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The appearance of a fixed dark spot in the visual field (scotoma) is a symptom found in a wide variety of pathologies, both related and unrelated to the eyesight. This is usually a very alarming finding for patients and you must consult an ophthalmologist as soon as possible, since it can be a sign of a serious illness that may, on occasions, have irreversible and incapacitating effects on the eyes.
An endless list of illnesses are associated with scotomas and a differential diagnosis will be given depending on the location of the scotoma and other factors, too.
When the scotoma appears in the centre of the visual field, we suspect a pathology of the retina, in the macula in particular, the most common being age-associated macular degeneration (AMD), both the atrophic and the exudative types (with a quicker and more aggressive development). It is very common and potentially serious, but when detected and treated early, patients can still have relatively good eyesight. Other causes of central scotomas can be macular holes, macular oedema, etc.
When the patient presents with a peripheral scotoma, we are probably looking at optic nerve problems (neuropathies) with an inflammatory or vascular cause or even the first sign of glaucoma, a very prevalent disease in our field, where highly chronic eye tension causes gradual, painless and irreversible damage of the optic nerve.
Brain lesions affecting the visual field may cause permanent scotomas, as opposed to those caused by ocular migraines which bring on a temporary scotoma (self-limited) called a “scintillating scotoma”. We must not forget about vascular diseases of the retina, (for example, venous obstruction or arterial occlusion) or retinal detachment which can be causes of peripheral scotomas.