[Physiological approach to peripheral neuropathy. Conventional nerve conduction studies and magnetic motor root...

In this communication, I first show some points we should mind in the conventional peripheral nerve conduction studies and later present clinical usefulness of motor root stimulation for peripheral neuropathy. CONVENTIONAL NERVE CONDUCTION STUDIES (NCS): The most important point revealed by the conventional NCSs is whether neuropathy is due to axonal degeneration or demyelinating process. Precise clinical examination with this neurophysiological information leads us to a diagnosis and treatment. Poor clinical examination makes these findings useless. Long standing axonal degeneration sometimes induces secondary demyelination at the most distal part of involved nerves. On the other hand, severe segmental demyelination often provokes secondary axonal degeneration at distal parts to the site of demyelination. These secondary changes show the same abnormal neurophysiological findings as those of the primary involvement. We should be careful of this possibility when interpreting the results of NCS. NCS of sensory nerves is not good at revealing demyelinating process. Mild temporal dispersion of potentials often reduces an amplitude of SNAP or loss of responses, which usually suggests axonal degeneration, because of short duration of sensory nerve potentials. MOTOR ROOT STIMULATION IN PERIPHERAL NEUROPATHY: Magnetic stimulation with a coil placed over the spine activates motor roots and evokes EMG responses from upper and lower limb muscles. The site of activation with this method was determined to be where the motor roots exit from the spinal canal (intervertebral foramina) (J Neurol Neurosurg Psychiatry 52 (9): 1025-1032, 1989) because induced currents are very dense at such a foramen made by electric resistant bones. In several kinds of peripheral neuropathy, this method has been used to detect a lesion at a proximal part of the peripheral nerves which can not be detected by the conventional NCSs. I present a few cases in whom motor root stimulation had a clinical merit. In a patient with neuralgic amyotrophy, motor root stimulation disclosed a conduction block between the cervical intervertebral foramen and brachial plexus which was not detected by conventional NCSs. Motor root stimulation clearly revealed demyelination in a patient with CIDP in whom sural nerve biopsy findings suggested axonal degeneration, that must be secondary to demyelination. In a patient with tomacular neuropathy, magnetic stimulation revealed conduction delay in the spinal nerve within the spinal canal (Clin Neurol (Jap), 28: 447-452, 1988). Based on the above results, combination of NCSs and magnetic motor root stimulation must brush up the neurophysiological approach to peripheral neuropathy.