What is glaucoma?
Glaucoma is a condition of the eye characterised by a raised pressure in the eyeball. Imagine the eyeball as an inflatable rubber ball which maintains its shape by the air pumped in. Here the eyeball’s shape is maintained by the pressure of a liquid, aqueous, which is made in the eye. Too much aqueous produced raises the eye pressure; too little reduces the pressure. If the pressure is higher than a certain level, changes occur both in the front and back of the eye which can lead to blindness if not controlled.
What is pressure (or tension) of the eyeball?
The pressure or, as it is medically, called, intracular tension is the pressure of the eyeball recorded by an instrument called the tonometer. The normal pressure should be 18-20 mm of mercury. A pressure of 21-23 mm is suspected glaucoma, while beyond 24 mm is definitely taken as glaucoma.
What are the methods of measuring intro-ocular pressure?
The simplest way of measuring the pressure is digital, which, in simple language, is gauging the pressure of the eye by using two fingers. Though an expert can make a fairly accurate judgement, it is obviously not accurate enough, and hence, certain instruments have been designed to help the doctor to measure pressure. The patient has a few drops put in the eye to dull the touch of the instrument.
(1) The Schiotz tonometer
An instrument which is put on the eye when the patient is in a sleeping position. A small plunger whose movement is suitably magnified, is placed on the eye. The pointer moves across the scale whose readings are translated into pressure. It is a simply designed instrument and has very wide usage all over the world. It has, as its disadvantages, the necessity of a sleeping position, and, owing to its weight, the tendency to sometimes give a false low reading in eyes which have myopia or have been operated.
(2) The applanation tonometer
A superbly accurate instrument which works on the principle of measuring the force required to flatten a known area of the cornea. It has a little optical cylinder in front which touches the eye but has a very light feel. This instrument is usually mounted on the slit microscope. There is no risk of scratching the eye with this instrument.
Here the pressure readings are recorded in a sitting position, and are free from error of any type.
(3) There are more sophisticated new units which use an air puff to measure the pressure using integrated electronic circuits: However, the accuracy of the aplanation tonometer is greatest and it is the accepted standardised technique of recording pressure.