Recent Advances in IOP
measurements
Dr Deepak Megur
Cataract and Glaucoma services
Megur Eye Care Centre
Bidar
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: Acceptable
(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 Tonometers
Principle
• 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 tonometer
Principle…
• 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 airpuff 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 airpuff 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…?