Background: Stimulation of the median nerve in the palm may be a useful addition to the conventional method of stimulation above the carpal tunnel for assessing a lesion of the media nerve through the carpal tunnel. It may be particularly beneficial in cases where the median sensory nerve action potential (SNAP) cannot be obtained because of advanced sensory deficits or in cases of acute carpal tunnel syndrome (CTS). It may also lead to the adoption of new normal values for amplitude measurements of the media nerve compound muscle action potential(CMAP) recorded from the abductor pollicis brevis muscle (APB). Currently, normal values vary from as low as a few millivolts (mV) to as high as 20 mV. Purpose: This primary purpose of this study is to establish a technique for stimulating the motor branch of the median nerve in the palm. In addition, we will report normal values for latency and amplitude and calculate the motor nerve conduction velocity (MNCV) across the carpal tunnel in the wrist to palm segment. Hypothesis: Our hypothesis was that there would be no statistical significant difference in the amplitude of the CMAP recorded from the APB between the stimulation site of the median nerve above the carpal tunnel and at the palmer site. Sample and Method: Healthy subjects with no history of injury or disease that may have affected the median nerves were recruited for the study. Subject’s age ranged between 26-44 years old. A single supramaximal stimulus to the median nerve was applied at the wrist at a point 8 cm proximal to the recording electrode located over the anatomic center of the abductor pollicis brevis muscle. A second stimulation site on the palm was identified at the point where the tip of the ring finger flexes to contact the thenar crease. The thrid stimulation site was located 8 cm proximal from the palm site along the course of the median nerve. The reference electrode was placed on the distal phalanx of the thumb and the ground electrode was firmly positioned over the dorsum of the hand. Both hands were stimulated in a random order. Results: Paired t-test showed no statistical difference in the amplitude of the CMAP between the stimulations of the median nerve at the wrist and at the palm. There was no statistical significance difference in amplitude, latency and MNCV between right and left hand of the subjects. Test re-test reliability was also determined for the stimulation sites. Interclass correlations coefficient (ICC) values were 0.98 for amplitude at the wrist, 0.96 for amplitude at the palm, 0.8 for latency at the wrist, 0.67 for latency at the palm and 0.79 for MNCV. Discussion: Palmar stimulation of the recurrent motor branch of the median nerve can be easily and rapidly performed with little discomfort to the patient. Adding palmer stimulation to the conventional testing procedures may demonstrate cases of acute CTS that have only conduction block and minimal slowing of median motor nerve fibers. Additional studies will be necessary to determine the extent to which these data are similar for individuals with CTS. Conclusion: Adding the palmer stimulation to the typical median nerve assessment through the carpal tunnel can provide valuable information. The testing procedures are reliable and aid in providing a more detailed description of median nerve function through the carpal tunnel.
