caused by the psychedelic drug
LSD There are two types of
muscle that control the size of the
iris: the
iris sphincter, composed of circularly arranged muscle fibers, and the iris
dilator, composed of radially arranged muscle fibers. The sphincter is controlled by nerves of the
parasympathetic nervous system, and the dilator by the
sympathetic nervous system. Sympathetic stimulation of the
adrenergic receptors causes the
contraction of the
radial muscle and subsequent dilation of the pupil. Conversely, parasympathetic stimulation causes contraction of the circular muscle and constriction of the pupil. 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 normally constricts the pupil) or overactivity of the
sympathetic nervous system (SNS). Pupil diameter also increases in reaction to cognitive tasks requiring memory and attention, and this phenomenon is used as an indicator of mental activation ('arousal') in psychophysiological experiments.
Drugs A
mydriatic is an agent that 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. Mydriatics typically also have a
cycloplegic effect, reducing or paralyzing the
accommodation reflex which may also be used for certain ophthalmic examinations or treatments, such as reducing painful
ciliary muscle spasm. One effect of administration of a mydriatic is intolerance to bright light (
photophobia). Purposefully-induced mydriasis via mydriatics is also used as a
diagnostic test for
Horner's syndrome. Mydriasis can be induced via modulation of
adrenergic or
cholinergic signalling. Drugs that can cause mydriasis include: •
Stimulants (typically monoaminergics) such as
amphetamines,
cocaine,
MDMA, and
mephedrone. •
Anticholinergics such as
diphenhydramine,
atropine,
hyoscyamine, and
scopolamine antagonize the
muscarinic acetylcholine receptors in the eye. Blocking acetylcholine receptors reduces the pupillary 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 pupillary 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 surgery. •
Serotonergics such as
LSD,
psilocybin mushrooms,
mescaline and
2C-B. These drugs are typically
hallucinogens. Similarly,
selective serotonin reuptake inhibitors can cause mydriasis. •
Dissociatives such as
dextromethorphan (an SSRI and sigma-1 agonist). • Certain
GABAergic drugs, such as
phenibut and
GHB. •
Adrenergic agonists, such as
phenylephrine and
cyclomydril. Adrenergic agonists may be used if strong mydriasis is needed in surgery.
Norepinephrine is a hormone and neurotransmitter that regulates the involuntary muscles of the
autonomic nervous system, including dilation of the
pupil aperture via the muscles of the
iris. Hence adrenergic agonists mimic the activity of norepinephrine, which is how they induce mydriasis. Natural release of the hormone
oxytocin can cause the pupils to dilate to a greater degree to stimulus but perhaps not directly causative of mydriasis. Long term effects of drugs can also cause mydriasis, for example
opioid withdrawal. It is not clear whether Marijuana use causes mydriasis, a survey of several studies produced conflicting findings and suggest the mydriatic effect can be related to other factors occurring with cannabis use.
Autonomic neuropathy Parasympathetic fibers travel with
cranial nerve III, the
oculomotor nerve, to innervate the circular layer of muscle of the eye (sphincter pupillae). Damage to this nerve typically manifests itself as mydriasis, because the
sympathetic supply to the pupil, which causes mydriasis, remains unaffected, and therefore unopposed. Multiple
central nervous system disorders e.g.
epilepsy,
stroke, and impending
brain herniation are known to lead to temporal mydriasis as well. A brain catastrophe, or a rapidly increasing brain mass, can cause compression of the oculomotor nerve.
Trauma 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 the normal
pupillary light reflex. ==References==