Glaucoma is a complicated disease. Our introduction to glaucoma explains the structure of the eye, what eye pressure is and how this is related to glaucoma diagnosis.
Glaucoma is the name of a group of eye diseases that damage the optic nerve. The optic nerve transfers visual information from the eye to the brain and if it’s damaged, it can result in sight loss and even blindness. There are three main types of glaucoma: primary, secondary, and developmental (glaucoma in babies and children).
Glaucoma is a complicated disease and diagnosis is not always easy. Several different tests will be needed to confirm how and why damage to the optic nerve has occurred. In many cases, glaucoma is associated with high pressure within the eye.
To understand how glaucoma affects us, it’s useful to understand the structure of the eye, and why keeping eye pressure within the normal range is so important.
The eye is shaped like a ball. The tough white outer coat is called the sclera and its surface is covered by a thin skin called the conjunctiva. At the front of the eye, the outer coat is clear and is called the cornea. Behind the cornea is the iris – the coloured part of the eye – with the pupil forming a hole in its centre. The lens of the eye, which helps to focus light is suspended behind the pupil.
The space between the cornea and the lens is filled with a clear fluid, called aqueous humour (or just aqueous). This is a clear, watery fluid that is continually produced inside the eye, and delivers nutrients and oxygen to all parts of the eye. The aqueous is responsible for maintaining the pressure in the eye (the intraocular pressure or IOP). Aqueous is different from tears, which are produced by glands outside of the eye and moisten the outer surface of the eyeball.
The pressure in the eye is determined by the balance between the production of aqueous inside the eye, and its drainage out of the eye. Aqueous is made in a ring of tissue that sits behind the iris called the ciliary body. It then flows through the pupil and drains away through tiny channels called the trabecular meshwork. The trabecular meshwork is found in the angle between the cornea and the iris (often called the drainage angle).
In a normal eye, there is a healthy balance between the production and drainage of fluid. The pressure may vary slightly at different times of the day, but it normally stays within a range that the eye can handle. In some cases, this balance is disturbed and the pressure rises above normal. This pressure rise is usually due to the flow of fluid out of the eye being restricted.
A certain level of pressure is needed for the eye to keep its shape, but if it gets too high, it can cause stress to the optic nerve leading to damage. Damage caused in this way is diagnosed as glaucoma.
Just to complicate diagnosis, glaucoma can also develop in some people when the IOP is within the normal range. This is called normal (or low) tension glaucoma.
Some people have a higher than normal IOP, but there is no detectable damage to the optic nerve. This is called ocular hypertension and needs to be monitored in case it develops into glaucoma in the future.
Age – Primary open angle glaucoma becomes more common with age. It’s fairly rare below age 40, but rises from about two in 100 over the age of 40 to more than one in 20 for those aged 80+.
Blood pressure – Very high blood pressure can lead to an increase in intraocular pressure. Low blood pressure can lead to insufficient blood supply to the optic nerve which can also cause problems.
Ethnicity – People of African-Caribbean origin have about a four-times higher risk of primary open angle glaucoma compared to those of European origin. People of east Asian origin are at higher risk of developing primary angle closure glaucoma.
Family history – There is at least a four-times higher risk of developing glaucoma if you have a close blood relative who has it. So if you have glaucoma, you should tell your relatives about the condition as they may need to be tested. More information can be found in our booklet Glaucoma and your relatives.
Short sight – People with short sight (myopia) are at increased risk of developing glaucoma.
Long sight – People with long sight are at increased risk of developing primary angle closure glaucoma.
Diabetes – People with diabetes may be at higher risk of developing glaucoma.
In the early stages, glaucoma might not present any symptoms. Most commonly, glaucoma affects your off-centre, or peripheral vision first and this can go unnoticed initially because your central vision, which we use for reading, recognising faces, watching tv etc, remains good. The only way to know if you are affected is to go for regular eye health checks at your optician/ optometrist. They will carry out a few painless tests, including measuring your IOP, to find out if you need referring to an ophthalmologist for further investigation.
Glaucoma is a life-long, chronic disease that cannot be cured. Any sight lost due to glaucoma cannot be regained. But the good news is, with effective medical treatment, the damage can be slowed or even stopped in its tracks.
Glaucoma was once a disease that would almost certainly lead to blindness. Huge advances in diagnosis, monitoring and treatment now mean that in the UK, the vast majority of people with glaucoma will keep useful sight for life.