Miosis

Not to be confused with meiosis, mitosis, or myositis.

Miosis (or myosis) is constriction of the pupil of the eye to less than or equal to two milimeters. This is a normal response to an increase in light but can also be associated with certain pathological conditions, microwave radiation exposure, and certain drugs, especially opioids.

The opposite, mydriasis, is the dilation of the pupil.

Miosis
Classification and external resources
File:Miosis.jpg Slight miosis of the left eye (there is also concurrent partial ptosis on the left, most likely part of Horner's syndrome)
ICD-10	H57.0
ICD-9	379.42
DiseasesDB	8243
MeSH	D015877

Physiology of the photomotor reflex

Light entering the eye strikes three different photoreceptors in the retina: the familiar rods and cones used in image-forming and the more newly discovered photosensitive ganglion cells. The ganglion cells give information about ambient light levels, and react sluggishly compared to the rods and cones. Signals from ganglion cells have three functions: acute suppression of the hormone melatonin, entrainment of the body's circadian rhythms and regulation of the size of the pupil.

The retinal photoceptors convert light stimuli into electric impulses. Nerves pertaining to the resizing of the pupil connect to the pretectal nucleus of the high midbrain, bypassing the lateral geniculate nucleus and the primary visual cortex. From the pretectal nucleus neurons send axons to neurons of the Edinger-Westphal nucleus whose visceromotor axons run along both the left and right oculomotor nerves. Visceromotor nerve axons (which constitute a portion of cranial nerve III, along with the somatomotor portion derived from the Edinger-Westphal nucleus) synapse on ciliary ganglion neurons, whose parasympathetic axons innervate the constrictor muscle of the iris, producing miosis. This occurs because sympathetic activity from the ciliary ganglion is lost thus parasympathetics are not inhibited. Image

Causes

Diseases

• Horner syndrome (a set of abnormalities in the nervous supply of the face due to damage to the sympathetic nervous system).
• Hemorrhage into pons (intracranial hemorrhage)
• Cluster Headaches with ptosis
• iridocyclitis

Drugs

• Opioids such as codeine, fentanyl, morphine, heroin and methadone
• Antipsychotics, including haloperidol, thorazine, olanzapine, quetiapine and others
• [cholinergic] agents such as acetylcholine
• Some cancer chemotherapy drugs, including camptothecin derivatives
• Mirtazapine, a noradrenergic and specific serotonergic antidepressant (NaSSA)
• Trazodone
• Some MAO Inhibitors.
• In some rare cases, when exposed to mustard gas.
• Organophosphates

Miotics

A miotic substance causes the constriction of the pupil of the eye (or miosis). It is the opposite of a mydriatic substance, which causes dilation of the pupil.

External links

• FP Notebook
• -1254817782 at GPnotebook

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