A ( surrounded by a field of normal or relatively well-preserved
Vision .
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, as may be caused by
Cortical Spreading Depression .]]
Every normal
Mammal ian eye has a scotoma in its field of vision, usually termed its
Blind Spot . The presence of this normal scotoma does not intrude into
Consciousness because it is very small, but it can be demonstrated to oneself by the simplest of clinical methods (such as the one in the
Blind Spot article).
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
Macula r 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 ."Possible Roles of Vertebrate Neuroglia in Potassium Dynamics, Spreading depression, and migraine", Gardner-Medwin, ''J. Exp. Biology'' (1981), 95, pages 111-127 (Figure 4). Less common, but important because sometimes reversible or curable by
Surgery , are scotomata due to
Tumor s 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.
