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.