Among nutritional disorders, those of the nervous system occupy a position of special interest and importance. The early studies of beriberi at the turn of the 20th century were largely responsible for the discovery of thiamine and consequently for the modern concept of diseases of nutritional deficiency. A series of notable achievements in the science of nutrition followed the discovery of vitamins. Despite such progress, diseases caused by nutritional deficiency, and particularly those of the nervous system, continue to represent a worldwide health problem of serious proportions. In communities where the diet consists mainly of highly milled rice there is still a significant incidence of beriberi. In some developing countries, deficiency diseases are endemic, the result of chronic dietary deprivation. And the ultimate effects on the nervous system of intermittent mass starvation, involving large portions of the African continent, remain an alarming medical and humanitarian crisis.
In addition to the occurrence in the developing world, it must be acknowledged that diseases of nutritional deficiency are not uncommon in the United States and other parts of the developed world. In addition to deprivation from poverty, other clinical circumstances in which nutritional deficiency can arise are alcoholism, dietary faddism, impaired absorption of dietary nutrients that occurs in conditions such as celiac sprue and pernicious anemia, and the wasting syndromes of cancer and HIV. In the United States, surgical excision of portions of the gastrointestinal tract for treatment of obesity has emerged has an important cause of nutritional deficiency. Finally, there are iatrogenic deficiencies induced by the use of vitamin antagonists or certain drugs, such as methotrexate or isonicotinic acid hydrazide (INH), which interferes with the enzymatic function of pyridoxine.
The term deficiency is used throughout this chapter in its strictest sense to designate disorders that result from the lack of an essential nutrient or nutrients in the diet or from a conditioning factor that increases the need for these nutrients. The most important of these are the vitamins, especially members of the B group—thiamine (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), pyridoxine (B6), biotin (B7), folic acid (B9), and cobalamin (B12). While some disorders can be attributed to a single vitamin deficiency, such as thiamine deficiency causing Wernicke disease and vitamin B12 deficiency causing subacute combined degeneration [SCD] of the spinal cord, other disorders are the result of multiple nutritional deficiencies. Characteristic of the nutritional diseases is the potential for involvement of both the central and peripheral nervous systems, an attribute shared only with certain metabolic disorders.
In many cases, vitamin deprivation occurs in the context of general undernutrition, and multisystem effects such as circulatory abnormalities and loss of subcutaneous fat and muscle bulk are usually associated. A total lack of vitamins, therefore, as occurs in starvation, is rarely associated with the classic deficiency syndromes of beriberi or pellagra. In other words, a certain amount of food is necessary to produce the disorders associated with a single vitamin deficiency. In a similar way, excessive intake of carbohydrates relative to the supply of thiamine favors the development of a thiamine-deficiency state. All deficiency diseases, including those of the nervous system, are influenced by factors such as exercise, growth, pregnancy, neoplasia, and systemic infection, which increase the need for essential nutrients, and by disorders of the liver and the gastrointestinal tract, which may interfere with the synthesis and absorption of these nutrients.