Glaucoma is divided into different subtypes. The thing that they all have in common is that the optic nerve is damaged.
The commonest type of glaucoma is Primary Open Angle Glaucoma (POAG). The IOP may be higher than normal (normal range 10-21mmHg) but not always. The patient has no symptoms. POAG is discovered when the eye pressures are checked and found to be high. Or when the optician looks at the back of the eye and suspects that the optic nerve looks damaged in a particular pattern.
The human eyeball is a closed chamber in which fluid (aqueous) is produced and drained away constantly. This internal circulation of fluid within the eye is not the same as your tears. In POAG, the drainage system does not filter the fluid sufficiently for reasons we still do not understand. Some people have optic nerves that are more susceptible to damage. In these patients even normal IOP can cause damage to the nerve. This may be because the blood supply to these susceptible optic nerves is compromised. Or the support for the optic nerve, as it travels into the back of the eye, is weak. Neither of these factors can be altered. So reducing the IOP in POAG is the only treatment option at the moment, even if the starting IOP is within the normal range.
In some patients, the drainage channels that filter the aqueous within the eye are too small. As we all get older we develop cataracts. The evolving cataract can push against the drainage channels and make them smaller and narrower. In some cases, they can be blocked off altogether. The IOP rises acutely causing acute glaucoma. This is an ophthalmic emergency and urgent eye pressure reduction is required. Otherwise, the vision will be lost forever.
It is important that the drainage angles are examined correctly when you see a glaucoma specialist. If we see narrow drainage angles before an acute attack occurs, we can do a laser treatment to reduce the chances of an acute attack. This is called a ‘peripheral iridotomy’. This is a tiny hole in the iris, the coloured part of your eye, akin to the hole at the top if your sink that stops the sink from overflowing. This is not a hole in the eye but is like an extra pupil at 12 o’clock in the iris. No one can see it and it is in a part of the iris that lies beneath the upper lid.
In some cases, laser alone does not allow enough space for fluid to drain. It may be necessary to remove the growing cataract to restore the space for aqueous fluid to drain efficiently. This means that we sometimes operate on cataracts for IOP rather than for vision.
Ms Laura Crawley is a Consultant Ophthalmologist at Clinica London. Her special expertise is in treating glaucoma patients as well as patients with glaucoma and cataracts. She has a lot of experience in treating glaucoma and has published extensively in scientific journals and on medical education. She still does a lot of emergency operations at the emergency department at the Charing Cross and Western Eye Hospitals for the NHS. At Clinica London, she is responsible for glaucoma patients and glaucoma patients with cataracts. She also sees patients with general eye problems.