Glaucoma—1: Diagnosis

doi:10.1136/bmj.328.7431.97

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P T Khaw,
A R Elkington

Introduction

The glaucomas are a range of disorders with a characteristic type of optic nerve damage. The glaucomas are the second commonest cause of blindness in the world, and the commonest cause of irreversible blindness. The most effective way of preventing this damage is to lower the intraocular pressure.

Normally, the ciliary body secretes aqueous, which flows into the posterior chamber and through the pupil into the anterior chamber. It leaves the eye through the trabecular meshwork, flowing into Schlemm's canal and into episcleral veins. The flow and drainage can be obstructed in several ways.

Normal aqueous drainage and possible sites of obstruction

Symptoms and signs

Raised intraocular pressure

The intraocular pressure is determined by the balance between aqueous production inside the eye and aqueous drainage out of the eye through the trabecular meshwork. Each normal eye makes about 2 μl of aqueous a minute—that is, about 70 l during the course of a lifetime. Normal intraocular pressure is 10-21 mm Hg, but it can drop as low as 0 mm Hg in hypotony and can exceed 70 mm Hg in some glaucomas.

The rate at which raised intraocular pressure causes optic nerve damage depends on many factors, including the pressure and whether glaucomatous damage is early or advanced. In general, pressures of 20-30 mm Hg usually cause damage over several years, but pressures of 40-50 mm Hg can cause rapid visual loss and also precipitate retinovascular occlusion.

Haloes around lights and cloudy cornea

The cornea is kept transparent by the continuous removal of fluid by the endothelial cells. When the pressure rises quickly (acute closed angle glaucoma), the cornea becomes waterlogged, causing a fall in visual acuity and creating haloes around lights (like looking at a light through frosted glass).