Tropicamide

Mechanism of Action
Tropicamide binds to the receptors in the muscles of the eye involved in controlling the pupil size and the shape of the lens (muscarinic receptor M4) and blocks them. Then, it acts by blocking the responses of the iris sphincter muscle to the iris and ciliary muscles to cholinergic stimulation, thus producing pupil dilation and cycloplegia.


Clinical Uses

Tropicamide is given to patient in form of eye drops to dilate the pupil and relax the lens so that eye examinations such as cycloplegic refraction and fundus examination of the eye. Tropicamide may also be used before and after eye surgery. The preparation (1% in 3ml) acts in 15-30 minutes, and the duration of activity is approximately 3-8 hours.


Ocular Effects

Tropicamide produces dilation of the pupil and paralysis of the ciliary muscle (cycloplegia) which prevents the eye from accommodating for near vision.

Some adverse ocular effects associated with the use of tropicamide include transient stinging, blurred vision, photophobia, superficial punctate keratitis and increased intra-ocular pressure.


Systemic Effects

Some adverse systemic effects associated with the use of tropicamide include dryness of the mouth, tachycardia (very fast heart rate), headache, allergic reactions, nausea, vomiting, pallor (paleness), central nervous system disturbances and muscle rigidity. Psychotic reactions, behavioral disturbances, and vasomotor or cardio-respiratory collapse in children have been reported with the use of tropicamide.


Indications and Contraindications

Indication: When mydriasis and cycloplegia of rapid onset and brief duration are required.

Contraindications: Patients with angle-closure glaucoma or narrow angles where mydriasis may precipitate attack of angle-closure glaucoma must not use tropicamide. Patients with allergies to tropicamide or any ingredient in the formulation must not use tropicamide.


Intra-Ocular Pressure (IOP)

Use of tropicamide can result in increase of IOP of users with open angle glaucoma. It is caused by a reduction in aqueous outflow, resulting in imbalance between aqueous drainage and production. Thus, aqueous accumulates in eye, increasing IOP.

The risk of inducing acute glaucoma following mydriasis with tropicamide alone is estimated to be about 0.3%, excluding those with history of glaucoma and shallow anterior chamber angles. This is because it is moderately selective for M4 receptors and the muscarinic receptors in the trabecular meshwork are mostly M2 and M3. Mydriasis with tropicamide alone is SAFE even in people with chronic glaucoma.