Mydriasis is an excessive dilation of the pupil due to disease, trauma or drugs. Normally, the pupil dilates in the dark and constricts in the light. A mydriatic pupil will remain excessively large, even in a bright environment. Sometimes colloquially referred to as a "blown pupil."
The opposite, constriction of the pupil, is called miosis.
The mechanism of mydriasis depends on the agent being used. It usually involves either a disruption of the parasympathetic nerve supply to the eye (which causes contraction of the pupil), or over activity of the sympathetic nervous system (SNS).
In cases of head injury or orbit trauma (eye injury), the iris sphincter (the muscle responsible for closing the pupil) or the nerves controlling it can be damaged, reducing or eliminating reactivity to light.
Cocaine inhibits the reuptake of noradrenaline (norepinephrine) within a nerve synapse. When a solution of cocaine is dropped into the eye, noradrenaline is no longer reabsorbed by neurons, and its levels increase. Noradrenaline, the neurotransmitter for the SNS, causes dilation of the pupil. Mydriasis is used as a diagnostic test for Horner's Syndrome, in which it is initiated using a similar mechanism, though cocaine is not usually used in this procedure. Many other drugs such as amphetamines and psychedelic drugs (LSD, psychedelic mushrooms, mescaline, and MDMA) are also known to cause mydriasis.
Opiate withdrawal can cause dilated pupils in some people. Opiates normally lead to miosis or the constriction of the pupils.
A mydriatic is an agent which induces dilation of the pupil. Drugs such as tropicamide are used in medicine to permit examination of the retina and other deep structures of the eye, and also to reduce painful ciliary muscle spasm (see cycloplegia). One effect of administration of a mydriatic is intolerance to bright light.
Safety and efficacy of bilateral topical application of rocuronium bromide for mydriasis in European kestrels (Falco tinnunculus).(Original Studies)(Report)
Mar 01, 2012; Abstract: The avian iris is mainly composed of striated muscle fibers, and the topical parasympatholytic drugs used in mammals to...