Abnormal parameters of magnetically evoked motor-evoked potentials in patients with cervical spondylotic myelopathy

Background context: Magnetic stimulation (MS), which is used to evaluate motor pathways, is helpful in evaluating cervical spinal cord compression (cervical myelopathy [CM]). Previous studies have shown that the central motor conduction time (CMCT), which is the time taken for the nerve impulses to reach the cervical spinal roots after the stimulation of the motor cortex, is prolonged in CM. However, the duration of motor-evoked potentials (MEPs) in CM has not been studied in detail. Purpose: To compare the duration, CMCT and amplitude of MEPs by MS between patients with clinical and magnetic resonance imaging (MRI) features of CM and a control group. Study design/setting: A cross-sectional study done at Teaching Hospital, Peradeniya, Sri Lanka. Patient sample: Consecutive patients with clinical features of cervical spondylotic myelopathy, without coexisting neurological abnormality. Methods: Transcranial and cervical spinal magnetic stimulation were performed on 21 patients with clinical and MRI features of spondylotic CM (mean age, 43.5years; range, 36-63 years; 9 men) and 17 healthy volunteers (mean age, 39.05 years; range, 23-54yrs; 6 males) using a circular coil with a Magstim 200 stimulator. MEPs were recorded over abductor digiti minimi muscle on both hands. Results: Seventeen patients had upper motor neuron (UMN) features in all four limbs; in the others, both lower limbs and one upper limb were affected. The upper limbs with UMN features had shorter duration MEPs compared with the control group. The CMCT and the total motor conduction time were also delayed in the CM group. All three differences were very highly significant (t=5.75, -3.76, 5.27; p<.001). The amplitudes showed no significant difference between the two groups (t=1.27, p=.208). Conclusion: This study shows that in addition to the CMCT, the duration of MEPs is also useful in evaluating patients with CM using MS.