Vrms/T quantitation: usefulness in patients with neuropathies

This study evaluates the sensitivity of root mean square voltage (Vrms)/turns (T) ratios in neuropathies. Data were recorded with concentric needle electrodes from the biceps brachii, first dorsal interosseous, tibialis anterior, and calf muscles. Recordings were analyzed from 35 normal subjects as well as 24 consecutive patients referred for evaluation of neuropathic disorders. Direct tension measurements were not required. Reasoning from studies in normal subjects in which Vrms/T values were related to measurements of relative tension, the data were recorded during strong muscle contractions of less than 80% of maximum tension as monitored by mean amplitude values. For all 5 muscles, mean Vrms/T values > or = 1.0 calculated from four separate needle sites were significantly more frequent in the patients than controls (p < 0.012-0.0001). There were 13 muscles in which relevant conduction studies were abnormal and Vrms/T > or = 1.0 were the only abnormality of the needle EMG examination. In addition, a Vrms/T of > or = 1.0 was present in 10/14 muscles in which abnormalities might have been expected but where other needle EMG parameters were unremarkable and relevant nerve conductions normal. This study indicates that Vrms/T measurements can add meaningfully to the electrodiagnostic evaluation of neurogenic dysfunction.