Describe the formation, circulation and functions of aqueous humour. Add a note on glaucoma.

Formation and Circulation

• Volume 0.25 mL; formed at 2–3 µL/min; turned over about every 100 minutes
• Formed by the ciliary processes — chiefly by active secretion involving carbonic anhydrase and Na+–K+ ATPase; partly by ultrafiltration and diffusion

Ciliary processes → Posterior chamber → Through the pupil → Anterior chamber → Trabecular meshwork at the iridocorneal angle → Canal of Schlemm → Episcleral veins

• About 85% drains by this conventional route; 15% by the uveoscleral route

Composition

• Similar to plasma but with very little protein (0.02% vs 7%) — hence its optical clarity
• Higher in ascorbate, lactate and chloride; lower in glucose, urea and bicarbonate
• Blood–aqueous barrier keeps protein out; inflammation breaks it → aqueous flare seen on slit-lamp examination

Intraocular Pressure

• Normal 10–21 mmHg, mean about 16 mmHg
• Determined by the balance between formation and drainage
• Measured by tonometry — Goldmann applanation is the gold standard
• Shows diurnal variation, highest in the early morning

Glaucoma

Applied Aspects

• Glaucoma is the leading cause of irreversible blindness worldwide — the field loss cannot be recovered, so early detection is everything
• Treatment — prostaglandin analogues (↑ uveoscleral outflow), β-blockers and carbonic anhydrase inhibitors (↓ formation), pilocarpine; laser or surgery
• Mydriatics are contraindicated in a shallow anterior chamber — they may precipitate acute angle closure
• Mannitol is given intravenously in acute attacks to draw fluid out osmotically
• Steroid-induced glaucoma — prolonged topical steroids raise intraocular pressure in susceptible people; a reason never to use them casually for a red eye
• Screening after 40, and earlier with a family history, is recommended because open angle glaucoma is symptomless until late