Fine structural evaluation of altered Schmidt-Lanterman incisures in human sural nerve biopsies.

Fine structural alterations of Schmidt-Lanterman incisures (SLI) were investigated in a series of 242 unselected sural nerve biopsies that had been examined for diagnostic purposes. The series included cases with Friedreich's ataxia, HSAN I, HMSN I-III, HMSN VI, tomaculous neuropathy, metachromatic leukodystrophy, ceroidlipofuscinosis, dysproteinemic neuropathies, and myotonic dystrophy, in addition to several neuropathies less-specifically classified as either of a predominantly demyelinating, axonal, or neuronal type. The following classification of SLI alterations is proposed: (A) abnormal inclusions; (B) changes in shape and dimension; and (C) modes of disintegration. Abnormal inclusions comprised membranous whorls, uniform and pleomorphous lysosome-like bodies, and accumulation of granular substances at the site of the major dense line, or granular deposits at the site of the intraperiod line of the myelin sheath. Variations of incisural shape and dimension included folding, dilatation, and pocket formation (compartmentalization). Disintegration at incisures comprised a fine, vesicular and a gross, vacuolar type. Various combinations of these changes were observed. The most frequent change consisted of membranous whorls, detected in SLI of 89 biopsies. They were most prominent in chloroquine neuropathy where they occurred in SLI as well as in the adaxonal and abaxonal cytoplasm of Schwann cells. Compartmentalization of the myelin sheath at incisures associated with formation of myelin loops was a frequent feature in myotonic dystrophy. It is concluded, that changes of incisural ultrastructure are sensitive indicators of human neuropathies offering clues to the type of the underlying pathomechanism.