The normal eye
The eyeball is filled with a watery fluid called aqueous humour, which creates pressure and helps maintain the shape of the eyeball. The aqueous humour that fills the anterior chamber (front part of the eye) is produced by the ciliary body, just behind the iris. The fluid drains back into the bloodstream through sieve-like channels called the trabecular meshwork. Normally, the amount of fluid produced is balanced by the amount draining away, so the pressure in the eye stays constant.
In most people with glaucoma, it is raised pressure inside the eye that causes optic nerve damage. The increase in pressure usually happens because the aqueous humour in the anterior chamber doesn’t drain away properly. However, the cause of glaucoma is not completely understood. Some people have high internal eye pressure but do not develop optic nerve damage, while in others the nerve becomes damaged despite normal internal eye pressure.
Types of glaucoma
There are two main types of glaucoma, open angle glaucoma and closed angle glaucoma.
Open angle glaucoma
Open angle glaucoma is the most common form and results when the trabecular meshwork becomes blocked over several years, leading to gradual increase in pressure and worsening of vision. The build up of pressure inside the eye is painless as well as slow. But, if it isn’t treated, as the optic nerve is gradually damaged, the field of vision is reduced, so that eventually only a small area of central vision remains (tunnel vision) before sight is lost completely.
Most people do not notice any symptoms until they have some loss of vision, and by this time the optic nerve may be damaged. This is why regular screening tests are important for people over the age of 40.
Closed angle glaucoma
Closed angle glaucoma (also called acute glaucoma) is much less common. It involves a sudden, complete blockage of the trabecular meshwork, and can result in permanent blindness if not treated promptly. Symptoms usually affect only one eye, which becomes red and extremely painful.
This may be accompanied by a headache, blurred vision and vomiting. Closed angle glaucoma needs immediate treatment in hospital. This usually involves medicine to reduce the pressure in the eye, followed by laser treatment or surgery (see Treatment, below).
The other eye is usually treated at the same time, because there is a strong chance it will develop the same problem. If treated quickly there can be almost complete and permanent recovery of vision. There are two other uncommon types of glaucoma: secondary glaucoma, in which the rise in internal eye pressure is the result of another eye condition, and congenital glaucoma due to a physical abnormality of the eye present at birth
Risk factors for primary open-angle glaucoma include increasing age and a family history of glaucoma. It is more common in Afro-Caribbean people. There has also been a suggestion that diabetes is a risk factor for primary open-angle glaucoma, though there is no clear evidence of this.
Screening tests are important to detect glaucoma before there is significant loss of vision. An optometrist can do these during a sight test. Ideally, everyone over 40 should have their eyes checked for glaucoma every two years.
There are three types of simple, painless tests that an optometrist can do:
- Look at the back of the eye using a special torch (ophthalmoscope). Damage to the optic nerve can be seen at the back of the eye.
- Measure the internal pressure of the eye (tonometry). This is done with a device that just touches the front surface of the eyeball (cornea).
- Visual Fields Test: Measure the field of vision, by showing a sequence of spots of lights and asking which can be seen.
People who are found to have glaucoma can be referred to an ophthalmologist for assessment and treatment.
Treatment for glaucoma aims to lower the internal pressure of the eye and prevent any loss of vision, or any further deterioration in vision. Options include medicine given as eye drops, laser treatment and surgery. Treatment cannot reverse any existing optic nerve damage, so it won’t improve sight if that has already deteriorated.
There are several different types of medicine available for treating glaucoma. They help lower the internal pressure of the eye by increasing the flow of fluid out through the trabecular meshwork, or by reducing production of fluid. It is important that any prescribed eye drops are used every day as advised by your Ophthalmologist, or the pressure in the eye will rise again.
There are combination eye drops available that might be more convenient for those who need to use two different medicines. Side-effects: all the eye drops can cause some irritation to the eyes. They can cause itching and dry eyes. Side-effects of other types of medicine may include taste disturbance, darkening of the iris, thickening and lengthening of eye lashes, headaches and blurred vision.
A laser can be used to open the holes in the trabecular meshwork. The procedure is usually quick and causes little discomfort. No general anaesthetic is required and there is no need for an overnight stay in hospital. The success of this type of treatment varies from person to person, and it may be necessary to continue using eye drops after laser treatment, though often at a lower dose.
The effectiveness of newer medications for glaucoma has recently reduced the incidence of traditional surgery for glaucoma. However, surgery is an option when medicines and/or laser treatment have failed to adequately lower pressure in the eye.
The operation is straightforward and involves creating a tiny opening in the eye wall, which allows fluid to escape into the loose tissue under the conjunctiva (the clear membrane that covers the white of the eye). From there it is absorbed back into the bloodstream. The operation often makes further treatment with eye drops or tablets unnecessary, but approximately one in five people need to keep using them.