The Clinical Significance of Somatosensory and Motor Evoked Potential in Myelopathy

ABSTRACT

BACKGROUND: Myelopathies are classified as intramedullary and extramedullary one on the basis of location of lesion. Though there are some characteristic clinical findings which can differentiate extramedullary from intramedullary lesions, it is not easy to tell extramedullary from intramedullary lesions in complicated cases. We performed this study to figure out the relationships between anatomic location (intramedullary or extramedullary), clinical parameters (motor and sensory symptoms and signs), and electrophysiologic findings in myelopathy. We also investigated the diagnostic value of motor evoked potential (MEP) compared with that of somatosensory evoked potential (SSEP) in myelopathy. The amplitude changes of compound muscle action potential (CMAP) after motor cortex stimulation have a lot of intra-individual and inter-individual variability, so were not used to determine pyramidal tract dysfunction in clinical ground. We analyzed CMAP amplitude changes in myelopathy. METHODS: Fifty-six patients with myelopathy demonstrated in MRI were studied. We defined abnormal CMAP amplitude changes as intra-individual inter-side amplitude ratio more than 50%. RESULTS: Lower MEP showed abnormal findings in 93% of tests and lower SSEP, 37% (p<0.05). The correla-tions between clinical parameters and electrophysiologic findings were higher in lower MEP (71%) than lower SSEP (42%) (p<0.05). CONCLUSIONS: The results of this study suggest that MEP is more useful than SSEP for detecting spinal cord dysfunction but as false positivity of lower MEP was considerable, MEP and SSEP are reciprocal diagnostic method for myelopathy. SSEP and MEP do not have significant diagnostic values in differentiating extramedullary from intramedullary myelopathy. The inter-side CMAP amplitude difference may indicate subclinical spinal cord dysfunction.