Alpha-2 Agonists, Pilocarpine and Rho-Kinase Inhibitors
Prostaglandin analogues, beta-blockers and carbonic-anhydrase inhibitors carry most of the day-to-day work of lowering eye pressure. But the glaucoma cabinet holds three more classes, and each earns its place for a specific reason. One has a dual action and a terrifying trap in babies. One is almost never used long-term — yet it is the single drop that can save an eye in acute angle closure. And one is the first genuinely new mechanism in decades. Learn what each does to aqueous humour, and the whole cabinet starts to make sense.
A 68-year-old man with open-angle glaucoma comes back three months after his ophthalmologist added a third drop, brimonidine, to control a pressure that latanoprost and timolol together were not quite reaching. His pressure is finally at target. But he has a complaint: for the last few weeks both eyes have been red, itchy and a little swollen along the lids, and he feels oddly sleepy in the afternoons. On the slit lamp there are fine bumps — follicles — lining the inner surface of his lower lids: a follicular conjunctivitis. Nothing is infected. This is an allergy to the drop itself, the commonest reason brimonidine gets stopped, and the daytime drowsiness is the same drug reaching his brain. In one patient you meet both faces of the alpha-2 agonist: a useful pressure-lowering add-on, and a medicine the eye and the body slowly tire of.
A quick reminder: pressure is a plumbing balance
Intraocular pressure is set by a tap and two drains. The ciliary body secretes aqueous humour — the tap. It leaves the eye by two routes — the drains. The conventional (trabecular) route runs through the trabecular meshwork into Schlemm's canal, and it carries the great majority of outflow. The unconventional (uveoscleral) route seeps out through the ciliary muscle. Every glaucoma drug lowers intraocular pressure (IOP) by turning down the tap, opening a drain, or both — this framework is laid out in full in the Glaucoma-overview chapter. Prostaglandin analogues open the uveoscleral drain; beta-blockers and carbonic-anhydrase inhibitors turn down the tap. The three classes here each attack the balance in their own way, and one of them does two things at once.
Alpha-2 agonists: the dual-action add-on
Brimonidine and apraclonidine stimulate alpha-2 adrenergic receptors on the ciliary body. These drugs belong to the same pharmacological family as clonidine, the central alpha-2 agonist met in the Autonomic Nervous System section — which is exactly why they can make a patient drowsy. In the eye their action is dual. Stimulating alpha-2 receptors on the ciliary body reduces aqueous production (turning down the tap), and it also increases uveoscleral outflow (opening the unconventional drain). Reducing inflow and boosting outflow at the same time is an unusually efficient pair of moves, and there is a further, more contested claim: a possible neuroprotective effect on the retinal ganglion cells, which some evidence suggests may protect the optic nerve independently of pressure. That claim is not settled, and pressure-lowering remains the reason the drops are prescribed.
Clinically they are used mostly as an add-on, layered onto a prostaglandin and a beta-blocker when a single agent is not enough. Apraclonidine in particular has a special short-term job: blunting the sharp IOP spike that can follow laser procedures such as laser trabeculoplasty or peripheral iridotomy, given around the time of the laser to prevent a dangerous post-laser pressure rise. What limits the alpha-2 agonists long-term is not the eye pressure — it is tolerance of the drop. Ocular allergy is common: the follicular conjunctivitis seen in the opening scene, with red, itchy, swollen lids, develops in a substantial minority of chronic users and is the usual reason the drug is abandoned. Systemically, because a meaningful fraction is absorbed and the molecule reaches the brain, patients can get drowsiness, dry mouth and low blood pressure (hypotension) — the familiar central alpha-2 signature.
The single most important safety fact about brimonidine is not an eye problem — it is a brain one, in the very young. In infants and small children the drug crosses the immature blood–brain barrier readily and can cause profound CNS depression: apnoea, bradycardia, hypotension, hypothermia and unresponsiveness. Brimonidine is therefore contraindicated in infants and young children, and a swallowed bottle is a genuine paediatric emergency. Whenever you see an alpha-2 agonist near a baby, think of clonidine — the same central depression, the same danger.
- Alpha-2 agonists (brimonidine, apraclonidine) stimulate alpha-2 receptors on the ciliary body.
- Dual action: reduce aqueous production AND increase uveoscleral outflow.
- Mainly an add-on; apraclonidine also blunts the IOP spike after laser procedures.
- Ocular allergy / follicular conjunctivitis is common and often limits long-term use.
- Systemic: drowsiness, dry mouth, hypotension — the clonidine family signature.
- Contraindicated in infants/young children: crosses the BBB → apnoea, bradycardia, CNS depression.
Cholinergic miotics: pilocarpine and the trabecular pull
Pilocarpine is a direct muscarinic agonist — it acts on the eye's cholinergic muscles. It works on the same muscarinic receptors covered in the Autonomic Nervous System section, and it targets two muscles the parasympathetic system controls. First, it contracts the ciliary muscle. Because that muscle's tendons insert into the trabecular meshwork, its contraction physically pulls the meshwork open, widening the trabecular pores and increasing conventional outflow — the drain is stretched wide. Second, it contracts the iris sphincter, producing miosis (a small pupil). That second action is the one that makes pilocarpine special: constricting the pupil pulls the peripheral iris taut and away from the drainage angle, physically unblocking an angle that a bunched-up iris was closing. This is precisely why the drug is central to angle closure — a link explored in the Acute angle-closure chapter — and it is the mirror image of the mydriatics discussed in the Pupil/mydriatics chapter, which dilate the pupil and can crowd a narrow angle shut.
Picture the drainage angle as a bath drain, and the peripheral iris as a loose curtain hanging beside it. When the pupil is wide, the curtain bunches up over the drain and blocks it — that is angle closure. Pilocarpine pulls the curtain tight and hauls it clear of the drain. At the same time, the ciliary muscle it contracts acts like a hand tugging on guy-ropes stitched into the drain's mesh cover, stretching the mesh open so water runs out faster. One drug, two mechanical pulls — the curtain off the drain, and the mesh stretched wide.
So why is a drug this clever now rarely used for chronic glaucoma? Because the same muscle contractions that lower pressure also make the eye uncomfortable and the vision worse. The persistent miosis leaves the pupil small, so vision is dim and blurred, especially at night or in low light. Contracting the ciliary muscle causes a dull brow ache from ciliary spasm, worst in the first weeks. That spasm also drives the eye into accommodation, producing a myopic shift — blurred distance vision that fluctuates. And by pulling on the retina through the vitreous base, miotics carry a small but real risk of retinal detachment, particularly in already-myopic eyes. Modern once-daily prostaglandins do the same pressure-lowering job with a fraction of the burden, so pilocarpine has been pushed to the margins of chronic care.
Pilocarpine's marginal role in chronic glaucoma hides its one indispensable use: acute angle-closure glaucoma. Here an eye is red, rock-hard and painfully high-pressure because the iris has slammed the drainage angle shut. Once the pressure has been brought down enough for the iris sphincter to respond (very high pressure can paralyse it), a drop of pilocarpine constricts the pupil, pulls the iris off the angle and helps reopen the drain — buying time until a definitive laser iridotomy makes a permanent bypass. The drug that is too troublesome for everyday use is the one you reach for in the emergency.
Rho-kinase inhibitors: the first drain-directed new class in decades
For years every new glaucoma drop worked on the tap or on the uveoscleral drain, and none directly targeted the trabecular meshwork itself — the site where most glaucomatous outflow resistance actually lives. Rho-kinase (ROCK) inhibitors changed that. Netarsudil is the leading agent. By inhibiting Rho-kinase, the drug relaxes the smooth-muscle-like tone of the trabecular meshwork, softening the diseased mesh so aqueous drains through it more easily, and it also lowers episcleral venous pressure — the back-pressure the aqueous must overcome to leave the eye. Both effects push in the same direction: more conventional outflow. Its side effects are mostly cosmetic and local rather than dangerous: conjunctival hyperaemia (red eye, the commonest and the usual reason for stopping), corneal verticillata (a harmless whorl-like deposit in the cornea, visible on examination and reversible), and small subconjunctival haemorrhages that look alarming but are benign.
Fixed combinations: stacking by mechanism
Most glaucoma is controlled by starting one drop and adding others until the pressure reaches target. Because each class works by a different mechanism, their effects add up: turn down the tap with a beta-blocker or a carbonic-anhydrase inhibitor, open the uveoscleral drain with a prostaglandin, open the trabecular drain with a Rho-kinase inhibitor, and pull on both drain and tap with an alpha-2 agonist. Combining two drugs that share a mechanism gains little; combining across mechanisms gains a lot. To keep regimens simple and improve adherence, many pairs come as fixed-combination drops in a single bottle — timolol paired with dorzolamide, brimonidine or a prostaglandin, and netarsudil paired with latanoprost. Fewer bottles and fewer instillations mean patients actually take the medicine, which for a silent, symptomless disease is half the battle. The guiding principle is simple: never stack two drops from the same class — layer classes that attack the pressure balance from different sides.
Alpha-2 agonists — brimonidine (Alphagan), apraclonidine (Iopidine): reduce production + increase uveoscleral outflow; watch for ocular allergy and the infant contraindication. Cholinergic miotics — pilocarpine: contracts ciliary muscle (opens trabecular meshwork) and iris sphincter (miosis, pulls iris off the angle); niche in chronic disease, indispensable in acute angle closure. Rho-kinase inhibitors — netarsudil (Rhopressa/Rhokiinsa): relax the trabecular meshwork + lower episcleral venous pressure; conjunctival hyperaemia and corneal verticillata are the signature findings. Fixed combos such as netarsudil–latanoprost bottle two mechanisms together.
- Pilocarpine (muscarinic agonist): ciliary muscle contraction opens the trabecular meshwork → more conventional outflow.
- Its miosis pulls the peripheral iris off the drainage angle — the key to acute angle closure.
- Miotic side effects: dim/night vision, brow ache from ciliary spasm, myopic shift, risk of retinal detachment.
- Netarsudil (Rho-kinase inhibitor): relaxes the trabecular meshwork + lowers episcleral venous pressure.
- Rho-kinase side effects: conjunctival hyperaemia, corneal verticillata, small subconjunctival haemorrhages.
- Fixed combinations stack classes by mechanism to lower IOP and improve adherence — never duplicate a class.
- Prescribing brimonidine anywhere near an infant or toddler — it crosses the blood–brain barrier and can cause apnoea, bradycardia and CNS depression; it is contraindicated in the very young.
- Forgetting pilocarpine in acute angle closure — or, conversely, giving a mydriatic to a narrow-angle eye and precipitating an attack.
- Mistaking brimonidine's follicular conjunctivitis or netarsudil's conjunctival hyperaemia and corneal verticillata for infection and treating with antibiotics — they are drug effects that resolve on stopping.
A 3-month-old infant is brought to the emergency department after accidentally swallowing a few drops from a parent's glaucoma bottle. He is now floppy, unresponsive, with a slow heart rate and shallow breathing. Which drop is the most likely culprit?
- Alpha-2 agonists (brimonidine, apraclonidine) have a dual action — reduce aqueous production AND increase uveoscleral outflow; useful add-ons and for post-laser IOP spikes.
- Their limits are ocular allergy (follicular conjunctivitis), systemic drowsiness/dry mouth/hypotension, and an absolute contraindication in infants (BBB → apnoea, bradycardia, CNS depression).
- Pilocarpine (muscarinic agonist) opens the trabecular meshwork via ciliary-muscle contraction and pulls the iris off the angle via miosis — little used chronically (dim vision, brow ache, myopic shift, detachment risk) but key in acute angle closure.
- Netarsudil (Rho-kinase inhibitor) relaxes the trabecular meshwork and lowers episcleral venous pressure (hyperaemia, corneal verticillata); fixed combinations stack classes by mechanism.
- Kanski's Clinical Ophthalmology: A Systematic Approach — Glaucoma: medical management.
- Bartlett & Jaanus. Clinical Ocular Pharmacology — Adrenergic agonists, cholinergic agents, and Rho-kinase inhibitors.
- American Academy of Ophthalmology, Basic and Clinical Science Course (BCSC) — Glaucoma.
- Rang & Dale's Pharmacology — The autonomic nervous system and ocular pharmacology.
- British National Formulary (BNF) — Drugs used in glaucoma.
- European Glaucoma Society, Terminology and Guidelines for Glaucoma.
