Unmyelinated nerve fibers in sural nerve in pure autonomic failure.

We examined sural nerve biopsy specimens from 7 patients with pure autonomic failure (PAF). The mean unmyelinated nerve fiber density in these patients was 40% less than in age-matched controls. Increased numbers of clusters of collagen pockets not containing unmyelinated axons were the most prominent finding in PAF. This appears to reflect recent dropout of a group of sympathetic efferents and suggests grouping of unmyelinated fibers by modality at the level of the sural nerve trunk.