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scotoma

Did you mean: scotoma, blind spot

 
Dictionary: sco·to·ma   (skə-tō') pronunciation
 
n., pl. -mas or -ma·ta (-mə-tə).

An area of diminished vision within the visual field.

[New Latin scotōma, from Late Latin, dim sight, from Greek skotōma, dizziness, from skotoun, to darken, from skotos, darkness.]

scotomatous sco·to'ma·tous adj.
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Medical Dictionary: sco·to·ma
 
(skə-tō')
n., pl. -mas or sco·to·ma·ta (-mə-tə).
  1. An area of diminished vision within the visual field.
  2. See blind spot (sense ).
 
Obscure Words: scotoma
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[fr. Gk skotoma, dimness of vision < skotos, darkness]  /skuh TOE muh/  
Path.  an obscuration of part of the visual field (also fig.)
 
Wikipedia: Scotoma
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Scotoma
Classification and external resources
ICD-10 H53.4, H53.1
ICD-9 368.4, 368.12
MeSH D012607

A scotoma (Greek for 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.

Example image showing normal field of vision.
Example image showing small, deep central scotoma, as may be caused by age-related maculopathy.
Example image showing a peripheral ring scotoma, as may be caused by retinitis pigmentosa.
Example of a scintillating scotoma, as may be caused by cortical spreading depression.

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 disc. When both eyes are open, visual signals that are absent in the blind spot of one eye are provided from the opposite visual cortex for the other eye, even when the other eye is closed. The absence of visual imagery from the blindspot does not intrude into consciousness with one eye closed, because the corresponding visual field locations of the optic discs in the two eyes differ.

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 degrees of visual angle.

The term scotoma is also used metaphorically in psychology to refer to an individual's inability to perceive personality traits in themselves that are obvious to others.

Contents

Presentation of pathological scotoma

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 because of the normal reduced Optical resolution in the peripheral visual field.

Causes

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.[1] 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 obvious to the person experiencing it, 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. Similarly, scotomata may develop as a result of the increased intracranial pressure that occurs in malignant hypertension.

See also

Detection

Types

References

  1. ^ "Possible Roles of Vertebrate Neuroglia in Potassium Dynamics, Spreading depression, and migraine", Gardner-Medwin, J. Exp. Biology (1981), 95, pages 111-127 (Figure 4).

 
 

Did you mean: scotoma, blind spot

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Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved.  Read more
Medical Dictionary. The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company Read more
Obscure Words. © 2008 by Michael A. Fischer http://home.comcast.net/~wwftd Read more
Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Scotoma" Read more