Magnetic stimulation in the assessment of peripheral nerve disorders

Relatively pain-free excitation of both superficial and deep nerves in the assessment of nerve conduction velocity is the main advantage of magnetic stimulation over conventional electrical stimulation. General utility of this technique has often been called into question by a number of authors because of difficulties in obtaining supramaximal responses or in determining the exact site of impulse generation when stimulating a peripheral nerve distally. Meanwhile, magnetic stimulation of the cervical and lumbar roots has become a routine procedure for the assessment of peripheral conduction time and is combined with transcranial magnetic stimulation of the motor cortex in the assessment of central conduction time. Recent developments in magnetic coil and stimulator design have improved the focality of the stimulus, so that selective supramaximal stimuli can be delivered to commonly studied peripheral nerves in the upper and lower limbs, both at proximal and distal segments. Furthermore, the introduction of small figure-8-shaped coils enables safe diagnosis of chronic compression syndromes with exact assessment of conduction velocities over short distances of peripheral nerves. The ease of application of magnetic stimulation and the absence of pain for the patient make magnetic stimulation a particularly attractive method for also investigating patients with demyelinating polyneuropathies, and will certainly replace conventional electrical stimulation in the near future.