Glaucoma is not just one disease , but a group of diseases that all involve optic nerve damage. Most of these types of glaucoma involve high eye pressure and are classified by the underlying problem that blocks the drainage channels.
In the US, Europe and Africa, open angle glaucoma is most common. The aqueous channels appear normal but fail to allow the aqueous fluid to pass through (similar to a clogged filter) so that the eye pressure builds up. Research suggests that a genetic defect is responsible for this malfunction of the aqueous channels in most patients with Primary Open Angle Glaucoma.
Up to one third of patients with open angle glaucoma have eye pressures that are in the “normal” range (10-21) but they still get glaucoma damage. This condition is called Low Tension Glaucoma or Normal Pressure Glaucoma.
Some of these patients have a very thin cornea (clear window at the front of the eye) that makes their eye pressure measure lower than it actually is. Others may have a weak optic nerve that can be damaged even at normal eye pressures. Still others may have episodes of very low blood pressure that cause damage to the optic nerve indistinguishable from glaucoma.
In contrast, angle closure glaucoma is more common in Asia than in the U.S.A. Here the front part of the eye is small and the iris (the brown or blue part of the eye) covers the channels of drainage (like a sheet of paper covering the bathtub drain). This can happen suddenly with pain and immediate loss of vision, or more commonly develop slowly without pain or noticeable changes in sight.
The tendency to develop both kinds of blocked channels is often inherited and may affect several members in one family. In most patients, this blockage and high pressure is not caused by the blood pressure or anything one eats or drinks, although physical inactivity, obesity and smoking can make established glaucoma worse, as can poorly controlled high blood pressure and diabetes.
Infants and children may also develop glaucoma. Any infant or child with unusually large eyes, cloudy eyes or who cannot open his eyes (light sensitivity with excessive tearing) needs to have immediate evaluation by an ophthalmologist.
Some medications can cause or worsen glaucoma. Use of steroids (like prednisone, cortisone, or dexamethasone) for more than a few weeks can cause high eye pressures in 30-40% of normal people . This occurs more frequently if you have family members with glaucoma (a “family history” of glaucoma) and even more frequently if you have glaucoma. While this is most common with topical eye drops and ointments, it can occur with steroids taken by any method (by mouth, skin cream, nasal spray, inhalers and joint injections).