Electromyographic features.
EMG reveals spontaneous, continuous, irregularly occurring doublet, triplet or multiplet single motor unit discharges, firing at a high intraburst frequency (30-300 Hz) (Fig. 2). In addition, fibrillation potentials and fasciculations are often present, the former indicating the discharge of single muscle fibres. The shape or size of the motor unit during a train of discharges can vary, due to the depolarization of different muscle fibres supplied by the same motor nerve (Fig. 3). Several different motor units may be seen to be discharging in the same EMG recording. Most spontaneous discharges occur in distal muscles and usually no more than 10 different motor unit (or partial motor unit) discharges are seen (Hart et al. 2002). Characteristically, electrical stimulation of the nerve results in increased spontaneous activity seen as after-discharges (Fig. 4), and voluntary muscle contraction can provoke spontaneous motor unit activity lasting several minutes. Neuromyotonic discharges are characteristically present during sleep. Using electrophysiological techniques, it has been possible to determine the site of origin of the spontaneous electrical activity. Regional curarization abolishes the discharges, whereas general anaesthesia does not. This suggests that they arise from peripheral nerve. Several accounts have shown a reduction in the number of discharges following local anaesthetic nerve block at the elbow or knee, indicating that the ectopicfoci lie in the proximal segments of the nerve and its root. However, other reports show no diminution of spontaneous electrical activity after proximal nerve block, even when blocked at the wrist, suggesting a more distal discharge generation site. It is most probable that the generator sites lie anywhere along the whole length of the nerve, from the root to the terminal arborizations.
EMG reveals spontaneous, continuous, irregularly occurring doublet, triplet or multiplet single motor unit discharges,firing at a highintraburst frequency.
Other patients with milder symptoms of muscle cramps, twitching, and stiffness, who share similar autoimmune associations as autoimmune neuromyotonia, have EMG features similar to the cramp fasciculation syndrome, but without the doublet, triplet, or multiplet motor unit discharges usually seen in neuromyotonia.These EMG features reflect quantitative rather than qualitative differences between the diverse clinical syndromes of cramp fasciculation syndrome at one end, and acquired neuromyotonia at the other, more severe end of a spectrum of autoimmune peripheral nerve hyperexcitability (Hart et al. 2002).

Immunological findings

Immunoprecipitation assays detect anti-VGKC antibodies in the serum of about 40% of patients with acquired neuromyotonia, in 80% if there is an associated thymoma. Associated autoimmune disorders and other autoantibodies can be detected in approximately 50% of neuromyotonia patients, most notably antiacetylcholine receptor antibodies, indicating coexistent myasthenia gravis in about 20%.

© 2002 Blackwell Science Ltd

Figure 2 A 25s continuous needle EMG recording from medial gastrocnemius. Motor units are seen to fire spontaneously and irregularly as doublets (B), triplets (A), and multiplets (C), with intraburst frequencies of up to 120 Hz. The prolonged discharge in the middle of the recording consists of rapidly fi ring triplets of more than one motor unit (D).
Figure 3 Sections from longer continuous EMG recordings illustrating decremental patterns occurring within high frequency bursts of neuromyotonia.