Tonometry

The intraocular pressure (IOP)

The measurement of IOP (ocular tension) should be made in all suspected cases of glaucoma and in routine after the age of 40 years

•Normal IOP range is 10-21 mm of Hg with an average tension of 16 ± 2.5 mm of Hg.

• When IOP is less than 10 mm of Hg, it is called hypotony. An IOP of more than 21 mm of Hg should always arouse suspicion of glaucoma and such patients should be thoroughly investigated.

TONOMETERThe intraocular pressure (IOP) is measured

with the help of an instrument called Tonometer.

Two basic types of Tonometers available

are:-

1.indentation or Impression Tonometer.2. Applanation Tonometer

Indentation Tonometry•Indentation or (impression) Tonometry is based on the fundamental fact that a plunger will indent a soft eye more than a hard eye.

•The indentation tonometer in current use is that of Schiotz.

Schiotz Tonometer

Because of its simplicity, reliability, low price and relative accuracy, it is the most widely used tonometer in the world.

Schiotz Tonometer

It consists of:-•Handle for holding the instrument in

verticalposition on the cornea;

• Footplate which rests on the cornea;

• Plunger which moves freely within a shaft in the footplate;

•Bent lever whose short arm rests on the upperend of the plunger and a long arm which acts asa pointer needle. The degree to which the plungerindents the cornea is indicated by the movementof this needle on a scale;

• Weights: a 5.5 g weight is permanently fixed tothe plunger, which can be increased to 7.5 and 10gm.

Technique of Schiotz Tonometry•Before Tonometry, the footplate and

lower end of plunger should be sterilized.

Sterilized By

• Dipping the footplate in ether, absolute alcohol,acetone or by heating the footplate in the flame of spirit.

CONT…..•After anaesthetising the cornea with

2-4 per cent topical xylocaine, patient is made to lie supine on a couch and instructed to fix at a target on the ceiling. Then the examiner separates the lids with left hand and gently rests the footplate of the tonometer vertically on the centre of cornea. The reading on scale is recorded as soon as the needle becomes steady .

It is customary to start with 5.5 gm weight.

However, if the scale reading is less than

3, additional weight should be added to the

plunger to make it 7.5 gm or 10 gm.

• In the end, tonometer is lifted and a drop of antibiotic is instilled.

•A conversion table is then used to derive the intraocular pressure in mm of mercury (mmHg) from the scale reading and the plunger weight.

CONT………..

Advantages of Schiotz tonometer •cheap, handy and easy to use.

Disadvantage

•It gives a false reading when used in eyes with abnormal scleral rigidity. False low levels of IOP are obtained in eyes with low scleral rigidity seen in high myopes and following ocular surgery.

ERRORS OF INDENTATION TONOMETRY•Error inherent in the instrument

•Error due to contraction of extraocular muscles

•Error due to accommodation

•Error due to ocular rigidity

•Error due to variation in corneal curvature

•Errors in scale reading

•Blood volume alteration

Applanation Tonometry•The concept of applanation tonometry

wasintroduced by Goldmann is 1954.

It is based on Imbert-Fick law which states that the pressure inside a sphere (P) is equal to the force (W) required to flatten its surface divided by the area of flattening (A); i.e., P =W/A.

Cont…….

The commonly used applanation tonometers areGoldmann tonometer.

• Currently, it is the most popular and accurate tonometer. It consists of a double prism mounted on a standard slit-lamp. The prism applanates the cornea in an area of 3.06 mm diameter.

Technique•After anaesthetising the cornea with a

drop of 2 per cent xylocaine and staining the tear film with fluorescein patient is made to sit infront of slit-lamp.

•The cornea and biprisms are illuminated with cobalt blue light from the slit-lamp.

•Biprism is then advanced until it just touches the

apex of cornea. At this point two fluorescent semicircles are viewed through the prism.

• Then, the applanation force against cornea is adjusted until the inner edges of the two semicircles just touch. This is the end point.

•The intraocular pressure is determined by multiplying the dial reading with ten.

Technique of applanation tonometry

Perkin’s applanation tonometer1. This is a hand-held tonometer utilizing

the same biprism as in the Goldmann applanation tonometer.

2. It is small, easy to carry and does not require slit lamp.

3. It requires considerable practice before, reliable readings can be obtained.

Perkin’s applanation tonometer

3. Pneumatic Tonometer• In this, the cornea is applanated by

touching its apex by a silastic diaphragm covering the sensing nozzle (which is connected to a central chamber containing pressurised air).

• In this tonometer, there is a pneumatic-to-electronic transducer, which converts the air pressure to a recording on a paper-strip, from where IOP is read.

End point of applanation tonometry. (A) too small; (B) too large; (C) end point

4. Pulse air tonometer

Tonometer that can be used with the patient in any position.

5. Tono-Penis a computerised pocket tonometer. It

employs a microscopic transducer which applanates the cornea and converts IOP into electric waves.

Digital tonometry•A rough estimate of IOP can be made by

digital tonometry.

•For this procedur patient is asked to look down and the eyeball is palpated by index fingers of both the hands, through the upper lid, beyond the tarsal plate.

Cont….• One finger is kept stationary which feels

the fluctuation produced by indentation of globe by the other finger .

• It is a subjective method and needs experience.

•When IOP is raised, fluctuation produced is feeble or absent and the eyeball feels firm to hard. When IOP is very low eye feels soft like a partially filled water Bag.

Technique of digital tonometry

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