Diagnosis of incomplete conduction block of spinal cord from skin surface using spinal cord evoked magnetic fields

We previously reported the usefulness of neuromagnetic recordings for the diagnosis of disorders in peripheral nerves or the spinal cord. However, there have been no reports on incomplete conduction block of the spinal cord, which is clinically common in conditions such as cervical myelopathy. Here, we estimated the usefulness of measuring spinal cord evoked magnetic fields for evaluating incomplete conduction block.

Incomplete conduction block models of the spinal cord of the rabbit were established using a Fogarty balloon catheter that was inserted into the epidural space of the cervical spine. Electrical stimuli were applied to the lower thoracic spinal cord with an epidural catheter electrode. Spinal cord evoked potentials were recorded using epidural electrodes. Spinal cord evoked magnetic fields were recorded over the skin surface of the neck using a biomagnetometer.

The decrease in the conduction velocity and amplitude at the compression site could be detected by spinal cord evoked potentials from the epidural space, confirming the spinal cord lesion. The waveforms of the magnetic fields showed a biphasic configuration. The distribution of magnetic fields showed a characteristic quadrupolar pattern propagating from caudal to cranial. After compression, the amplitude and the conduction velocity of the magnetic fields decreased, and the distribution of magnetic fields were attenuated and decelerated near the compression site especially in the trailing magnetic fields. Diagnosis of the incomplete conduction block was thus possible.

We report the first measurement of the spinal cord evoked magnetic field in the intact spinal cord from the skin surface and that it can be applied to incomplete conduction block of the injured spinal cord. The use of a biomagnetometer is promising as a less-invasive method for clinically evaluating spinal cord function.