Miosis, or myosis, is excessive constriction of the pupil. The term is from Ancient Greek μύειν mūein, "to close the eyes". The opposite condition, mydriasis, is the dilation of the pupil. Anisocoria is the condition of one pupil being more dilated than the other.
|Miosis caused by high doses of opiates. Patient also shows ptosis of both eyelids and an inattentive look at the camera, a sign of altered level of consciousness caused by the sedative effect of the drug.|
- Senile miosis (a reduction in the size of a person's pupil in old age)
- Horner's syndrome
- Hemorrhage into pons (intracranial hemorrhage)
- Hereditary disorders
- Cluster Headaches with ptosis
- Fatal familial insomnia
- Opioids such as fentanyl, morphine, heroin and methadone (the notable exception being pethidine)
- Products containing nicotine such as cigarettes, chewing tobacco or gum.
- Imidazolines such as clonidine, naphazoline, oxymetazoline and tetrahydrozoline
- Antipsychotics, including risperidone, haloperidol, chlorpromazine, olanzapine, quetiapine
- Cholinergic agents such as acetylcholine
- Acetylcholinesterase inhibitors
- Serotonin antagonists, such as Ondansetron (an anti-emetic) known by its brand name Zofran
- Some cancer chemotherapy drugs, including camptothecin derivatives
- Mirtazapine, a noradrenergic and specific serotonergic antidepressant (NaSSA)
- Some MAO Inhibitors.
- Pilocarpine eye drops and all other parasympathomimetics
- In some rare cases, when exposed to mustard gas
Physiology of the photomotor reflexEdit
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 photosensitive ganglion cells have multiple functions including 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 involved in 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 iris sphincter muscle, producing miosis.
- Farlex medical dictionary citing:
- Miller-Keane Encyclopedia & Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition.
- Saunders Comprehensive Veterinary Dictionary, 3 ed.
- Seidel HM, Ball JW, Dains JE, Benedict GW (2006-03-29). Mosby's Guide to Physical Examination. Mosby. ISBN 978-0-323-03573-6.
- Farlex medical dictionary citing: Millodot: Dictionary of Optometry and Visual Science, 7th edition.
- Farlex medical dictionary citing: Mosby's Medical Dictionary, 8th edition.
- Hou RH, Scaife J, Freeman C, Langley RW, Szabadi E, Bradshaw CM (June 2006). "Relationship between sedation and pupillary function: comparison of diazepam and diphenhydramine". British Journal of Clinical Pharmacology. 61 (6): 752–60. doi:10.1111/j.1365-2125.2006.02632.x. PMC 1885114. PMID 16722841.