Do cerebral potentials to magnetic stimulation of paraspinal muscles reflect changes in palpable muscle spasm, low back pain, and activity scores?

Objective: Previous studies have shown that cortical-evoked potentials on m agnetic stimulation of muscles are influenced by muscle contraction, vibrat ion, and muscle spasm. This study was carried out to determine whether thes e potentials correlate with palpatory muscle spasm,patient symptoms, and di sability in patients with low back pain. Methods: A prospective observational study was performed on 13 subjects wit h a history of low back pain visiting an orthopedic hospital-based clinic. Patients were screened for serious pathologic conditions by an orthopedic s urgeon. The patients were then evaluated for the presence of muscle spasm b y one of the investigators who was blinded to the results of the evoked pot ential studies. Patients were asked to complete a low back pain visual anal ogue scale (VAS) and a Roland-Morris Activity Scale (RMAS). Cortical-evoked potentials were recorded with a magnetic stimulator placed over the lumbar paraspinal muscles with the patient in the prone position. The palpatory e xamination, VAS, RMAS, and the cortical potentials were repeated after 2 we eks of therapy commonly used to reduce muscle spasm. Results: The patients demonstrated a significant decrease in low back pain VAS and RMAS scores after treatment compared with before treatment. There w as a reduction in the amount of palpatory muscle spasm in 11 of 13 cases. T he cortical potentials before treatment were attenuated compared with previ ously reported controls and showed a significant increase before and after treatment in the amplitude of these potentials with multivariate analysis o f variance. There was significant correlation between the changes in cortic al potentials after treatment and the changes noted in paraspinal muscle sp asm and VAS and RMAS scores. Conclusions: This study confirms the previous report that the amplitude of cerebral-evoked potentials on magnetic stimulation of paraspinal muscles is depressed in the presence of palpable muscle spasm. The close correlation among these potentials, paraspinal muscle spasm, and clinical symptoms sugg ests that the measurement of muscle activity may be more important in the a ssessment of low back pain than is commonly accepted.