What is glaucoma?
Glaucoma is a complicated disease in which damage to the optic nerve leads to progressive, irreversible vision loss.
How the eye works
The eyeball is like a balloon and to maintain its shape, the pressure inside has to be higher than atmospheric pressure.
This pressure is maintained by water being pumped into the eyeball at a steady rate and it is controlled by the outflow.
The best way to understand this is to imagine the eye like a sink. The taps are open at a constant rate but to keep the water level constant, you vary the size of the drain.
In the eye, the taps are behind the iris, the coloured part of the eye and the drain is like a sieve in an area between the coloured part and the clear window at the front of the eye. The sieve opens and shuts to let the pressure remain constant.
With age, the sieve stops working properly so the pressure is not controlled and can build up. The normal pressure is between 14 and 24 mm Hg. Let’s assume on average it is 18 mm Hg.
Normally blood has to come into the eye against this pressure. The normal blood pressure at this point is 58 mm Hg. So, for each pulse the eye gets 40 units of blood.
How does glaucoma affect this?
If the pressure in the eye goes up to say, 28 mm Hg, and then only 30 units of blood get into the eye. The eye starts to get starved of blood. Slowly, the nerves in the eye get damaged and die. The effect of this is that only the nerves which are living can transmit messages to the brain. Where the nerves are dead, there is no transmission to the brain and these become blind areas. As the disease advances, these become bigger.
As the pressure goes to 38, the veins close down and there is a risk of blockage of the vein. This causes loss of vision in the upper or lower half of the eye.
In a particular type of glaucoma, called narrow angle glaucoma, the pressure can go up to 58 or more. At this pressure, the blood supply to the eye is cut off resulting in severe headache, vomiting and loss of vision.
What are the effects and treatments of glaucoma?
There are two main types of glaucoma. The open angle type starts in the 50’s and gets worse over time. It can be treated with drops. This is equivalent to the drain becoming clogged but by turning the taps down, it can cope.
The second type, that can cause loss of sight within hours, is called narrow angle glaucoma. It often causes migraine like headaches before a full blown attack of angle closure. This is equivalent to the drain suddenly becoming blocked. This would normally be treated with laser which makes a new drain hole in the coloured part of the eye and is the same as the safety drain at the upper end of sink to stop it overflowing.