Describe the formation, circulation and functions of aqueous humour and the factors affecting intraocular pressure.

Definition and Normal Values

• Diurnal variation of 3 to 5 mmHg, highest in the early morning; a variation greater than 8 mmHg is abnormal
• Measured by tonometry — Goldmann applanation is the standard; Schiotz indentation and non-contact air-puff tonometers are alternatives
• Corneal thickness affects the reading — a thick cornea gives a falsely high value and a thin one a falsely low value, so central corneal thickness is measured alongside

Aqueous Humour — Formation and Drainage

Secreted by the ciliary epithelium of the ciliary processes into the posterior chamber, at about 2–3 microlitres per minute → Chiefly by active secretion (about 80%) involving carbonic anhydrase and Na+/K+ ATPase; the rest by ultrafiltration and diffusion → Flows through the pupil into the anterior chamber → Drains at the angle by two routes → conventional (about 80–90%) — trabecular meshwork → canal of schlemm → aqueous veins → episcleral veins → uveoscleral (about 10–20%) — through the ciliary muscle into the suprachoroidal space; the route increased by prostaglandin analogues

• Composition — resembles plasma but with far less protein (about 0.02%), and more ascorbate, lactate and chloride; the blood–aqueous barrier maintains this difference
• Functions — maintains intraocular pressure and the shape of the globe; nourishes the avascular cornea and lens and removes their waste; provides a transparent refractive medium; carries ascorbate as an antioxidant

Factors Affecting Intraocular Pressure

Applied Aspects

• Glaucoma is an optic neuropathy with characteristic disc cupping and field loss; raised pressure is the chief modifiable risk factor but is neither necessary nor sufficient — normal-tension glaucoma exists, as does ocular hypertension without damage
• Open-angle glaucoma — increased resistance at the trabecular meshwork; painless and insidious, taking peripheral vision first, so it is often advanced before the patient notices
• Angle-closure glaucoma — the peripheral iris blocks the angle; presents acutely with severe pain, a red eye, haloes, a fixed mid-dilated pupil and vomiting; a sight-threatening emergency
• Never dilate the pupil in a shallow anterior chamber without assessment, since antimuscarinics can precipitate an acute attack
• Steroid eye drops must not be sold or used without supervision; steroid-induced glaucoma is a preventable cause of blindness and is a real problem where they are available over the counter
• Glaucoma is a leading cause of irreversible blindness in India, and vision already lost cannot be restored — which makes case-finding by pressure measurement and disc examination after the age of 40 genuinely worthwhile
• A normal pressure does not exclude glaucoma, and a raised pressure does not establish it — the disc and the visual field decide the diagnosis