Acute Angle-Closure Glaucoma: The Ophthalmic Emergency
Most glaucoma is a slow thief, stealing sight over years without a symptom. Acute angle-closure glaucoma is the opposite: an eye that goes from normal to blinding in hours. The drainage angle slams shut, aqueous has nowhere to go, and the pressure inside the globe climbs to levels that can strangle the optic nerve in a single afternoon. It is one of the few true emergencies in ophthalmology — and one of the most satisfying pharmacology stories in medicine, because the treatment attacks the pressure from four directions at once. It is also a trap: the wrong eye drop, or the wrong systemic drug, can trigger the whole crisis.
A 63-year-old woman comes to the emergency department at midnight with the worst headache of her life, vomiting into a bowl. The triage nurse is thinking migraine, maybe a bleed. But her right eye is scarlet, and she says the ceiling lights have coloured rings around them and her vision on that side has gone misty. When you shine a torch you find a pupil stuck half-open — a fixed, mid-dilated oval that will not react. You gently press the closed eyelids: the right globe feels rock-hard, like a marble, next to the soft give of the left. The tonometer confirms it: intraocular pressure of 62 mmHg, when normal is under 21. This is not a headache. Her drainage angle has closed, and the optic nerve is on the clock.
The plumbing: where aqueous is made and where it drains
The eye is a sink with a tap that never turns off and a single drain. The ciliary body behind the iris continuously secretes aqueous humour — the clear fluid that nourishes the lens and cornea and inflates the front of the eye. That fluid flows forward through the pupil into the anterior chamber, then drains out at the angle where the iris meets the cornea, through a sieve called the trabecular meshwork. Production and drainage are balanced, and the balance sets the intraocular pressure (IOP). As the Glaucoma-overview chapter explains, chronic open-angle glaucoma is a drain that has slowly silted up while staying open. Angle closure is a different accident entirely: the drain is not clogged — it is physically covered over, in an instant, by the iris itself.
Pupillary block: how the angle slams shut
The trigger, in most cases, is pupillary block. In a susceptible eye the lens sits close against the back of the iris. When the pupil is mid-dilated, that contact becomes a seal: aqueous made behind the iris can no longer slip forward through the pupil. Fluid then piles up behind the iris and balloons it forward like a spinnaker, until the peripheral iris bows against the cornea and smothers the trabecular meshwork. Now aqueous is still being manufactured every minute, but the only exit is sealed. Pressure has nowhere to go, so it soars — commonly to 50–80 mmHg, three to four times normal. That is why angle closure favours particular eyes: small, hypermetropic (long-sighted) eyes with a shallow anterior chamber, older patients whose lens has thickened with age, and people of East Asian descent, in whom the anatomy is more common. A narrow angle is the loaded gun; dilation pulls the trigger.
Precipitants a prescriber must know
The mid-dilated pupil is the danger zone — and many everyday things dilate a pupil. A pupil dilates in dim light (the classic story is symptoms starting in a darkened cinema), during stress or excitement (a surge of sympathetic tone), and — the part that belongs to pharmacology — in response to drugs. Topical mydriatics and cycloplegics are the obvious culprits: an anticholinergic drop like tropicamide or atropine, or a sympathomimetic like phenylephrine, given to examine the fundus, can tip a narrow angle over the edge. This is exactly why the Mydriatics chapter warns you to assess the angle before dilating. But systemic drugs matter just as much, because a patient may never connect a tablet to their eye. Anticholinergic and adrenergic drugs taken by mouth can dilate the pupil enough to close a susceptible angle: tricyclic antidepressants, some antihistamines and antiemetics, ipratropium nebulisers, and adrenergic agents. A special case is topiramate, which causes angle closure by a completely different route — not pupillary block, but a drug-induced ciliary body effusion that pushes the whole iris-lens diaphragm forward and shallows the chamber, typically bilaterally and within the first two weeks of starting the drug. These links to the Toxicology / Autonomic material are worth memorising: the drugs that dilate are the drugs that endanger.
Think of the anterior chamber as a bathtub with the tap left running and the plughole set right at the rim. Normally the water drains as fast as it fills. Now imagine a loose rubber mat (the iris) floating in the tub. Dim the lights and the mat drifts up and flops over the plughole. The tap is still running, but the drain is sealed — and because it is a rigid tub with nowhere to expand, the pressure behind that mat rises brutally fast. The fix is not to turn off the tap alone; it is to pull the mat off the drain and, if you can, to punch a small permanent hole in the mat so water can always get past it. That hole is the laser iridotomy.
Recognising the crisis at the bedside
The presentation is dramatic and unilateral. There is sudden, severe pain in and around the eye, often with a boring headache on the same side. The autonomic storm from a pressure of 60-plus produces nausea and vomiting so prominent that patients are sometimes admitted to a surgical ward for an acute abdomen before anyone looks at the eye. The vision is blurred and, characteristically, the patient sees coloured haloes around lights — the sky-high pressure forces the cornea to swell with fluid (corneal oedema), and the waterlogged cornea diffracts light into rings. On examination the eye is red with a diffuse, deep injection, the cornea looks hazy rather than crystal-clear, and the pupil is fixed and mid-dilated, often vertically oval, because the iris sphincter has been paralysed by ischaemia. And the sign you can elicit with nothing but your fingertips: the globe is stony hard. Compare it to the other eye and the difference is unmistakable.
A fixed, mid-dilated pupil in a red, painful eye with a rock-hard globe is acute angle-closure until proven otherwise — and it is one of the few times "the pupil won't react" is good news for the diagnosis rather than a sign of a brain catastrophe. The reason the pupil is stuck mid-way and not fully blown is mechanical and ischaemic: the pressure has choked the blood supply to the iris sphincter, paralysing it in a half-constricted position. That same ischaemia explains a treatment quirk you will meet in a moment — pilocarpine, the drug meant to constrict this pupil, may simply not work until you have first brought the pressure down.
- Angle closure = the peripheral iris physically covers the trabecular meshwork, usually via pupillary block, so aqueous cannot drain.
- IOP rockets to 50–80 mmHg — the optic nerve is threatened within hours.
- Predisposed eye: small, hypermetropic, shallow anterior chamber, older lens, East Asian ancestry.
- Triggered by pupil dilation: dim light, stress, mydriatic/anticholinergic and sympathomimetic drops.
- Systemic culprits: tricyclics, some antihistamines/antiemetics, adrenergics; topiramate acts by a different ciliary-effusion mechanism (bilateral).
- Classic triad: severe pain + nausea/vomiting, red eye with haloes and blurring, fixed mid-dilated pupil and a rock-hard globe.
The emergency drug attack: lower the pressure from every angle
This is applied pharmacology at its purest — hit production, volume, inflammation and the angle all at once. Because time equals optic nerve, you do not try one drug and wait; you deploy several mechanisms in parallel, then reassess pressure within an hour. First, cut aqueous production. A topical beta-blocker such as timolol switches off the ciliary body's beta-driven secretion (the same drug and mechanism you met in the Beta-blocker chapter of chronic glaucoma, now used acutely). A carbonic anhydrase inhibitor blocks the enzyme that supplies bicarbonate for aqueous formation — given here not as an eye drop but systemically, oral or intravenous acetazolamide, for a fast, whole-eye effect, exactly the CAI you met in the CAI chapter. A topical alpha-2 agonist such as brimonidine or apraclonidine both reduces production and improves outflow. Second, pull volume straight out of the eye. An intravenous hyperosmotic agent — mannitol — raises the osmolarity of the blood so that water is drawn osmotically out of the vitreous into the circulation, shrinking the eye's contents and dropping the pressure fast; this is the same osmotic-diuresis principle covered in the Cardiovascular / diuretics material. Third, calm the storm: a topical corticosteroid reduces the intense inflammation the pressure spike provokes. Position the patient supine, which lets the lens fall back and can help deepen the angle.
Opening the angle: pilocarpine and its ischaemic catch
The four moves above buy time by lowering pressure, but none of them re-opens the drain. That job belongs to pilocarpine — a directly acting muscarinic (cholinergic) agonist, the miotic you met in the Alpha-agonist / miotic chapter. Pilocarpine contracts the iris sphincter to constrict the pupil (miosis), and a constricted iris is pulled taut and away from the drainage angle, physically peeling the peripheral iris off the trabecular meshwork and re-establishing outflow. That is the elegant part. Here is the catch, and it is a favourite exam point: at the very high pressures of an acute attack, the iris sphincter is ischaemic and paralysed, so pilocarpine often simply cannot make it contract. Give it too early and nothing happens. So the sequence matters — lower the pressure first with the production-blockers and mannitol, and only once the pressure has fallen enough for the sphincter to regain its blood supply will pilocarpine bite and pull the angle open. It is a drug that needs the ground prepared before it can work.
Sequence is everything in the acute attack: reduce production and osmotically shrink the eye first, and use miosis to re-open the angle second. Reaching for pilocarpine as your opening move, before the pressure has come down, is a classic error — the ischaemic sphincter simply won't respond, and you have lost minutes the optic nerve did not have to spare.
Reduce production: timolol 0.5% drop (beta-blocker); acetazolamide 500 mg IV or oral (CAI); brimonidine/apraclonidine drop (alpha-2 agonist). Reduce volume: mannitol 20% IV infusion (hyperosmotic). Reduce inflammation: prednisolone acetate drop (topical steroid). Open the angle: pilocarpine 1–2% drop (muscarinic miotic), once IOP has fallen. Position: lie the patient flat. Definitive fix: laser peripheral iridotomy — and prophylactic iridotomy of the fellow eye, whose identical narrow anatomy makes it the next attack waiting to happen.
Definitive treatment and protecting the fellow eye
Drugs are the fire brigade; they put out the acute blaze but do not rebuild the wiring. The definitive cure addresses the pupillary block directly: a laser peripheral iridotomy makes a tiny hole through the peripheral iris with a YAG laser, giving aqueous a permanent shortcut from behind the iris to in front of it. That bypass equalises the pressure across the iris, so it can no longer bow forward and seal the angle — the loaded gun is unloaded. Because the anatomy that predisposed one eye is bilateral, the fellow eye carries a high risk of its own attack, so it receives a prophylactic iridotomy before it ever becomes symptomatic. Once the eye is quiet, some patients still need long-term pressure-lowering drops or lens surgery, but the iridotomy is what breaks the mechanism. Surgical detail belongs elsewhere; what a prescriber must carry away is the pharmacological logic and the sequence.
- Reduce production: topical timolol (beta-blocker), systemic acetazolamide (CAI, oral/IV), alpha-2 agonist (brimonidine/apraclonidine).
- Reduce volume: IV mannitol draws water osmotically out of the vitreous — the same osmotic-diuresis principle as the diuretics chapter.
- Reduce inflammation: a topical corticosteroid calms the pressure-driven inflammation.
- Open the angle: pilocarpine (miotic) pulls the iris off the meshwork — but only works after IOP falls, because the sphincter is ischaemic.
- Position the patient supine to let the lens fall back and deepen the angle.
- Definitive treatment is laser peripheral iridotomy — plus prophylactic iridotomy of the fellow eye.
- Dilating a suspected narrow angle. Never instil a mydriatic (tropicamide, atropine) or sympathomimetic into an eye with a shallow chamber to "get a better look" — you can precipitate the very attack you are trying to exclude. Assess the angle first.
- Reaching for pilocarpine first. At an IOP of 60+ the iris sphincter is ischaemic and won't respond; lower the pressure with CAI, beta-blocker and mannitol before expecting miosis to open the angle.
- Mistaking it for migraine or a surgical abdomen. The headache and vomiting can dominate, but a red, painful eye with haloes, a hazy cornea, a fixed mid-dilated pupil and a rock-hard globe is angle closure — miss it and the eye can be lost.
A 65-year-old long-sighted woman presents with a severe painful red right eye, vomiting, haloes around lights, and a fixed mid-dilated pupil; IOP is 64 mmHg. After starting IV acetazolamide, topical timolol and IV mannitol, which drug re-opens the drainage angle — and why must it come after the others?
- In a predisposed eye (narrow, hypermetropic, older) pupillary block seals aqueous behind the iris, which bows forward and covers the trabecular meshwork; IOP rockets to 50–80 mmHg.
- Presents as sudden severe unilateral eye pain, headache and vomiting, a red eye with haloes and blurring, a fixed mid-dilated pupil and a rock-hard globe — an emergency.
- Emergency drugs lower IOP from every angle: reduce production (timolol, systemic acetazolamide, alpha-2 agonist), pull out volume (IV mannitol), calm inflammation (topical steroid), then open the angle with pilocarpine — which fails until the pressure drops because the sphincter is ischaemic.
- Never dilate a suspected narrow angle; watch for drug precipitants (anticholinergics, sympathomimetics, tricyclics, some antihistamines, topiramate). Definitive cure is laser peripheral iridotomy, plus prophylactic iridotomy of the fellow eye.
- Kanski's Clinical Ophthalmology: A Systematic Approach — Glaucoma: primary angle-closure.
- American Academy of Ophthalmology, Basic and Clinical Science Course (BCSC), Section 10: Glaucoma.
- Bartlett & Jaanus, Clinical Ocular Pharmacology — agents lowering intraocular pressure; drug-induced angle closure.
- Rang & Dale's Pharmacology — the eye: aqueous dynamics, carbonic anhydrase inhibitors, muscarinic agonists and osmotic diuretics.
- Royal College of Ophthalmologists / NICE guidance — acute primary angle closure: emergency management.
- European Glaucoma Society Terminology and Guidelines for Glaucoma — angle-closure glaucoma.
