Uni- and bipolar surface recording of human nerve responses

Nerve conduction studies are of great clinical value in diagnosing nerve pathology and injury. In most neurophysiological laboratories nerve conduction recordings are routinely made with surface bipolar electrodes. More rarely, needle electrodes are used. A bipolarly recorded nerve action potential (NAP) is the difference between 2 unipolar NAPs recorded from each pole of a bipolar electrode and a remote reference. The delay between the 2 separated unipolar NAPs depends on the distance between the poles and the conduction velocity (CV). When the delay between the unipolar NAPs was increased (partly through simulation), the following changes of the bipolar NAP occurred: (1) total amplitude was maximal at a certain delay but decreased if the delay was shorter or longer, (2) latencies of the first positive and the first negative peaks increased slightly until their amplitudes reached maximal values, (3) a second negative peak of increasing latency appeared with longer delays, (4) latencies of later peaks increased linearly, and (5) total area increased nonlinearly. All parameters of the bipolarly recorded NAP (except the onset of the first positive phase) changed with increasing delay. The significance of this is that if NAPs are recorded with a bipolar electrode, standard values obtained for each nerve cannot be transferred between laboratories if different interelectrode (interpolar) distances are used. Furthermore, the assumption that a fixed interelectrode distance allows comparisons between reference values and patient data is incorrect.