|Classification and external resources|
Example image showing small, deep central scotoma, as may be caused by age-related maculopathy
A scotoma (Greek σκότος/skótos, darkness; plural: scotomas or scotomata) is an area of partial alteration in the field of vision consisting of a partially diminished or entirely degenerated visual acuity that is 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 photoreceptor cells, 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. The absence of visual imagery from the blind spot does not intrude into consciousness 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 one's 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 5×7 degrees of visual angle.
Scotoma is also a symptom of retinal damage from exposure to high-powered lasers.
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.[contradictory] 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.
Common causes of scotomata include demyelinating disease such as multiple sclerosis (retrobulbar neuritis), damage to nerve fiber layer in the retina (seen as cotton wool spots) due to hypertension, 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 they are 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 bitemporal paracentral scotoma, and later, when the tumor enlarges, the scotomas extend out to the periphery to cause the characteristic bitemporal hemianopsia. 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 textbook bitemporal 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.
- Binasal hemianopsia
- Bitemporal hemianopsia
- Blind spot
- Cortical spreading depression
- Scintillating scotoma
- "Possible Roles of Vertebrate Neuroglia in Potassium Dynamics, Spreading depression, and migraine", Gardner-Medwin, J. Exp. Biology (1981), 95, pages 111-127 (Figure 4).