Miotics

Miotics constrict the pupil and ciliary muscle. Pilocarpine is the main agent and is the oldest glaucoma drug. It still has a role in acute angle closure, prevention of angle closure and in resource-limited care, but is now rarely first-line because of frequent dosing and visual side effects.

Mechanism. Pilocarpine is a muscarinic M3 receptor agonist. Contraction of the ciliary muscle pulls on the scleral spur and opens the trabecular meshwork, increasing conventional outflow; contraction of the iris sphincter constricts the pupil and pulls the iris away from the angle in angle closure. Carbachol has muscarinic and cholinesterase-inhibiting activity.

Pharmacokinetics. Topical onset 10 to 30 minutes, peak 1 to 2 hours, duration 4 to 8 hours, so dosing up to four times daily. Oral pilocarpine is absorbed well, with a half-life of about 1 hour.

• Pilocarpine 1, 2 and 4 percent drops, and 4 percent gel at night: the usual agent.
• Carbachol 0.01 percent intraocular: miosis during cataract surgery.
• Acetylcholine intraocular: miosis in surgery.
• Oral pilocarpine 5 mg: dry mouth in Sjogren syndrome and after head and neck radiation.
• Low-dose pilocarpine 1.25 percent: presbyopia, where available.

Regimens.

• Acute angle closure: after systemic pressure-lowering therapy has started and pressure begins to fall, pilocarpine 2 percent (1 percent in children and lightly pigmented irises) in the affected eye, repeated, and then one drop in the other eye as prophylaxis, until laser iridotomy.
• Chronic angle closure and prophylaxis before iridotomy: pilocarpine 1 or 2 percent three or four times daily.
• Open-angle glaucoma: 1 to 4 percent three or four times daily, as add-on only.
• Dry mouth: oral pilocarpine 5 mg three times daily with meals (maximum 30 mg daily).
• Presbyopia: 1.25 percent once or twice daily where licensed.

Selection. Reserve for angle closure, after laser iridotomy when residual narrow angles remain, aphakic or pseudophakic glaucoma in rare cases, and when no other agent is available. Not a first choice for open-angle glaucoma.

Contraindications. Uveitis, neovascular glaucoma, malignant (aqueous misdirection) glaucoma, acute iritis, and any condition where a small pupil is undesirable; asthma, severe chronic obstructive pulmonary disease, bradycardia and heart block (oral drug especially); peptic ulcer, urinary obstruction.

Effects. Brow ache and headache (marked in younger patients), blurred and dim vision, induced myopia, pupil constriction in cataract, ciliary spasm, retinal detachment (rare; examine the peripheral retina first), conjunctival and lid irritation, sweating, salivation, bronchospasm, bradycardia, nausea and diarrhoea.

Interactions. Reduces the effect of mydriatics and anticholinergics; additive effect with other cholinergic drugs and beta blockers (bradycardia).

Monitoring. Pressure, pupil, angle width and retina at follow-up; ask about night vision, brow ache and driving.

Special groups. Pregnancy and breastfeeding: use with care. Children: used in specialist hands for angle closure. Older adults: reduced night vision and falls risk; counsel on dim light. Region: pilocarpine is cheap and often the only miotic available, but four-times-daily dosing lowers adherence; use is mostly short term until laser or surgery.