The generator site in acquired autoimmune neuromyotonia

To investigate the origin of ectopic activity in neuromyotonia (NMT).

We studied two patients. In addition to routine studies, we tested synchronicity of spontaneous discharges in different motor units in simultaneous recordings made with two needle electrodes in the first dorsal interosseus muscle. Time-locked fasciculations in these double recordings would represent abnormal ectopic activity initiated in a nerve trunk with ephaptic stimulation of a nearby axon. In patient 1, this research protocol was applied once, 15years after regular intravenous immunoglobulin (IvIg) treatment. Patient 2 was investigated before and 1year after IvIg.

Both patients improved after IVIg, mirrored by a striking decrease in the amount of spontaneous activity on electromyography. Moreover, our technique did not detect synchronous spontaneous activity (time-locked fasciculations) on the second assessment, although this was predominant before treatment in patient 2.

In NMT, abnormal discharges originate both in distal axonal branches and in more proximal segments. It appears that IvIg is more effective in blocking antibody activity in proximal axonal segments, perhaps related to factors such as blood-nerve barrier, temperature or differing ion channel distributions.

Treatment effects can shed light on the origin of abnormal activity in NMT.