Glaucoma is another complication of uveitis as well as the long term use of steroid eye drops. Specialists test eye pressure at each appointment to ensure it doesn’t rise. If it does rise, it is known as ocular hypertension, which can lead to glaucoma if not quickly and properly treated.
Ocular hypertension means that pressure in the eye is higher than the normal range of between approximately 10 – 21 mm Hg. but there are exceptions. Children’s eyes can tolerate a higher pressure without damage occurring to the optic nerve for a longer period than an adult’s eyes.
Ocular hypertension is not the same as having glaucoma. High pressure is treated in the hope that this will prevent glaucoma and the optic nerve will remain undamaged.
Glaucoma damages the optic nerve and the damage is irreversible; sight lost cannot be recovered. The optic nerve contains over a million fibres, and these fibres carry images of what is seen to the brain. In glaucoma, the fibres become damaged and die. As more and more die, what we see is affected.
During the initial stages of glaucoma, most people don’t know they have it. Even as the disease progresses, people are still unaware, because the changes in vision are so slight to begin with that we might not even notice them, especially if a second, unaffected eye, is filling in the visual field gaps. By the time a patient does notice, considerable damage has already been done to the optic nerve.
What causes glaucoma is not yet fully understood, but one of the biggest risk factors for its development is high intraocular pressure. The average normal range of pressure is between 10 and 21mm Hg. Pressure varies through the course of the day (diurnal changes) and occasionally patients may be asked to attend the eye clinic at different times of the day to follow these changes. The decision to begin treating for raised intraocular pressure (ocular hypertension) is not always simple, and when pressure is in the low twenties, there may be monitoring and a reduction, or a change in the type of steroid being used, before a decision is taken to use pressure lowering drops or tablets.
If pressure is much higher, for example over 30, glaucoma drops may be used immediately to try and prevent glaucoma from developing. Doctors treating uveitis patients with pressure lowering drops are stringent about measuring intraocular pressure regularly, because some types of uveitis are associated with increased intraocular pressure, and because other patients who are treated with the various forms of steroid are steroid responders. Being a steroid responder means that steroid raises your intraocular pressure.
It is also important that intraocular pressure remains consistent because spikes in pressure, either upwards or downwards, may also cause damage to an optic nerve. These spikes in pressure sometimes occur during, or after, surgical procedures.
A normal intraocular pressure is measured when the production of the aqueous humor is optimally balanced with how it drains from the eye. The aqueous is a watery fluid produced continuously by the ciliary body, which bathes the cornea and the lens, nourishing them. (Tears are a completely different type of fluid). This fluid leaves the eye through a drainage system called the trabecular meshwork. Proper drainage through this network is needed to keep the intraocular pressure within a normal range.
Uveitis patients are at increased risk of developing glaucoma. Frequently, this is secondary glaucoma, that is, glaucoma caused by another disease (the uveitis) and/or drugs (corticosteroid).
The uveitis entities which carry a risk of glaucoma developing include anterior uveitis, herpetic uveitis, JIA type uveitis, Sarcoidosis, Fuch’s syndrome, posterior uveitis, Posner-Schlossman syndrome and idiopathic uveitis. Some of the reasons for this are described below.
Anterior uveitis affects the production of the aqueous and its drainage. If synechiae have formed, drainage is affected. In addition, the drainage system, the trabecular meshwork, may become clogged with inflammatory cells which have settled in it.
Uveitis may also alter the composition of the aqueous and affect pressure through increased protein content and the presence of prostaglandins. Corticosteroid decreases the rate at which the aqueous drains.
Emedicine’s website provides a useful discussion of Drug-Induced Glaucoma.