Visual Symptomatology Symptoms Attributable to Optic Nerve Fiber Damage

Visual loss is a common mode of presentation. Although it can be assumed that the majority of tumors that present this way are nonfunctioning adenomas, all adenomas, particularly prolactinomas in men, frequently giant somatotrophs and rarely, Cushing's tumors and thyrotrophs can present with visual loss. A common symptom of chiasmal compression is the patent's tendency to bump into objects on the side of the temporal field loss, either unilaterally or bilaterally. When central vision is affected early, usually through direct compression of the optic nerve particularly where there is a 'post- fixed' chiasm, patients complain of blurred vision often with scotomatous central field loss. On occasion, severe visual loss is discovered in one eye when the "good" eye is covered, leading to a false report of sudden visual loss. That the loss has been more gradual is usually suggested by the presence of optic atrophy, which takes a month or more to develop.

It is noteworthy that patients do not complain that the abnormal visual field is black but rather that it is blank: they are unaware of visual stimuli. A positive complaint of blackness is more likely to accompany retinal disease and not lesions of the optic nerve fibers at any level.

Several unusual symptoms may be encountered as a consequence of bitemporal hemianopias. Postfixational blindness occurs when there is a bitemporal field defect that precisely splits the middle of the field (1,2). In these circumstances, when a patient converges on a relatively close object, more distant objects fall into the blind hemifields behind the object of attention and therefore are not seen. Symptomatically this is manifested, for instance, by difficulty in cutting finger nails—focusing on the scissors results in disappearance of the nails—or occasionally in driving when focusing on near objects, such as the driving instruments, causes important distant objects, such as traffic lights or pedestrians, to disappear.

The presence of postfixational blindness is readily demonstrated by confrontation. The patient is asked to focus on one of two objects held 30 cm from the face. The other object is then moved toward the examiner. When it enters the blind fields, the patient no longer sees it.

A second group of symptoms derives from the phenomenon of retinal slip. The relationship between the fields contributed by each eye is normally preserved by a neurophysiologic mechanism that depends on the stimulation of corresponding points in the homonymous hemifields. With a bitemporal field loss this is impossible. The patient's visual perception depends on the function of two nasal fields that are not locked together. Slippage can thus occur. In the vertical plane, the slippage results in a tendency when reading to drop to the next lower line. In the horizontal plane, slippage can result in words either running into each other or expanding: door may become dor or dooor. Slippage can have most unfortunate consequences, as when a patient who was a bank clerk was reprimanded for making errors on several orders of magnitude because he lost or added several zeros to clients' balance sheets without being aware of what was happening.

Unusually, an abnormal increase in separation of the nasal fields can be reported as diplopia. Clearly its cause must be distinguished from defects in eye movements.

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