More generally, mydriasis also refers to the natural dilation of pupils, for instance in low light conditions or under sympathetic stimulation.

Mydriasis is frequently induced by drugs for certain ophthalmic examinations and procedures, particularly those requiring visual access to the retina.

Sympathetic stimulation of the adrenergic receptors causes the contraction of the radial muscle and subsequent dilation of the pupil.

It usually involves either a disruption of the parasympathetic nerve supply to the eye (which normally constricts the pupil) or overactivity of the sympathetic nervous system (SNS).

Pupil diameter also increases in reaction to cognitive tasks requiring memory and attention, and this phenomenon is used as an indicator of mental activation (‘arousal’) in psychophysiological experiments.

Multiple central nervous system disorders e.g. epilepsy, stroke, and impending brain herniation are known to lead to temporal mydriasis as well.