Mydriasis is an abnormal dilatation of the pupil, either unilateral or bilateral. Causes include:
Oculoparasympathetic paresis, from lesions at the EdingerWestphal nucleus or anywhere along the course of the oculomotor (III) nerve (usually unilateral)
Tonic enlargement of the pupil (Holmes-Adie pupil, usually unilateral)
Sympathomimetic drugs, e.g., adrenaline (usually bilateral) Later stages of central cephalic herniation
If only one pupil appears large (anisocoria), it is important to distinguish mydriasis from contralateral miosis, when a different differential will apply (e.g., Horner’s syndrome).
Kawasaki A. Approach to the patient with abnormal pupils. In: Biller J (ed.). Practical neurology (2nd edition). Philadelphia: Lippincott Williams & Wilkins, 2002: 135-146