Diseases in this age period have a diversity of manifestations, yet each disease tends to have a certain characteristic pattern of neurologic expression, as though the pathogenetic mechanism were acting more selectively on particular systems of neurons. Such affinities between the disease process and certain anatomic structures raise the question of pathoclisis, that is, specific vulnerability of particular neuronal systems to certain morbid agents. Stated another way, for each disease there is a common and relatively stereotyped clinical syndrome and a small number of variants; conversely, certain other symptoms and syndromes are rarely observed with a given disease. At the same time, however, it is clear that more than one disease may cause the same syndrome.
In deference to these principles, the diseases in this section are grouped according to their most common mode of clinical expression, as follows: 1. The progressive cerebellar ataxias of childhood and adolescence 2. The familial polymyoclonias and epilepsies 3. Extrapyramidal syndromes of parkinsonian type 4. The syndrome of dystonia and generalized choreoathetosis 5. The syndrome of bilateral hemiplegia, cerebral blindness and deafness, and other manifestations of focal cerebral disorder 6. Strokes in association with inherited metabolic diseases 7. Metabolic polyneuropathies 8. Personality changes and behavioral disturbances as manifestations of inherited metabolic diseases
It is advantageous to be familiar with these groupings. Like the age of onset and the distinctions between gray and white matter diseases of earlier onset, this scheme facilitates clinical diagnosis. One word of caution: It is a mistake to assume that the diseases in these categories affect one and only one particular part of the nervous system or to assume that they are exclusively neurologic. Once the biochemical and genetic abnormalities are discovered, they are usually found to implicate cells of certain nonneurologic tissues as well; whether or not the effects of such involvement become symptomatic is often a quantitative matter. Also, one encounters mixed neurologic syndromes in which tremor, myoclonus, cerebellar ataxia, seizures, and choreoathetosis are present in various combinations; it is then difficult to decide whether a movement disorder is of one type or another.
The Progressive Cerebellar Ataxias of Late Childhood and Adolescence
In the preceding section, it was pointed out that there is a large group of diseases, some with a known metabolic basis, in which an acute, episodic, or chronic cerebellar ataxia becomes manifest in early childhood. Here the discussion of the cerebellar ataxias is continued, with reference to those forms that begin in late childhood and adolescence. In these later age periods, the number of ataxias of proven metabolic type diminishes markedly. Most of them, of chronic progressive type, are part of the late-onset lipid storage diseases. Of the other cerebellar ataxias of late childhood and adolescence, only the Bassen-Kornzweig acanthocytosis, late-onset GM2 gangliosidosis, Refsum disease, ataxia-telangiectasia, and a genetic fault in vitamin E metabolism fall into the category of truly metabolic disease. Refsum disease is so clearly a polyneuropathy (cerebellar features only in exceptional cases) that it is presented in Chap. 43. Ataxia-telangiectasia is usually encountered in late childhood, but the ataxia may begin as early as the second year of life; therefore it has been described in the preceding section with the ataxias of early childhood.