Miosis, or myosis (), is excessive constriction of the pupil.[ Farlex medical dictionary citing:
]
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Miller-Keane Encyclopedia & Dictionary of Medicine, Nursing, and Allied Health, 7th ed.
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Saunders Comprehensive Veterinary Dictionary, 3rd ed.
[ Farlex medical dictionary citing: Millodot: Dictionary of Optometry and Visual Science, 7th ed.][ Farlex medical dictionary citing: Mosby's Medical Dictionary, 8th ed.] The opposite condition, mydriasis, is the dilation of the pupil. Anisocoria is the condition of one pupil being more dilated than the other.
Causes
Age
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Senile miosis (a reduction in the size of a person's pupil in old age)
Diseases
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Ehlers Danlos Syndrome
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Horner's syndrome
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Hemorrhage into pons (intracranial hemorrhage)
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Hereditary disorders
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Cluster headaches with ptosis
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Iridocyclitis
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Fatal familial insomnia
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Aphakia
Drugs
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such as fentanyl, morphine, heroin and methadone (the notable exception being pethidine)
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Products containing nicotine such as cigarettes, chewing tobacco or gum
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such as clonidine, naphazoline, oxymetazoline and tetrahydrozoline
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Antipsychotics, including risperidone, haloperidol, chlorpromazine, olanzapine, quetiapine
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Cholinergic agents such as acetylcholine
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Acetylcholinesterase inhibitors
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Serotonin antagonists, such as ondansetron (an anti-emetic) known by its brand name Zofran/ Emiston in BD
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Some cancer chemotherapy drugs, including camptothecin derivatives
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Mirtazapine, a noradrenergic and specific serotonergic antidepressant (NaSSA)
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Some MAO inhibitor
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Pilocarpine eye drops and all other parasympathomimetics
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In some rare cases, when exposed to mustard gas
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Organophosphates
Physiology of the photomotor reflex
Light entering the eye strikes three different photoreceptors in the
retina: the familiar
Rod cell and
Cone cell 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 photosensitive ganglion cells have multiple functions including acute suppression of the hormone
melatonin, entrainment of the body's
and regulation of the size of the pupil.
The retinal photoceptors convert light stimuli into electric impulses. Nerves involved in the resizing of the pupil connect to the Pretectum of the high Mesencephalon, 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 Motor neuron axons run along both the left and right . 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 iris sphincter muscle, producing miosis.
See also
External links
