Diagnostic value of turns-amplitude analysis in neuromuscular disease

We describe a new method of turns-amplitude analysis of electromyography interference patterns, modified from the original St»lberg's method. Ten samples were obtained from one muscle at maximum voluntary contraction. As indices of amplitude (A), the average of the three highest and three lowest values from the ten samples was calculated. Similar indices were calculated for turns (T) and the T/A ratio. A total of six indices were evaluated. Criteria for abnormalities in these six indices were determined through a control study of healthy volunteers. The test-retest reliability of the six indices was confirmed in the extensor digitorum communis muscle, and the Pearson product-moment correlation coefficients ranged from 0.767 to 0.907 (P<0.01). The clinical usefulness of this method was evaluated in 21 patients with mild or moderate neuromuscular disorders. Abnormalities in the T/A indices were found in 77.8%–100% of the patients, a higher incidence of abnormality than for the other indices. Two of the three criteria were found to be abnormal in 80% of patients with myopathy and in 78.6% of those with neuropathy. The diagnostic sensitivity of our method in these patients was 79.2% (19 of 24 studies [in 3 patients with myopathy, two muscles were examined]). The clinical usefulness of our method of analysis appears to be similar to that in other reports of turns-amplitude analyses.