Usefulness of motor evoked potentials in compressive myelopathy

Motor Evoked Potentials elicited by transcranial magnetic stimulation were recorded from 1 degree Dorsal Interosseus for the upper limbs and from Extensor Digitorum Brevis for the lower limbs in 42 subjects with compressive myelopathy (36 in the cervical region and 6 in the dorsal region), radiologically defined by Nuclear Magnetic Resonance (NMR) imaging, with no clinical and radiological signs of radiculopathy. Central motor conduction abnormalities in the cortex-C8 and cortex-L5 tract were compared with clinical signs of motor impairment and with NMR findings. The subjects with medullar hyperintensity of NMR signal at the compression site level, showed a central conduction time (C.C.T.) prolongation in at least one side in the 70% of cases in the cortex-C8 tract and in the 95% of cases in the cortex-L5 tract, while in the cases with no compression site alterations of signal, C.C.T. abnormalities were observed only in 25% in the cortex-C8 tract and in 42% in the cortex-L5 tract. In 12 subjects (10 with cervical compression and 2 with dorsal compression) we compared Motor Evoked Potentials before, two weeks and two months after surgical decompression. The C.C.T. cortex-C8 and cortex-L5 improved at least in one side in 11 out of the 12 subjects, showing a good correlation with clinical recovery. In the examined patients we found a latency reduction statistically significant between the first and second investigation, with a trend to stabilization in the following controls.