MRI detection of peripheral nerve denervation and regeneration: correlation with electromyography and histology

Background: Acute nerve lesions have been shown to cause hyperintense signals on T2 weighted MRI of the nerve. An animal study using a defined nerve lesion was performed to sequentially study MRI changes in the nerve in comparison to electrophysiological alterations and nerve histology in nerve degeneration and regeneration. Methods: Rat sciatic nerve was compressed by a tight suture which was opened after one week. Serial EMG and motor nerve conduction studies were conducted on the first four days after lesion, on day seven and then in weekly intervals parallel to MRI scanning. On defined time points, sciatic nerves were taken for histology. Results: Nerve hyperintensity on T2 weighted MRI was seen in the thigh already at 24h after denervation. Progression up to the lower leg level was present after 48h. Signal changes regressed after four weeks in the thigh, five weeks on the knee level and seven weeks in the lower leg. On EMG, spontaneous activity was found two days after lesion with a proximo-distal gradient which decreased from day 14 in the lower leg and day 35 in the foot. First voluntary activity on EMG occurred about 2 weeks before regression of MRI nerve hyperintensity. Compound muscle action potentials (CMAP) in the foot muscles reappeared seven weeks after lesion, increasing to 80% of the previous values after 13 weeks. On histology, axonal degeneration in the acute phase and, later, nerve oedema were found to parallel MRI changes. Conclusion: MRI is an excellent diagnostic tool to detect acute axonal nerve lesions, showing changes at the time as EMG. In addition, regeneration can be followed by MRI scanning which is in parallel with regression of histological alterations.