Glaucoma: Open-Angle vs Angle-Closure Guide

Glaucoma is a group of eye diseases in which the optic nerve — the cable carrying vision to the brain — is gradually damaged, usually because the pressure inside the eye (intraocular pressure, IOP) is too high. The eye constantly makes a fluid called aqueous humor; when its drainage is blocked, pressure builds and crushes nerve fibers.

Glaucoma is the leading cause of irreversible blindness worldwide, and it is dangerous because the commonest form has no symptoms until vision is already lost. Unlike cataract, the damage cannot be undone — treatment only stops it worsening. Knowing the two main types and their warning signs is what saves sight.

Key Concepts

• Intraocular pressure: normal is 10–21 mmHg, measured with a tonometer. Pressure above 21, or a pressure an individual's nerve cannot tolerate, drives damage.
• Aqueous humor dynamics: the ciliary body produces aqueous humor, which drains through the trabecular meshwork into Schlemm's canal. Anything narrowing this drain raises IOP.
• Open-angle glaucoma: the commonest type; the drainage angle looks open but the meshwork clogs slowly. Painless, stealing peripheral vision first and producing tunnel vision over years.
• Angle-closure glaucoma: the iris suddenly blocks the drainage angle — an emergency with severe eye pain, headache, halos around lights, nausea, hazy cornea and a mid-dilated pupil. Needs immediate pressure-lowering drugs and a laser hole in the iris (iridotomy).
• Cup-disc ratio: the optic disc's central cup enlarges as fibers die; a ratio above 0.5 or asymmetry between eyes is suspicious.
• Treatment ladder: drops first — prostaglandins (latanoprost, nightly), beta blockers (timolol), alpha agonists, carbonic anhydrase inhibitors — then laser, then surgery (trabeculectomy) for uncontrolled cases.

Formulas

• Target IOP: target = baseline IOP × (1 − desired reduction). Guidelines usually aim for 20–30% below the untreated baseline, lower for advanced damage.
• Achieved reduction: % reduction = (baseline − current) ÷ baseline × 100. Comparing this with the target shows whether the drops are working. IOP is measured in mmHg.

Worked Example

1. A 55-year-old man has untreated IOP of 28 mmHg in both eyes, cup-disc ratio 0.7 and early visual-field loss — open-angle glaucoma.
2. With a 25% reduction target: target IOP = 28 × 0.75 = 21 mmHg.
3. After one month of nightly latanoprost, IOP is 20 mmHg. Achieved reduction = (28 − 20) ÷ 28 × 100 = 28.6%.
4. Since 28.6% beats the 25% target, the single drop is working. He continues lifelong drops with visual-field tests every 6–12 months to confirm the nerve stays stable.

Common Misconceptions

• Myth: Glaucoma always hurts. Fact: only acute angle-closure is painful; open-angle glaucoma destroys vision painlessly — hence screening after 40.
• Myth: Treatment brings lost vision back. Fact: dead optic-nerve fibers never regenerate; treatment only preserves remaining vision.
• Myth: Drops cure it, so you can stop when you feel fine. Fact: drops control pressure only while used; stopping lets damage resume silently.

Real-World Applications

• Screening: everyone over 40, diabetics and those with family history need periodic IOP and disc checks.
• Emergency care: acute angle-closure gets immediate drugs plus laser iridotomy to save the eye.
• Low-vision rehabilitation: patients with advanced tunnel vision learn mobility and magnifier skills.

Glaucoma damages the optic nerve through high eye pressure: open-angle is a silent chronic thief of peripheral vision, angle-closure a painful emergency. Treatment targets a 20–30% IOP reduction with lifelong drops, laser or surgery — halting, but never reversing, the damage.