VISUAL-FIELD DEFECT

a blind spot (scotoma) or blind area within the normal bounds of vision. In most cases the blind spots or areas are persistent, but in some instances they may be temporary and shifting, as in the scotomata of migraine headache. Narrowing of the fields of vision may be due to syphilitic inflammation of the optic nerves or to glaucoma. In this type of defect the field of vision widens as the distance from the observed object increases. In hysteria another type of narrowed vision sometimes occurs, called tunnel vision, or shaft vision, in which the field does not widen with distance. Constriction of the visual field with enlargement of the natural blind spot at the centre of the field comes from papilledema, the abnormal collection of fluid in the optic disk. The optic disk is the point at which the optic nerve enters the eye; the small blind spot normally present in the centre of the visual field results from the absence of retinal visual cellsrods and conesin the optic disk. Blind spots in the interior of the visual fields can result from a number of other causes, including poisoning with wood alcohol or with quinine, diseases that attack the nerve sheaths, deficiency diseases, and atherosclerosis, a form of blood vessel disease in which fatty plaques form in the innermost layer of the vessels. Blindness in one-half of each of the visual fields is also encountered. This may be in corresponding halves of the fields (homonymous hemianopia) or in the inner or the outer halves (nasal or temporal hemianopia). Similarly, a quarter of each field may be involved. The extent and the location of the blind areas in the visual fields may be clues concerning the location of the lesion responsible. Bitemporal hemianopiablindness in the two outer halves of the visual fieldssuggests, for example, a lesion in the optic chiasma, the point at which the optic nerves from the two eyes meet and at which some of the nerve fibres from one retina cross over to the opposite side of the brain. A tumour of the pituitary may press upon the chiasma and have this effect.