scotoma

   Also known as negative scotoma. The term scotoma comes from the Greek noun skotos (darkness). It is used to denote an area or island of loss or impairment of vision, surrounded by a field of normal or relatively well-preserved vision. In accordance with the degree of loss of vision, scotomata may be divided into relative and absolute scotomata. The group of relative scotomata is subdivided into shallow and deep scotomata. In accordance with their localization within the field of vision, scotomata are commonly divided into central and peripheral (or paracentral) sco-tomata. When central scotomata extend towards the area of the blind spot, they are referred to as centro-cecal scotomata. When central scotomata have a ring shape, leaving both central vision and peripheral vision intact, they are referred to as peri-central scotomata. When peripheral scotomata appear as an extension of the blind spot, they are referred to as Seidel's scotomata. When peripheral scotomata have a ring shape, they are referred to as annular scotomata or ring scotomata. For cases of * achromatopsia that confine themselves to an island-shaped region of the visual field, the term * colour scotoma is used. Migraine attacks are often preceded by a * scintillating scotoma, which typically takes the form of a ball of light within the central field of vision, developing into a shimmering arc of white or coloured lights. This arc may go on to assume the shape of a * fortification spectrum. The retina's * blind spot is sometimes referred to as a physiological scotoma.

   References

   Lauritzen, M. (1994). Pathophysiology of the migraine aura: The spreading depression theory. Brain, 117, 199-210.

   Sacks, O. (1992). Migraine. Revisedandexpanded. New York, NY: Vintage Books.