Tonometer - Basics & Types

Tonometer- An instrument to check IOP. Different types explained with its basic Principle, Pros & Cons & its usage.

1. Tonometer
Rajeswari K
M.Optom

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opto Tonometer
Tonometer
Tonometry is the procedure performed to determine the
intraocular pressure (IOP)
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History
1826: William Bowman used digital tonometry as a
routine examination test.
1885: Maklakov designed an applanation tonometer. This
was used for a number of years till 1959.
1905: Hjalmar Schiotz produced his indentation
tonometer. This made tonometry a simple and routine
clinical test.
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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
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Types
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Direct InDirect
Static Dynamic
Contact Non Contact
Indentation
Application
Schiotz
GAT,Perkins,Tonopen
Airpuff
Ballistic
Digital

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Other Types
In a normal eye IOP becomes more during tonometry
Low-Displacement Tonometers.
Tonometers in which the IOP is negligibly raised during tonometry (less than 5%) are
termed as low-displacement tonometers.
Eg. Goldman's Applanation Tonometer.(raises IOP by only 3%)
High-Displacement Tonometers
Tonometers that displace a large volume of
fl
fluid and consequently raise IOP
significantly are termed as high- displacement tonometers.
Eg. Schiotz.
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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
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Shiotz Tonometry
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PARTS
Scale
Needle
Additional Weight
7.5g,10g,15g
Weight 5.5g
Foot plate
ROC 15mm
Plunger
3mm Diameter
Holder
Lever

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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.
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Friedenwald Conversion Table

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Procedure
Patient should be anaesthetised with 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 anaesthetised 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.
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Source of Error
Accuracy is limited as ocular rigidity varies from eye to eye.
Repeated measurements lower IOP.
Steeper or a thicker cornea causes greater displacement of
Schiøtz reads lower than GAT
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Advantage
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
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Connects to the slit lamp
Biprisms (measuring prism)
Adjusting Knob
Feeder arm
Control weight insert
(for calibration)
Housing
PARTS

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Principle
Applanation tonometry is based on the Imbert-Fick Law.
It 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
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Modi
fi
fied law
Cornea being aspherical, wet, and slightly inflexible fails to follow the law.
F is the force necessary to flatten its surface
here it is tear films
When applying force the first layer that gets applanated is tear film that exerts some
surface tension (S) for tonometry head.
This has to be added with the force
A is the area (here it is front surface of the cornea, which is not necessary)
The outer area of corneal flattening differs from the inner area of
fl
flattening (A1). It is
this inner area which is of importance.
Lack of flexibility(inflexible nature of cornea) requires force to bend the cornea (B)
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P = — ——
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F F+S
A A1+B
Law Modi
fi
fied Law

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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.
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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 room illumination is reduced.
A fixation light may be placed in front of the fellow eye.
The angle between the illumination and the microscope should be
approximately 60°
The tension knob is set at 1. If the knob is set at 0, the prism head may
vibrate when it touches the eye and damage corneal epithelium.
The 1 g position is used before each measurement.
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opto Tonometer
After instilling topical anaesthesia, cornea is made apparent by
instilling fluorescein and viewed in cobalt blue light.
The biprism should not touch the lids or lashes because this
stimulates blinking and squeezing as a protective mechanism.
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.
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Procedure

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Errors
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If the rings are too narrow
An excessively wide fluorescein ring can cause IOP to be
overestimated
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Too Low
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Circles not Coinciding
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Too close
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End Point
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Calibration
GAT should be calibrated periodically, at least twice in a
week.
If the GAT is not within 0.1 g (1 mmHg) of the correct
calibration, the instrument should be repaired.
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Sterilisation
Applanation tip should be soaked for 5-15 min in diluted
sodium hypochlorite, 3% H
2
O
2
or 70% isopropyl alcohol
Can also be wipped using alcohol pads
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Perkins Tonometer
It uses same prisms as Goldmann
It is handheld 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
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Tonopen
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Portable
It is particularly useful in community health fairs, on ward
rounds ,children, irregular surfaces,
Tono-Pen tends to overestimate the IOP in infants so its usefulness
in congenital glaucoma screening and monitoring is limited.
In band keratopathy where the surface of the pathology is harder
than normal cornea, the Tono-Pen tends to overestimate the IOP
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Non Contact Tonometer
Air puff tonometer
Commonly used and saves time.
A puff of air of known area is generated against cornea
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.
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