Confirmation of virtual unmyelinated fiber absence in hereditary sensory neuropathy type IV.

Sural nerves from two unrelated young boys were virtually without unmyelinated fibers (UFs). Small myelinated fibers (MFs) may also have been slightly reduced in number. Since no degeneration or regeneration is observed, UF absence is assumed to be congenital, due to either lack of neuron formation or to premature degeneration. The main clinical features of this inherited sensory neuropathy (previously identified by us as type IV) are the inherited nature and abnormality of nociception, of sweating, and of thermal regulation associated with mild mental retardation. Our findings confirm the congenital absence of UFs of cutaneous nerves in cases such as these and provide further evidence that this disorder has a different natural history and pathology than do the other three types of hereditary sensory neuropathy.