Motor-evoked potentials elicited from human erector spinae muscles by transcranial magnetic stimulation

Study Design. The compound muscle action potentials elicited from the erector spinae muscles by transcranial magnetic stimulation was studied in 15 healthy adults. Objective. To describe the recording procedure for consistent compound muscle action potentials elicited from the human erector spinae by transcranial magnetic stimulation and to establish the normal latency ranges of the responses, at respective spinal levels, for the practical use of this test. Summary of Background Data. Although recording of the compound muscle action potentials from limb muscles after transcranial magnetic stimulation has been extensively studied, the use of the erector spinae as the target muscle has not, probable because of the difficulty of consistently evoking compound muscle action potentials. Methods. Compound muscle action potentials from the erector spinae muscles were recorded wit the subject prone during tonic voluntary background contraction of the back extensor muscles at approximately 20% of maximum effort. The compound muscle action potentials were recorded concurrently at multiple levels after magnetic stimulation to the brain with a round coil centered over the vertex. Results. The onset and peak latencies (mean +/- SD) of the major negative potential increased progressively toward the caudal level from 13.4 +/- 2.2 msec and 17.6 +/- 2.2 msec at T5-T6 to 20.8 +/- 1.5 msec and 26.3 +/- 3.0 msec at L4-L5 in healthy subjects. The latencies changed significantly between two adjacent segments from T5-T6/T6-T7 through T12-L1/L2-L2. Conclusions. The compound muscle action potential was elicited from the voluntarily contracted erector spinae at all spinal levels from T5-T6 through L4-L5 in all subjects tested. This noninvasive test has potential for evaluating the functional integrity of the motor pathway in the thoracic spinal cord, the spinal nerves, or both.