Mydriasis

Mydriasis
Other names	Blown pupil[1]
Dilated pupils caused by mydriatic drops instilled for a dilated fundus examination
Pronunciation	/mɪˈdraɪ.əsɪs, maɪ-/[2]
Specialty	Ophthalmology, neurology

Mydriasis is the

Fixed, unilateral mydriasis could be a symptom of raised intracranial pressure. The opposite, constriction of the pupil, is referred to as miosis. Both mydriasis and miosis can be physiological. Anisocoria is the condition of one pupil being more dilated than the other.

Causes

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There are two types of

The mechanism of mydriasis depends on the agent being used. It usually involves either a disruption of the

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.[5]

Drugs

A mydriatic is an agent that induces

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.
• 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.^[6]
• hallucinogens. Similarly, selective serotonin reuptake inhibitors
  can cause mydriasis.
• Dissociatives such as dextromethorphan (an SSRI and sigma-1 agonist).
• Certain
  GHB
  .
• Adrenergic agonists, such as phenylephrine and cyclomydril.^[7] Adrenergic agonists may be used if strong mydriasis is needed in surgery.^[8] 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 mild to moderate mydriasis.^{[citation needed]}

Long term effects of drugs can also cause mydriasis, for example opioid withdrawal.

Autonomic neuropathy

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