Morphology and morphometry of peripheral nerves after spinal cord injury

The morphology of peripheral nerves sampled at the time of nerve transfer surgery was directly assessed in 13 participants (mean age at injury 27±8 years and motor level of C4 or below) within 15 months of their spinal cord injury (SCI). Donor nerves included teres minor (TM), posterior axillary (PAX), brachialis (BRAC), extensor carpi radialis brevis (ECRB) and supinator (SUP) nerves. Recipient nerves included those to triceps (TRI), posterior interosseus (PIN) and anterior interosseus (AIN). Fresh nerve samples were fixed in glutaraldehyde, processed and embedded in Araldite Epon. Multiple semi-thin sections from each nerve sample were cut, mounted onto slides, stained with methylene blue or toluidine blue, and imaged by light microscopy using an Olympus BX60 microscope. Glutaraldehyde-fixed nerve specimens were post-fixed with osmium tetroxide, stained with uranyl acetate and alkaline lead citrate and embedded in Epon/araldite. Ultrathin sections were cut and examined using a JEOL 1400 Series transmission electron microscope. Qualitative data were recorded from the nerve specimens according to various morphologic criteria. A comprehensive quantitative morphometric analysis of axon, fibre and myelin areas and diameters and g-ratios was also undertaken.

Over two-thirds of both donor and recipient nerves showed abnormalities that differed in appearance and aetiology. These included myelin thickening and folding (tomacula), demyelination, reduced density of large, myelinated axons, oedematous endoneurium, evidence of compression, and thickened epineurium and perineurium. Morphometric analysis showed differences in axon density and myelination in the sampled nerves, with significant nerve fibre loss in teres minor and posterior axillary nerves, and increased myelin area in the other nerves. The g-ratio was significantly reduced by the degree of myelin thickening. The morphological findings show evidence of widespread peripheral nerve pathology after spinal cord injury.