Intraoperative nerve action potential recording electrodes (above). Positive response obtained (arrowheads, below).

When the affected nerve is exposed during surgery, very sensitive electrical testing can be performed, which helps guide the surgeon. This testing is not possible through the skin before the surgery.

An example is when it is uncertain if a partially scarred nerve has functional nerve fascicles regenerating through it, which just need more time to reach their target muscles. Should you cut and replace this nerve with a graft, thereby potentially restarting this slow regeneration process? Microscopic inspection is not enough.

We use direct stimulation and recording across the injured segment to estimate the number of functional nerve fascicles that have regenerated across the region of injury. This is called a nerve action potential (NAP). If absent, then the nerve segment is replaced with a graft. If present, then scar tissue is removed and the chance of recovery is good.

Other available intraoperative electrical tests include evoked EMG recording to monitor nearby functional nerve fascicles during tumor (schwannoma) removal, and sensory evoked potentials with stimulation of the proximal nerve roots just outside the spine to confirm that the spinal rootlets are at least partially attached to the spinal cord. This latter test is important during brachial plexus surgery.