Motor evoked potentials elicited from erector spinae muscles in patients with thoracic myelopathy

A prospective study was conducted to test the utility of the motor evoked potentials (MEPs) from the erector spinae muscles as a measure to estimate the motor level of thoracic compression myelopathies in 13 consecutive patients.

To confirm whether this test is a useful addition to the neurological examination in non-invasively localizing the level responsible for the main functional change in mild to moderate thoracic myelopathy.

Department of Orthopaedic Surgery, Kochi Medical School, Kochi, Japan.

This electrophysiological study consisted of transcranial magnetic stimulation (TMS) of the brain and surface recording of MEPs from voluntarily contracted erector spinae muscles with the patient in the prone position. The recordings were obtained unilaterally from the same side as the lower-limb affected at 12 serial interspinous levels from T5-6 to L4-5. The results were compared to the MEP data from normal subjects and to neurological and MRI findings.

Multisegmental MEP studies demonstrated a focal conduction block in one patient, a single site of conduction delay in seven, and normal conduction in five. The conduction block was characterized by an abrupt reduction in amplitude of the MEPs. Examination of the sites conduction delay showed that the latency difference between the two adjacent levels was longer than the corresponding normal upper limit by 1.00+/-0.40 ms (range, 0.62-1.61 ms). The site of conduction abnormalities approximated to the compressive lesion site shown by MRI. All five patients with false-negative MEP findings had the lesion site at or caudal to the T10-11 vertebral level.

This method has the advantage of instantaneously testing multisegments of the thoracic spinal cord. The technique is of particular value in estimating the motor level of the lesions rostral to T10-11 vertebral level, which can not be achieved by clinical examinations or MEP recordings from the lower limb.