Uncompacted inner myelin lamellae in inherited tendency to pressure palsy

Nerves in patients with inherited tendency to pressure palsy (ITPP) are susceptible to degrees of traction or compression which in nonaffected persons do not induce neuropathic symptoms or deficits, conduction block of fibers, or electro-myographic changes characteristic of the disorder. Two observations suggest a wide-spread asymptomatic abnormality of nerves: 1) low conduction velocity of clinically unaffected nerves, and 2) focal thickenings (tomacula) on teased myelinated fibers of clinically unaffected sural nerves. Sural nerves from five patients and five healthy subjects were assessed for morphologic abnormality in ITPP that might account for the susceptibility of nerves to compression. Teased nerve fibers showed a higher frequency of segmental demyelination or remyelination, or both (p < 0.003). The mean frequency of fibers showing focal myelin thickenings was 57 ± 10% in ITPP and 0% in controls. In electron micrographs, regions of uncompacted myelin lamellae, usually affecting the innermost lamellae and extending for a variable distance aver-aging 9 ± 4 μm were seen in 11 ± 4% of fibers in ITPP. None were found in the control nerves. The finding of uncompacted myelin lamellae may suggest an abnormality of myelin composition or of interaction of Schwann cells and axons accounting for the increased susceptibility to pressure palsy, tomaculous formation, or demyelination. From electron microscopic evaluation of serial skip sections we infer that myelin of tomaculae is in continuity with intemodal myelin and is reduplicated (full-thickness or cleaved layers are longitudinally or circumferentially folded-back on themselves).