INTRODUCTION: Studies have shown that the chronic nerve compression (CNC) with compressive neuropathies induces changes in the structure and microvasculature of peripheral nerves. The notion that CNC induces segmental demyelination is poorly supported. Nerve teasing is a technique that neuropathologists have used to evaluate Schwann cell function and their ability to myelinate axons. We sought to use this rigorous technique to provide meticulous data about myelination and nerve injury.

METHODS: A nerve compression model was used with SD rats. A sterile one-inch silastic tube (ID=1.3mm) was atraumatically placed around right sciatic nerves. EMG/NCV studies were performed at the time of specimen harvest. Nerve samples, at one- or eight-month post-surgery, were fixed in gluteraldehyde, and osmium tetroxide. Under a surgical microscope, single myelinated axons were manually teased apart from the whole nerve using ultra-fine forceps. Internodal length (IL), axonal diameter (AD), and evaluation of myelin alterations were done digitally at both time points using ImagePro Express 4.5.

RESULTS: Consistent with previous data, there were no significant electrophysiological changes at the one-month time point. By eight-months, NCV consistently decreased to 65% of normal. CNC induces significant alterations in IL and myelin structure. Normal fibers exhibited thick healthy myelin without areas of demyelination with IL and AD of 1231 µm ± 107 and 5.45 µm ± 0.87, respectively. However, at one-month post surgery, the mean IL was reduced to 335 µm ± 96 with an AD of 5.26 µm ± 1.12. Each teased fiber exhibited segments of demyelination with remyelination and areas of normal healthy myelin. It was noted that demyelination began at the paranode and progressed into the internode. Axonal integrity was maintained in all fibers examined. At eight-months, fibers were not actively demyelinating. However, a reduction of IL to 397 µm ± 152 was noted in all segments of each fiber with an AD of 4.89 µm ± 1.03 .

CONCLUSIONS: This study confirms the hypothesis that CNC induces segmental demyelination. In addition to performing axonal counts and measuring myelin thickness, we recommend nerve teasing to provide invaluable information on Schwann cell role in response to nerve injury. CNC induces regions of segmental demyelination that were evident in teased fibers at one-month without signs of axonal death. As the naked healthy axon is exposed to the surrounding milieu, Schwann cells adhere and produce new, thinner, yet normal myelin. Our data supports that CNC induces segmental demyelination, and not Wallerian degeneration.