Mydriasis

Mydriasis is the dilation of the pupil, a condition in which the pupil becomes larger than normal for the surrounding lighting. In normal conditions the pupil constricts in bright light and dilates in dim light; mydriasis can be physiologic (due to darkness or age-related changes) or pathologic (due to disease or drugs) and may be unilateral or bilateral. A persistently dilated, nonreactive pupil is a red flag for serious neurologic or ocular conditions.

Physiology and mechanism: Pupil size is controlled by the iris sphincter muscle (parasympathetic, constriction) and the iris dilator muscle (sympathetic, dilation). Mydriasis occurs with increased sympathetic activity or blockade of parasympathetic input. Disruptions along the sympathetic pathways (from the hypothalamus to the eye) or damage to the parasympathetic pathways (such as CN III lesions) can produce abnormal dilation. Pharmacologic agents that stimulate the dilator or block the sphincter readily cause mydriasis.

Causes: Physiologic mydriasis occurs in low ambient light. Pharmacologic mydriasis is common after use of anticholinergic drugs (eg, tropicamide, atropine) or sympathomimetics (eg, phenylephrine). Pathologic causes include third nerve palsy or compressive lesions, brainstem or midbrain pathology, ocular trauma, acute angle-closure glaucoma (often with a fixed, mid-dilated pupil), Adie syndrome (tonic pupil with poor light response), and other neurologic disorders. Bilateral mydriasis without ptosis or visual loss may indicate a systemic cause or drug effect.

Evaluation and management: Determine if the dilation is reactive to light, accompanying ptosis, ophthalmoplegia, pain, or visual disturbance. Urgent evaluation is warranted for a fixed dilated pupil with neurologic signs or suspected acute angle-closure glaucoma. Management targets the underlying cause; some pharmacologic dilations resolve as drugs wear off, while neurologic or structural causes require specialist assessment and treatment.