Recent Advances in IOP measurements

Dr Deepak MegurCataract and Glaucoma services

Megur Eye Care CentreBidar

Karnataka

Tonometry Why…?

• Because….– “IOP is the only modifiable risk factor

in Glaucoma”

– Most frequently examined parameter in the follow up of a glaucoma pt.

What to use…?

• Goldmann Applanation Tonometry

• Gold Standard

TonoPen: AcceptableTonoPen: Acceptable (SEAGIG Asia Pacific Glaucoma guidelines)(SEAGIG Asia Pacific Glaucoma guidelines)

Factors affecting measured IOP.

• Central Corneal Thickness:– Thicker Corneas: Artificially elevated IOP– Thinner Cornea: Artificially decreased IOP

– Correction nomogram:• 525 Micron• 1 -3 mm hg per 40 micron deviation.

( SEAGIG Asia Pacific Guidelines 2004)

Central Corneal Thickness

• Measurement– Ultrasonic Pachymeter– Measured along / Before GAT– Time of measurement, waking hours

• CCT– Predicting Risk factor– Prognostic significance– Response to drugs

CCT independent IOP Measurements:

• Pascal’s Dynamic Contour Tonometry

• Contour Matching

• Ocular Response Analyzer:– Corneal Hysteresis– Less influenced by

• Corneal thickness and resistance

Pascal’s Dynamic Contour Tonometry.

• Slitlamp-mounted.. • Direct measurement of

pressure - no systematic errors from force-to-pressure conversion.

• Numerical display of result. • No mechanical calibration

required; self-calibrating. • Convenient disposable tip

prevents contamination and potential infection

• Battery operated - no cabling. • Visual control of eye/tip

interface through transparent SensorTip.

• Single button operation.

Pascal’s Dynamic Contour Tonometry

• Dynamic contour tonometry (DCT) is a novel method which uses principle of contour matching instead of applanation.

• This is designed to reduce the influence of biomechanical properties of the cornea on measurement.

• These include corneal thickness, rigidity, curvature, and elastic properties.

• It is less influenced by corneal thickness but more influenced by corneal curvature than the Goldmann tonometer

DCT -Principles

• It uses a miniature pressure sensor embedded within a tonometer tip contour-matched to the shape of the cornea.

• 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 and the PASCAL's computer calculates a change in pressure in concordance with the change in resistance

• The contour matched tip has a concave surface of radius 10.5 mm, which approximates to the shape of a normal cornea when the pressure on both sides is equal.

• The probe is placed adjacent to the central cornea.• The integrated piezoresistive pressure sensor

automatically begins to acquire data, measuring IOP 100 times per second.

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

• The device also measures the variation in pressure that occurs with the cardiac cycle. (Ocular pulse Amplitude)

I-Care Rebound Tonometer

• Contact tonometer• Rebound principle• Digital reading• Probes changed for

every patient

Rebound TonometersPrinciple

• Determine IOP by bouncing a small plastic tipped metal probe against the cornea.

• The device uses an induction coil to magnetise 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.

I-Care Rebound Tonometer

• Advantages:– Fast – No Anaesthetic

required– Pt friendly– children

• Affected by corneal properties.– Thickness– Hysteresis

Transpalpebral tonometry Diaton tonometer (BiCOM, Inc)

• measuring intraocular pressure through the Eyelid.

• Transpalpebral tonometry does not involve contact with the cornea and does not require sterilization of the device or topical anesthetic during routine use.

• only moderate correlation with those provided by applanation tonometry

• More affected by the corneal thickness than Goldmann tonometry.

Diaton tonometerPrinciple…

• The Diaton tonometer calculates pressure by measuring the response of a free falling rod

• the principle is based on Newton's second law, as it rebounds against the tarsal plate of the eyelid.

• The patient is positioned so that the tip of the device and lid are overlying sclera.

Non-contact tonometry or air-puff tonometry

• Non-contact tonometry:• It uses a rapid air pulse to applanate the

cornea.• Corneal applanation is detected via an

electro-optical system.• Intraocular pressure is estimated by

detecting the force of the air jet at the instance of applanation.

•

Non-contact tonometry or air-puff tonometry

• A fast and simple way to screen for high IOP. • However, modern non-contact tonometers have

been shown to correlate well with Goldmann tonomtery measurements

• Particularly useful – in children and other non-compliant patient groups.

• non-contact tonometry – which reduces the potential for disease

transmission…?