Diagnostic value of recruitment order in routine EMG

In this report a new technique of recording, employed for the first time in routine clinical electromyography (EMG) is described. Conventional analysis is extended to the order of recruitment pattern, and relates the new electrophysiological features to current data about anatomical reorganization of all active motor units (MUs) and their functional effectiveness in neuromuscular disorders. This new concept was tested in four groups of patients (two different myopathic disorders), and (two neuropathies), which were compared to normal subjects. The data obtained confirmed that in normal subjects isometric muscle contraction from weak to maximal effort, was characterized by recruitment of MUs in an orderly pattern according to the "size principle". In polymyositis patients representing the myopathy model a nearly normal range of functioning MUs was found, but their sizes were reduced to a greater extent in the low threshold population. Irrespective of their increased firing rate and variability, the orderly recruitment pattern was still maintained. A different picture was seen in patients with Duchenne dystrophy where the range of functioning MUs and their sizes were reduced with the characteristic lack of high amplitude MUs, (probably high threshold). In the neurogenic groups the range of functioning MUs was reduced but increased in size, and the pattern of recruitment evidently became disorderly. We also found a difference in the manner of disorderly recruitment between motor neuron range disease (MND) and polyneuropathy patients. In the MND group the surviving MUs create very narrow and probably only high threshold MUs with a tendency to their synchronization seen evidently during maximal effort. In the peripheral neuropathy group, the functioning MUs represented wider range of very different sizes, and they were often recruited asynchronously during maximal effort, giving lower amplitudes and rather higher density of record not proportional to the number of MUs lost.