Glaucoma

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Glaucoma is the result of impaired resorption of circulating aqueous, leading to increased pressure in the globe and damage to the retina

A very common syndrome in which there is increased intraocular pressure, glaucoma affects 2% of the population over the age of 40 years. It is important in that if it remains untreated, blindness develops.

There are two main clinical syndromes.

Chronic glaucoma occurs with gradual increase in intraocular pressure, leading to slow, gradual deterioration in visual acuity if untreated. Acute glaucoma is associated with rapid increase in intraocular pressure, which causes severe pain and redness in the eye, and rapid deterioration in visual function (this can be permanent if not urgently treated).

The effects of raised intraocular pressure are cupping of the optic disc, detected on fundoscopy, and degeneration of retinal ganglion cells. Clinically, there is progressive peripheral visual-field loss, leading to blindness in untreated cases. In acute glaucoma there is breakdown of the endothelium, leading to oedema of the cornea and formation of painful corneal bullae. In chronic glaucoma the sclera may stretch to form bulges termed staphylomas.

Glaucoma is caused by the failure of filtration of aqueous by the trabecular meshwork

There are several common causes of glaucoma, divisible into primary and secondary groups.

Primary glaucoma is caused by two main abnormalities in drainage of aqueous:

1 Closing up of the trabecular meshwork, which normally leads to the canal of Schlemm, can occur as a degenerative process, the incidence of which increases with age; it is mainly encountered in those over the age of 40 years, often being familial. Because the drainage angle is normal, this is termed primary open-angle glaucoma.

2 With age, patients who have a congenitally shallow anterior chamber develop narrowing of the angle between the iris and the cornea, causing functional blockage to aqueous drainage. This occurs particularly when the pupil is dilated, as the iris thickens with 
contraction. Acute attacks may therefore be precipitated by being in the dark. Because the drainage angle is abnormal, this is termed primary closed-angle glaucoma. Secondary glaucoma is caused by diseases that obstruct the drainage of aqueous. For example, there may be adhesions between the iris and cornea that are caused by uveitis or are secondary to vascular proliferation due to retinal ischaemia(secondary closed-angle glaucoma). Alternatively, there may be blockage of the trabecular meshwork by particulate material in the aqueous, especially degenerate lens material, pigment from melanocytic lesions, or macrophages accumulating in response to haemorrhage or inflammation (secondary open-angle glaucoma).

Congenital glaucoma, seen in childhood with enlargement of the globe, is very rare. It is mainly due to developmental defects in the drainage of aqueous.

Normal filtration of aqueous by the trabecular network. With age, the channels in the trabecular meshwork degenerate. However, aqueous continues to be produced, so the pressure in the eye increases. As the angle between the cornea and iris root is normal (open), this is termed open-angle glaucoma. If the anterior chamber is congenitally shallow, then the drainage angle is functionally closed. When the pupil dilates, for instance in low light, this further compromises the drainage of aqueous and may precipitate an acute attack of glaucoma. If abnormal tissue (e.g. new blood vessels) blocks the trabecular meshwork, then aqueous cannot drain and glaucoma may develop; this is termed secondary closed-angle glaucoma. If particulate matter (such as inflammatory cells) occludes the trabecular meshwork, it is termed secondary open-angle glaucoma, since the irido-corneal angle remains open.
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