Amplitude-dependent slowing of conduction in amyotrophic lateral sclerosisand polyneuropathy

The mechanism of motor nerve conduction slowing in amyotrophic lateral scle rosis (ALS) is thought primarily to be loss of large, fast-conducting motor fibers; this is less certain in axonal polyneuropathy, We compared motor c onduction studies in 64 patients with axonal polyneuropathy with 72 patient s with ALS. Compound motor action potential amplitude, distal motor latency , and conduction velocity were converted to a percentage of the upper or lo wer limit of normal and then represented as a square root (SQRT) transforma tion, plotted with SORT amplitude as the independent variable and SORT late ncy or SORT conduction velocity as the dependent variables. Regression anal ysis of the lower extremity nerve data showed that prolongation of latency and slowing of velocity were amplitude-dependent and were virtually identic al in ALS and polyneuropathy. In the upper extremity, amplitude-dependent p rolongation of latency was similar in both groups, but amplitude-dependent slowing of velocity was seen in ALS and not in axonal polyneuropathy. Our d ata support the hypothesis that the major mechanism of slowing is similar i n both polyneuropathy and ALS and is the loss of large, fast-conducting fib ers. However, the presence of distal but not proximal slowing in the upper extremity of axonal polyneuropathy suggests that additional mechanisms may be contributory. (C) 1999 John Wiley & Sons, Inc.