonometer

No contact directly with the eyeballThe test is done through the upper eyelidNo risk of infection during the testNo anesthesia drops and staining agentsComfortable for the patientIOP measuring in immobilized patients and in

children

Special conditions Corneal astigmatismCorneal EdemaKeratoconusFlat ACPenetrating KeratoplastyContact Lenses

Corneal astigmatism

Mires unequal Underestimating IOP –with the ruleOverestimating IOP – against the rule 1 mm Hg for every 4 D

Irregular astigmatism– unpredictableRecommendationsirregular corneas – Mackay Marg principle-Tono

Pen

Corneal EdemaCornea with epithelial or stromal edema –

easier to indentHence underestimation by 10 – 30 mm HgRecommendation- Mackay Marg T

Corneal ScarsIncreased corneal rigidity in the area of the

scar- increased IOPRecommendation- Mackay Marg T,

Pneumotonometer

keratoconusCorneal thinning- Low IOP measuredIncreased curvature- Low IOP measuredDecreased corneal rigidity – reduces overall

ocular rigidity – K value differs hence Schiotz is also inaccurate

Recommendation- Mackay Marg T, Tono Pen away from the cone

Flat ACUnreliable with applanation- errors upto 51

mm HgFlat A.C. post Trab. –

Overfiltration- has low IOPAqueous misdirection- high IOPDiagnosis difficult with tonometer due to

unreliabilityRecommendation- digital pressure

Laser Refractive Surgery LASIK – reshape – CCT decreases – falsely Low

IOP by applanation Peripheral Tono Pen & Goldmann readings

unchanged Central & peripheral Pneumotonometer

readings unchanged Post op steroid induced increase IOP may mask

the underestimated goldmann readings

Recommendations-Tono Pen or PneumotonometerCorrection factor(C) : P1-P2 P1 = pre op IOPP2 = post op IOP after 6 months and at least

off steroids for 1 monthTrue IOP= appl IOP + C

Penetrating KeratoplastyIrregularEdemaScarringAstigmatismRecommendation- Mackay Marg T considered

the best for irregular and scarred cornea, Tono Pen

Contact LensesApplanation – unreliable Recommendation- Pneumotonometer, Tono

Pen

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