pupildilation

Pupillary dilation, or mydriasis, is the enlargement of the pupil, the opening in the center of the iris. The pupil size is determined by two smooth muscles: the sphincter pupillae, which constricts the pupil under parasympathetic control, and the dilator pupillae, which widens it under sympathetic control. In dim light or during sympathetic activation, the pupil dilates to allow more light to reach the retina; in bright light, constriction reduces light entry.

Causes of dilation include normal physiological adaptation to low light, pharmacologic effects of certain eye drops and systemic medications, and various neurological or ocular conditions that affect the autonomic pathways. Common pharmacologic causes involve topical mydriatics used in eye examinations, such as phenylephrine (an alpha-adrenergic agonist) and tropicamide or cyclopentolate (muscarinic antagonists). Pathologic dilation can occur with conditions such as third nerve palsy, Adie’s tonic pupil, or diffuse brain injury; Horner syndrome typically causes pupil constriction rather than dilation.

Clinical assessment often involves evaluating pupil size and reactivity to light and accommodation, and comparing pupils under different illumination to identify anisocoria. A dilated, fixed pupil can be an urgent sign of intracranial mass effect or severe optic-nerve or oculomotor impairment, requiring prompt evaluation.

Medical uses include routine ophthalmologic examination and diagnostic procedures that require unobstructed visualization of the retina and optic nerve. Risks of pharmacologic dilation include photophobia, blurred vision, and potential for systemic effects or precipitating angle-closure glaucoma in susceptible individuals.