New method to measure central motor conduction time using transcranial magnetic stimulation and T-response

Measuring central motor conduction time (CMCT) is one of the useful methods to detect an impaired level of the spinal segment in cervical myelopathy patients. We modified a new technique to calculate the CMCT using tendon reflex latency (T-response) and investigated its accuracy. Motor-evoked potentials (MEPs) following transcranial stimulation were recorded in 19 patients with cervical myelopathy caused by a single level of spinal cord compression. CMCT was measured by subtracting the peripheral conduction time, which was calculated by using the T-response for the biceps brachii muscle (Biceps), the compound muscle action potentials (CMAPs) and the F-wave of the abductor digiti minimi muscle (ADM). In the control subjects, the mean value of CMCT of the Biceps and ADM was 3.8 and 7.0 ms, respectively. The accuracy of the determination of the CMCT for Biceps using T-response was investigated beforehand in the unilateral brachial plexus palsy patients and thoracic spinal cord myelopathy patients. The calculated CMCT (3.88+/-0.65 ms) for Biceps was close to the N2 latency (4.06+/-0.3 ms) of the evoked spinal cord potentials which were recorded from the epidural space on the C3-4 vertebral level following transcranial magnetic stimulation. The CMCT of both the Biceps and ADM was delayed in all cases of C1-2 cord compression. In patients with cord compression on the C3-4 level, two of four patients showed CMCT prolongation in Biceps. The prolongation of CMCT was observed only in ADM in patients with C4-5 or C5-6 cord compression. Measurement of the CMCT using T-responses was useful in proximal limb muscles. Comparison of the CMCT in Biceps and ADM could allow us to better detect the functional level diagnosis for compressive cervical myelopathy.