Mydriasis (aka blown pupil) is dilation of the pupil, usually defined as when having a non-physiological cause, but can be physiological.

• Physiological, i.e. due to pupillary reflex
• Non-pathophysiological causes include:
  • Disease
  • Trauma
  • Use of drugs, including:
    • Anticholinergics, such as Tropicamide
    • Alpha agonists, which are sympathetic drugs, such as Phenylephrine (i.e. mimics actions of epinephrine/adrenaline)

• 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 fibers of the iris which increases the pupillary aperture is referred to as a mydriasis
• More generally, mydriasis also refers to the natural dilation of pupils, e.g. in low light conditions or under sympathetic stimulation
• A fixed, unilateral mydriasis can be a Sx of raised intracranial pressure