There are dozens of types of glaucoma. The most common form is "POAG" or primary open angle glaucoma. This is distinct from another major type of glaucoma - angle closure glaucoma. Some glaucomas arise secondary to other eye conditions, for example, pseudoexfoliation syndrome or pigment dispersion syndrome.

Primary angle closure glaucoma is caused by contact between the iris and the drainage angle, which in turn blocks outflow of the fluid from the eye. This contact between iris and drainage angle is believed to gradually damage the function of the drain (the "trabecular meshwork") until it  is unable to drain fluid effectively, and then the eye pressure rises. In many cases, contact between iris and the meshwork causes the formation of scar tissue. These can cause permanent obstruction of aqueous outflow, and in some cases, the pressure may build up in the eye very rapidly causing pain and redness. The treatment for angle closure is initially for YAG iridotomy. Sometimes, laser iridoplasty may be performed, and otherwise rarely a surgical iridectomy is required for those with no cataract. Angle closure is often dealt with very effectively nowadays with cataract surgery as this procedure definitively opens up the drainage angle.

Developmental glaucoma

* Primary congenital glaucoma
* Infantile glaucoma
* Glaucoma associated with hereditary of familial diseases

Secondary glaucomas

* Inflammatory glaucoma
* Uveitis of all types
* Fuchs heterochromic iridocyclitis

* Phacogenic glaucoma
* Angle-closure glaucoma with mature cataract
* Phacoanaphylactic glaucoma
* Phacolytic glaucoma
* Subluxation of lens

* Glaucoma secondary to intraocular hemorrhage
* Hyphaema
* Haemolytic glaucoma

* Traumatic glaucoma
* Angle recession glaucoma: Traumatic recession on anterior chamber angle
* Postsurgical glaucoma
* Aphakic pupillary block
* Ciliary block glaucoma

* Neovascular glaucoma (see below for more details)
* Drug-induced glaucoma
* Corticosteroid induced glaucoma

Neovascular glaucoma is an uncommon type of glaucoma that is more difficult to treat. This condition may be caused by proliferative diabetic retinopathy (PDR) or central retinal vein occlusion (CRVO).

Prism used to measure eye pressure (Goldmann tonometry).

Although many cases of glaucoma are caused by high eye pressure, there are an increasing number of patients that develop damage to the optic nerve even with low eye pressures. Certainly, glaucoma is caused by high eye pressure, and also by other factors such as poor perfusion (i.e. blood supply) to the nerve. In other cases, glaucoma is caused by having a crowded drainage angle (narrow angle glaucoma).

There are many types of glaucoma; in most cases the exact cause of raised pressure is not identified infact. Sometimes glaucoma can be caused by injury to the eye, or by other eye conditions, or by the use of medications (e.g. steroids in just about any form). Generally, whatever the cause, glaucoma is treated similarly. Some types of glaucoma respond less well to medication, or less well to laser or surgery.

Visual field of a patient with glaucoma.

Glaucoma affects about 5% of those over the age of 65 (approximate estimate) and 10% of those over the age of 80. In other words, about 1 in 20 people over the age of 65 are affected with this condition, so it is a fairly common condition. Glaucoma is commoner in some communities, for example in those from West Africa, and those from the Caribbean. Glaucoma is commoner in some families, and many patients with glaucoma have family members with the condition. Many genes have been identified as causing glaucoma, however generally the condition is not passed on directly and often might skip a generation. Nevertheless, for example, if you have a brother or sister with the condition then the risk of developing glaucoma yourself is increased significantly.

Eye pressure can vary by several points (measured in mmHg) during the day, and infact is higher at night / when lying down. Normally, eye pressure is also higher in the morning on awakening. Drinking a large volume of water or fluid quickly can lead to a significant rise in eye pressure, in some individuals. The "water drinking test" used to be a test for glaucoma patients, but it is unreliable in general and not used nowadays. Nevertheless, patients with glaucoma might be advised not to drink very large volumes of fluid (eg large volumes of beer!) in a short period as this might cause a significant rise in eye pressure. Alcohol infact is not known to be harmful to the optic nerve, except if consumed in very large quantities. An occasional glass of wine particularly may be beneficial. A substance found in wine, resveratrol, is being investigation for its potential beneficial effect in glaucoma.

Eye pressure is also varies seasonally, and is higher (very slightly) in the winter months.

Goldmann tonometer tip for measuring IOP