The CSF protein content may be mildly or moderately elevated over a period of years. We retain the nomenclature Charcot-Marie-Tooth (CMT) disease (type 1), which shows slowed nerve conduction as a consequence of a disorder of myelin. A distinctive feature of hereditary neuropathy is the uniformity of the electrophysiologic changes, for example, a similar degree of slowing of nerve conduction velocity in all the nerves, a feature that distinguishes this group from most acquired neuropathies. The distinction between the demyelinating and axonal types of inherited neuropathies is based on the motor nerve (typically ulnar or median nerve) conduction velocities in the arms, with slowing to velocities below 38 m/s defining the demyelinating category.
To the reader not immersed in neuromuscular diseases, the classification, nomenclature, and number of genes that give rise to this group of diseases is dizzying. One approaches the affected patient, however, in a more circumscribed way by noting that the dominantly inherited demyelinating types are designated CMT1 and the dominant axonal types, as Charcot Marie type 2 (CMT2). The group of myelinopathies with onset in infancy (also called Dejerine-Sottas disease or congenital hypomyelinating neuropathy), are subsumed under CMT3, whereas most recessively inherited neuropathies (both axonal and demyelinating) are termed CMT4. There are, in addition, forms with intermediate degrees of conduction slowing that are not easily classified. An older nomenclature uses the term hereditary motor-sensory neuropathies, or HMSNs, for the main CMT types. The hereditary sensory neuropathies (HSNs) and the hereditary sensory autonomic neuropathies (HSANs) are considered separate entities. This category includes a variety of metabolic neuropathies and types that do not fit into the CMT classification.
Recent genetic findings have in some ways simplified the matter of classification and have permitted the creation of a nosology that more or less parallels the clinical one. The systems in Tables 43-2 and 43-6 represent an attempt to conciliate the clinical and genetic data.
Of this large and varied group, only the sensorimotor Charcot-Marie-Tooth type is the one likely to be seen with any regularity by neurologists and general physicians. This group has become quite large as more individual mutations are discovered, but a small number accounts for the majority of cases. The commonly encountered varieties are indicated in italics in Table 43-6. The major forms of hereditary neuropathy are designated as subcategories of CMT disease, grouped by patterns of inheritance and described in more detail further on.
Inherited Polyneuropathies of Mixed Sensorimotor-Autonomic Types