Mydriasis
From Wikipedia, the free encyclopedia
An abnormally dilated pupil. | |
ICD-10 | H57.0 |
ICD-9 | 379.43 |
OMIM | 159420 159410 106240 |
DiseasesDB | 8603 |
Mydriasis is an excessive dilation of the pupil due to disease or drugs. Although the pupil will normally dilate in the dark, it is usually quite constricted in the light. A mydriatic pupil will remain excessively large, even in a bright environment. It is occasionally referred to as a "blown pupil."
Constriction of the pupil is called miosis.
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[edit] Mechanism
There are two types of muscle that control the size of the iris: circular muscle and radial muscle. The former is innervated by the parasympathetic nervous system, the latter by the sympathetic nervous system. Sympathetic stimulation of α1 adrenergic receptors causes the contraction of the radial muscle, and subsequent dilation of the iris. Conversely, parasympathetic stimulation causes contraction of the circular muscle and constriction of the iris.
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 overactivity of the sympathetic nervous system (SNS).
[edit] Causes
[edit] Pathological
The parasympathetic nervous supply which causes constriction of the pupil, or miosis, is supplied by cranial nerve III, the oculomotor nerve. Damage to this nerve typically manifests itself as mydriasis, because the sympathetic supply to the pupil which causes mydriasis remains unaffected, and therefore unopposed.
[edit] Drugs
Atropine blocks muscarinic acetylcholine receptors. Acetylcholine (ACh) is the neurotransmitter of the parasympathetic nervous system and blocking its action means the pupil cannot constrict.
Cocaine inhibits the reuptake of noradrenaline 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. Many other drugs such as amphetamines and psychedelic drugs (LSD, psychedelic mushrooms, mescaline, MDMA) are also known to cause mydriasis. The mydriasis caused by cocaine is used as a diagnostic test for Horner's Syndrome, in which the pupil is not dilated by cocaine.
Antihistamines and tricyclic antidepressants may cause mydriasis.
[edit] Mydriatics
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). The effect of administration of a mydriatic is or was sometimes called "drops in eyes".