Glaucoma is the name given to a group of conditions that cause damage to the optic nerve and if left untreated this damage can lead to loss of vision. The damage is caused by an increase in the pressure at the front of the eye, and this pressure is transmitted to the delicate optic nerve at the back of the eye. The optic nerve is responsible for passing information from the retina to the brain and if this link is damaged then the vision is affected.
Fluid at the front of the eye (aqueous humour) is produced in a part of the eye called the ciliary body, which is situated behind the iris (the coloured part of the eye). The fluid continuously flows from the ciliary body then through the pupil to enter the front part of the eye where it drains away through the trabecular meshwork, which is situated as a ring around the outer edge of the iris.
Different types of glaucoma
There are four main types of glaucoma which are:
- Primary open angle glaucoma (POAG) also known as chronic open angle glaucoma
- Acute angle closure glaucoma
- Secondary glaucoma
- Developmental glaucoma
The diagram below illustrates the difference between open angle glaucoma on the left and closed angle glaucoma on the right. The ‘angle’ refers to space between the back of the cornea and the front part of the iris. As the trabecular meshwork is at the junction where the two meet, the more space there is at this point, the easier it is for the aqueous to drain away. As can be seen in the diagram on the right, the angle is closed and so the aqueous cannot drain away and the pressure increases inside the eye.
Primary open angle glaucoma
Primary open angle glaucoma (POAG) is the most common type of glaucoma. It develops slowly and occurs when the drainage of aqueous fluid is reduced due to a narrowing of the drainage holes in the trabecular meshwork. This causes an increase in the pressure inside the eye and this damages the optic nerve. As the build-up of pressure is slow, there is no pain associated with POAG despite the fact that the peripheral vision is being lost.
POAG becomes more common with increasing age. It is uncommon below the age of 40, and affects 1% of the population over 40 and 5% of the population over 65. The risk of POAG is greater in people of African origin and in families where a close relative already has glaucoma. There is an increased risk of glaucoma in diabetics.
Opticians are able to detect glaucoma by performing screening tests. These include checking the intraocular pressure, assessing the shape and size of the optic nerves and by examining the visual fields. If any of these tests are abnormal then the optician will recommend a referral to an eye doctor (ophthalmologist). It is recommended that eye tests should be carried out every two years. If there is a family history of glaucoma then annual eye tests are free for people in this at risk group.
The treatment of POAG is overseen by ophthalmologists. The first line of treatment is the use of eye drops. There are many different sorts of eye drops and they all act by lowering the pressure inside the eye. If drops cannot lower the pressure enough, then laser treatment can be used or surgery if the pressure is excessively high, despite the use of eye drops. The operation to lower the pressure is called a trabeculectomy.
Angle closure glaucoma
Angle closure glaucoma is much less common than POAG, occurring in approximately 0.1% of the population. This type of glaucoma is quite different to POAG: the pressure rises inside the eye very quickly to a very high level. As a result, the vision becomes blurry and the eye is red and painful. The reduced vision is caused by the cornea becoming waterlogged and hazy. This change can also give rise to halos appearing around lights. It is possible to have mild (or ‘sub-acute’) attacks where the pressure settles back down to normal. The clue that these attacks have taken place is by the presence of halos around lights with mild or no pain.
The treatment of angle closure glaucoma is an emergency. It is vital to reduce the pressure inside the eye as quickly as possible to prevent permanent damage to the eye and loss of vision. This treatment is carried out in hospital and involves the use of pressure lowering drops and intravenous medications. Once the pressure has been successfully lowered, then laser treatment is required to prevent the pressure from building up again in the future. The laser treatment works by creating a tiny hole in the iris which acts as an alternative route for the aqueous fluid inside the eye to flow, when the normal route through the pupil is blocked. The laser treatment is carried out in the other eye as a preventive measure.