Mydriasis Mydriasis Mydriasis () is the dilation of the pupil, usually having a non-physiological cause, or sometimes a physiological pupillary response. Non-physiological causes of mydriasis include disease, trauma, or the use of drugs. Normally, as part of the pupillary light reflex, the pupil dilates in the dark and constricts in the light to respectively improve vividity at night and to protect the retina from sunlight damage during the day. A "mydriatic" pupil will remain excessively large even in a bright environment. The excitation of the radial fibres of the iris which increases the pupillary aperture is referred to as a mydriasis.
Mydriasis instance, the adrenoceptor agonist phenylephrine (e.g. Cyclomydril) is used if strong mydriasis is needed for a surgical intervention. Alternatively, anticholinergics (in particular, antimuscarinics) such as atropine, hyoscyamine, and scopolamine antagonize the muscarinic acetylcholine receptors in the eye. Blocking acetylcholine receptors, reduces the pupilary muscles' ability to constrict and causes dilation (which is critical in eye surgery procedures such as cataract surgery which require uninterrupted access to the inner eye via the pupilary aperture, thus requiring that the eye be both paralyzed and anesthetized before the procedure can go ahead. The antimuscarinic, tropicamide, may be used as a mydriastic agent during