Miostat (Carbachol)

Carbachol is a cholinergic agonist used in ophthalmology to induce pupil constriction and improve aqueous outflow.

• Intraocular surgery: induces rapid and sustained miosis, improving surgical field clarity, reducing the risk of iris trauma, and facilitating intra‑ocular lens placement.
• Glaucoma: used as an adjunctive miotic to increase trabecular outflow, thereby helping to lower intraocular pressure in patients with open‑angle or secondary glaucoma.
• Ophthalmic diagnostic procedures: produces controlled pupil constriction, allowing precise evaluation of the anterior chamber angle and lens positioning during gonioscopy or laser procedures.

• Carbachol acts as a direct cholinergic agonist that binds to muscarinic (M3) receptors on the sphincter pupillae and ciliary muscle in the eye. Activation of these receptors triggers intracellular calcium release, causing contraction of the sphincter muscle (producing miosis) and contraction of the ciliary muscle, which opens the trabecular meshwork and enhances aqueous humor outflow. The resulting pupil constriction also improves depth of focus for surgical procedures.

Carbachol may cause ocular and systemic reactions following intraocular administration.

Common Side Effects

• Blurred vision: temporary loss of visual acuity due to pupil constriction.
• Eye discomfort or pain: mild irritation at the site of administration.
• Redness of the conjunctiva: mild hyperemia.
• Increased tear production: transient lacrimation.

Serious or Rare Side Effects

• Acute intraocular pressure rise: may precipitate angle‑closure glaucoma.
• Allergic reaction: swelling, itching, or rash around the eye.
• Corneal edema: swelling of the cornea leading to visual disturbance.

Prior to using Carbachol eye preparation, clinicians should observe several safety considerations.

• Assess baseline intraocular pressure: ensure patient does not have uncontrolled glaucoma.
• Screen for hypersensitivity: avoid in individuals with known allergy to carbachol or similar compounds.
• Use caution in narrow‑angle glaucoma: risk of precipitating angle closure.
• Monitor for systemic cholinergic effects: such as bradycardia or bronchospasm, especially in vulnerable patients.
• Avoid concurrent use with other miotics unless directed: to prevent excessive miosis.
• Store at room temperature: keep in original container, protect from light and moisture.

Carbachol can interact with other ophthalmic or systemic agents, affecting its efficacy or safety.

Major Interactions (Avoid)

• Systemic cholinesterase inhibitors (e.g., physostigmine): additive cholinergic stimulation may cause severe systemic effects.
• Concurrent use of other miotic agents: may lead to excessive pupil constriction and ocular discomfort.

Moderate Interactions (Monitor Closely)

• Topical beta‑blocker eye drops: combined effect may overly lower intraocular pressure.
• Anticholinergic medications (e.g., atropine): may reduce the miotic response of carbachol.
• General anesthetic agents used during eye surgery: may potentiate ocular cholinergic activity.