Evolution of motor and sensory deficits in amyotrophic lateral sclerosis estimated by neurophysiological techniques

Although amyotrophic lateral sclerosis is a degenerative disease of the upp er and lower motor neurons, there is evidence that the disease can affect o ther systems, including the sensory system. On the other hand, within the m otor neuron pool there is possibly a predilection of the degenerative proce ss for the motor neurons fibers with the fastest conduction velocity (MNFCV ). We studied these two aspects of the disease in a group of 50 patients by prospectively assessing several sensory indices and by studying the select ivity of the spinal motor neuron loss. At baseline, nerve conduction studie s and somatosensory evoked potentials showed abnormal slowing in the periph eral and central sensory pathways. Thermal thresholds for heating were elev ated but were normal for cooling. In more than 60% of the patients at least one of the sensory tests studied was abnormal. However, except for a signi ficant decrease in the amplitude of the sensory nerve action potentials of the sural nerves, these afferent dysfunctions were not progressive over the follow-up period of 6 months, in contrast to the marked deterioration in m otor functions. Three different statistical models were applied to evaluate the presence of demyelination, selective loss of MNFCV, or the purely rand om degeneration of fast- and slow-conducting motor neurons. These data indi cate a selective loss of the MNFCV and suggest that subclinical abnormaliti es of the sensory system in ALS are often present but almost nonprogressive . Furthermore, the amyotrophic lateral sclerosis disease process seems pref erentially to affect MNFCV.