Paraspinal muscle evoked cerebral potentials in patients with unilateral low back pain

Cerebral somatosensory evoked potentials (SEPs) were elicited by magnetic stimulation of paraspinal muscles unilaterally at the L2 and L5 levels in 20 healthy subjects and 16 patients with low back pain and unilateral muscle spasm. A magnetic coil with a mean diameter of 4.7 cm was placed tangentially to the skin. The stimulus strength was sufficient to induce a visible muscle twitch without producing muscle contraction in the legs. The potentials recorded over the scalp consisted of several components (P30, N40, P55, N70, and P90) and were elicited in all subjects. In both healthy and patient subjects, paraspinal muscle evoked potentials were readily elicited. Vibration applied to paraspinal muscles, as well as voluntary contraction of paraspinal muscles, was associated with attenuation of the evoked potentials. This finding suggests that muscle spindle receptors provide the afferent input responsible for the early components of the magnetically evoked cerebral potentials. In patients with unilateral muscle spasm, the amplitudes of P30-N40, N40-P50, and P50-N70 were decreased significantly on the affected side when compared with values on stimulation of the unaffected side, as well as those obtained from control subjects. The cerebral evoked potentials returned to normal amplitude when the muscle spasm subsided following a period of time and after the application of spinal manipulative therapy. The technique has potential for quantitative evaluation of muscle spasm in low back pain.