Congenital hypomyelination polyneuropathy. Pathological findings compared with polyneuropathies starting later in life.

Biopsies from patients with congenital hypomyelination polyneuropathy (Group I) and with late infantile (Group II) and juvenile (Group III) forms of hereditary motor and sensory neuropathy (HMSN) type III were compared, using morphometric methods and ultrastructural analysis. In congenital polyneuropathies (Group I), myelin sheaths were practically absent and onion bulbs, essentially made of multiple laminae of double layered basement membrane, surrounded every axon in the size range of normal myelinated axons. The number of these axons was markedly reduced. Serial sectioning of an isolated fibre showed that the territory of successive clusters of Schwann cell nuclei was considerably reduced when compared with the biopsies in Groups II and III and with normal controls. Fibres without myelin surrounded by multiple layers of basement membrane represented between 25 and 50 per cent of the entire population of fibres in the size range of myelinated fibres in Group II and were practically absent in Group III. The number of "myelinated' fibres (that is, fibres with myelin, and amyelinate or demyelinated fibres) was normal in Groups II and III. Although there is no indication that congenital hypomyelination onion bulb polyneuropathy is a separate entity, it can be considered as a subtype of Dejerine-Sottas disease (HMSN type III). In this disease there is a gradient of severity both in clinical expression and in the disorder of Schwann cells. In the severe congenital form, all Schwann cells are affected and are incapable of forming myelin. The diminution of the number of nerve fibres in the "myelinated' fibre size range, whether or not related to a prenatal involvement of Schwann cells, is another expression of the gravity of this form. The proportion of amyelinate fibres, i.e. with Schwann cells incapable of forming myelin, becomes less in the more benign late infantile and juvenile forms of the disease in which a process of demyelination and remyelination takes place. The literature on the congenital neuropathies, a heterogeneous assembly of diseases, is reviewed.