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An important segment of neurologic medicine, and one that is seen with great frequency in general hospitals, are disorders in which a global disturbance of cerebral function (encephalopathy) results from failure of some other organ system—heart and circulation, lungs and respiration, kidneys, liver, pancreas, and the endocrine glands. Unlike the diseases considered in Chap. 37, in which a genetic abnormality affects the metabolic functions of many organs and tissues including the brain, the cerebral disorders discussed in this chapter are strictly secondary to derangements of the visceral organs themselves. They stand at the interface of internal medicine and neurology.
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Relationships of this type, between an acquired disease of some thoracic, abdominal, or endocrine organ and the brain, have rather interesting implications. In the first place, recognition of the neurologic syndrome may be a guide to the diagnosis of the systemic disease; indeed, the neurologic symptoms may be more informative and significant than the symptoms referable to the organ primarily involved. Moreover, these encephalopathies are often reversible if the systemic dysfunction is brought under control. Neurologists must therefore have an understanding of the underlying medical disorder, for this may provide the means of controlling the neurologic part of the disease. In other words, the therapy for what appears to be a nervous system disease lies squarely in the field of internal medicine—a clear reason why every neurologist should be well trained in internal medicine. Of more theoretical importance, the investigation of the acquired metabolic diseases provides new insights into the chemistry and pathology of the brain. Each visceral disease affects the brain in a somewhat different way and, because the pathogenic mechanism is not completely understood in any of them, the study of these metabolic diseases promises rich rewards to the scientist.
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Table 40-1 lists the main acquired metabolic diseases of the nervous system according to their most common modes of clinical expression. Not included are the diseases caused by nutritional deficiencies and those caused by exogenous drugs and toxins, which can be considered metabolic in the broad sense; these are discussed in the following chapters.
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