Silent period measurement in the differentiation of central demyelination and axonal degeneration

The investigators recorded the silent period from the abductor pollicis brevis and gastrocnemius soleus muscles in normal subjects and in patients with multiple sclerosis and motor neuron disease. In the multiple sclerosis group the termination of the silent period recorded from the leg was significantly delayed, and the arm and leg silent period differences were increased threefold as compared with normal subjects. Silent period measurements in the motor neuron disease group were normal. The silent period probably participates in a "long loop reflex", and its delay in multiple sclerosis could therefore be due to a block in central conduction resulting from demyelination. This would not occur in motor neuron disease, in which axonal degeneration is the primary lesion. The techniques used are relatively simple and are a valuable means of differentiating central demyelination from axonal degeneration in man.