Magnetic stimulation of the peripheral nervous system: local versus generalized disorders

Magnetic stimulation was used to excite lumbar and cervical roots, enabling the measurement of proximal motor conduction besides the routine measurement of distal conduction velocity (CV). Latencies to abductor pollicis brevis (APB) and tibial anterior (TA) on both sides were used to estimate peripheral conduction velocity using body height; in addition, the uniformity of conduction was determined using the ratio of lumbar and cervical latencies (L/C ratio). In 124 controls the CV showed a clear inverse relation with age. After correction for this factor a high correlation between left and right CV remained. The L/C ratio--indicating the uniformity of conduction--was plotted against the CV, which made it possible to view both generalized and focal slowing in one diagram. An application of this approach is shown in patients with generalized slowing (demyelinating polyneuropathy), focal slowing (lumbar stenosis) and multifocal slowing (acute and chronic inflammatory demyelinating polyneuropathy). Most disorders could be accurately deduced from their place in the diagram. A special variable (distance between left and right data) was introduced to characterize the special case of multifocal slowing. It is concluded that this method easily provides an idea of global and/or focal peripheral conduction slowing and that it might be useful besides routine EMG as an additional tool.