Epidurally recorded spinal cord evoked potentials in patients with cervical myelopathy and normal central motor conduction time measured by transcranial magnetic stimulation.

OBJECTIVE: Use epidural recording of evoked spinal cord potentials (ESCPs) to investigate the pathology of cervical spondylotic myelopathy (CSM) in patients with normal central motor conduction time (CMCT) in upper and lower limbs. METHODS: A total of 75 patients with CSM were studied. All patients were examined before surgery for motor evoked potentials (MEPs) following transcranial magnetic stimulation (TMS). They were also evaluated during surgery by epidurally recorded ESCPs following stimulation of the median nerve, brain and spine. RESULTS: Seven patients (9%) showed normal CMCT in upper and lower limbs upon TMS examination. Only the ESCPs following median nerve stimulation (MN-ESCPs) were abnormal in these patients. In 5 of the 7 patients, a marked block in conduction of MN-ESCPs was observed at the C3-4 intervertebral level. The remaining two patients showed attenuation in the amplitude of MN-ESCPs at mid-cervical levels. CONCLUSIONS: We present 7 cases of CSM with negative CMCT findings. From the MN-ESCP results, we surmise that the pathology of CSM with normal CMCT is due predominantly to dysfunction of sensory systems involved in the upper limbs. SIGNIFICANCE: Examination by TMS is useful in the diagnosis of CSM but the possibility of negative CMCT findings upon TMS must be borne in mind. Multi-functional evoked spinal cord responses demonstrate that lesions in the sensory system are the major underlying pathology.