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A scotoma (Greek: darkness; plural: "scotomas" or "scotomata") is an area or island of loss or impairment of visual acuity surrounded by a field of normal or relatively well-preserved vision.
Every normal mammalian eye has a scotoma in its field of vision, usually termed its blind spot. This is a location with no photoreceptors, where the retinal ganglion cell axons that comprise the optic nerve exit the retina. This location is called the optic disk. The blindspot does not intrude into consciousness because the corresponding visual field locations of the optic disks in the two eyes differ: The visual signals that are absent in one eye are sent to cortex by signals from the other eye.
The presence of the scotoma can be demonstrated subjectively by covering one eye, carefully holding fixation with the open eye, and placing an object(such as your thumb) in the lateral and horizontal visual field, about 15 degrees from fixation (see the blind spot article). The size of the monocular scotoma is surprisingly large - 5x7 deg of visual angle.
Additional recommended knowledge
Symptom-producing or pathological scotomata may be due to a wide range of disease processes, affecting either the retina (in particular its most sensitive portion, the macula) or the optic nerve itself. A pathological scotoma may involve any part of the visual field and may be of any shape or size. A scotoma may include and enlarge the normal blind spot. Even a small scotoma that happens to affect central or macular vision will produce a severe visual handicap, whereas a large scotoma in the more peripheral part of a visual field may go unnoticed by the bearer due to the normal reduced visual resolution in the peripheral visual field.
Common causes of scotomata include demyelinating disease such as multiple sclerosis (retrobulbar neuritis), toxic substances such as methyl alcohol, ethambutol and quinine, nutritional deficiencies, and vascular blockages either in the retina or in the optic nerve. Scintillating scotoma is a common visual aura in migraine. Less common, but important because sometimes reversible or curable by surgery, are scotomata due to tumors such as those arising from the pituitary gland, which may compress the optic nerve or interfere with its blood supply.
Rarely, scotomata are bilateral. One important variety of bilateral scotoma may occur when a pituitary tumour begins to compress the optic chiasm (as distinct from a single optic nerve) and produces a bi-temporal hemicentral scotomatous hemianopia. This type of visual field defect tends to be very eloquent symptom-wise but often evades early objective diagnosis, as it is more difficult to detect by cursory clinical examination than the classical or text-book bi-temporal peripheral hemianopia and may even elude sophisticated electronic modes of visual field assessment.
In a pregnant woman, scotomata can present as a symptom of severe preeclampsia, a form of pregnancy-induced hypertension.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Scotoma". A list of authors is available in Wikipedia.|