Use of peripheral nerve action potentials for intraoperative monitoring

Following a physical injury to peripheral nerve, clinical evaluation and the use of conventional EMG/NCS is often unable to determine whether axons are crossing the site of injury before severe changes in distal tissues occur. The INAP recording identifies functional axons within neuromas before other signs of reinnervation have developed. In clinically complete lesions, recording an INAP across the injury indicates the presence of regenerating axons, and neurolysis of the encasing connective tissue is recommended. If an INAP is absent, resection of the dense neuroma is usually undertaken with end-to-end suture or graft. A present INAP indicates clinically significant regenerating axons even with large distances to target tissue. An absent INAP for injuries far from target tissue indicates a poor prognosis. In clinically incomplete lesions, INAP recording is of no value over the clinical examination and EMG/NCS. INAP can aid peripheral nerve tumor resection by identification of intact nerve fascicles. INAP responses are obtained by placing platinum-iridium bipolar stimulating electrodes proximal to the injury. The INAP is then recorded by distal electrodes. A standard EMG/NCS instrument with an isolated stimulation unit can be used with the appropriate gain and time base settings. Stimulus intensity required for a supramaximal response is usually less than 75 V at 0.05 msec duration. Frequency bandpass is similar to that for conventional EMG/NCS studies. Electrodes must elevate the nerve during the recording. Artifact from 60 Hz line frequency and stimulus are common problems.