Everything about Mydriasis totally explained
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ICD9 = |
DiseasesDB = 8603 |
OMIM = 159420 |
OMIM_mult = |
Image = Dilated_pupils_2006.JPG |
Caption = An abnormally dilated pupil.
}}
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.
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 pupil. Conversely, parasympathetic stimulation cause 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 over activity of the
sympathetic nervous system (SNS).
Causes
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.
Traumatic
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.
Drugs
Atropine blocks
muscarinic acetylcholine receptors.
Acetylcholine (ACh) is the
neurotransmitter of the
parasympathetic nervous system and blocking its action means the pupil can't constrict.
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's initiated using a similar mechanism, though cocaine isn't 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.
Antihistamines and
tricyclic antidepressants may cause mydriasis.
Mydriatic drops
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.
Physiological Response Indicating Interest
Pupillary response may also indicate interest in the subject of attention or sexual stimulation.
Further Information
Get more info on 'Mydriasis'.
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