Simultaneous impairment of vision in the two eyes, with central or centrocecal scotomas, is often caused by nutritional deficiency. This occurrence is observed most often in the chronically alcoholic or malnourished patient. Impairment of visual acuity evolves over several days to weeks, and examination discloses bilateral, roughly symmetrical central or centrocecal scotomas, with the peripheral fields being intact. With appropriate treatment (nutritious diet and vitamins B) instituted soon after the onset of visual loss, recovery is possible. If treatment is delayed, patients are left with varying degrees of permanent defect in central vision and pallor of the temporal portions of the optic discs. This disorder has been referred to as “tobacco-alcohol amblyopia,” the implication being that it is caused by the toxic effects of tobacco or alcohol or both. In fact, the problem is more likely one of nutritional deficiency and is more properly designated as deficiency amblyopia or nutritional optic neuropathy (see Chap. 41). The same disorder may be seen under conditions of severe dietary deprivation (see Chap. 41) and in patients with vitamin B12 deficiency.
A subacute optic neuropathy of possible toxic origin was described in Jamaican natives. It was characterized by bilaterally symmetrical central visual loss and had additional features of nerve deafness, ataxia, and spasticity in some cases. A similar condition is described periodically in other Caribbean countries, two decades ago in Cuba, where an optic neuropathy of epidemic proportions was associated with a sensory polyneuropathy. A nutritional etiology, possibly contributed to by tobacco use (putatively cigars in the Cuban epidemic), was the likely cause of these outbreaks (see Sadun et al and The Cuba Neuropathy Field Investigation Team report). A putative role of exposure to cyanide, either from smoking or consumption of cassava, has been a feature of some of these epidemics.
Impairment of vision because of methanol intoxication is abrupt in onset and characterized by large symmetrical central scotomas as well as symptoms of acidosis. Treatment is directed mainly to correction of the acidosis and possibly, the administration of fomepizole. The same may occur with ethylene glycol ingestion. In contrast, the subacute development of central field defects is attributable to toxins and other prescribed therapeutic agents, notably ethambutol, linezolid, isoniazid, streptomycin, chloramphenicol, methotrexate, and chlorpropamide. Infliximab and other TNF-alpha blocking medications can occasionally precipitate optic neuritis and other demyelinating syndromes of the central or peripheral nervous system. The main drugs reported to have a toxic effect on the optic nerves are listed in Table 12-3 and have been catalogued more extensively by Grant.
Hereditary Abnormalities of the Optic Nerve
Several optic neuropathies have a recognized genetic etiology. As the number of identified pathogenic genetic variations increases, it becomes more challenging to match specific phenotypes and genotypes. Leber hereditary optic neuropathy, a maternally inherited mitochondrial disorder, is an infrequent but important cause of blindness that usually presents in young adults. It may simulate the more common inflammatory optic neuropathies that also cause a relatively abrupt onset of visual loss, and in some cases also shows some degree of recovery (see “Hereditary Optic Atrophy of Leber” in Chap. 36). The visual field defect typically takes the form of a severe cecocentral scotoma.