Myokymia, muscle hypertrophy and percussion “myotonia” in chronic recurrent polyneuropathy

Abstract

Three unusual features were observed in a patient with chronic relapsing polyneuropathy: myokymia, muscle hypertrophy, and prolonged contraction in response to muscle percussion. Low nerve conduction velocity and conduction block were demonstrated in all motor nerves tested, indicating a demyelinating peripheral neuropathy. Myokymia was caused by spontaneous motor unit activity which was shown to originate in peripheral nerves, since it persisted after nerve block and was abolished by regional curarization. Muscle hypertrophy was attributed to increased peripheral nerve activity, and the prolonged contraction of muscle in response to direct percussion was attributed to irritability of intramuscular nerve terminals.

A benign motor neuron disorder: Delayed cramps and fasciculation after poliomyelitis or myelitis

“…Speculations about the mechanism of true muscle hypertrophy following denervation include work hypertrophy [21 and "stretch" hypertrophy [341 of the remaining fibers. Similar enlargement in cases of neuropathy with pseudomyotonia [20] or myokymia [36] has been attributed to continuous muscle work because hypertrophy is also seen in myotonia congenita and Schwartz-Jampel syndrome. Human denervation hypertrophy seems to occur exclusively in longstanding conditions of partial denervation.…”

Conduction block and continuous motor unit activity in chronic acquired demyelinating polyneuropathy

“…Patients with acquired inflammatory neuropathies may have CMUA as an associated feature. 4,11,13,18,19,29 The proposed mechanism implicates areas of focal demyelination which may serve as sites of ectopic impulse generation and, if multiple, may generate reverberating discharges producing self-sustained reexcitation. 3 Clinically, involuntary muscle contraction persisting during sleep and usually associated with an un- dulation of the overlying skin is a constant feature of these syndromes.…”

“…Acquired and hereditary demyelinating neuropathies occurring in association with CMUA have been described. 4,10,11,13,18,19,29 We describe a case of CMUA occurring in association with a chronic acquired demyelinating neuropathy in which the involuntary motor unit activity was particularly prominent in muscles affected by chronic motor conduction block.…”

Neuromyotonia, systemic lupus erythematosus and acetylcholine-receptor antibodies

“…Different hypotheses have been proposed in the last 10-15 years to explain the origin of the continuous muscular activity in NMT. Valenstein et al [23] have hypothesized that the activity of the sensory afferent fibres would excite the motor terminations in the distal parts of the nerve in an ephaptic way, and the muscle hypertrophy found in these patients would be caused by the continuous exercise from the spontaneous activity; Coers et al [4] believe that NMT is a particular expression of partial motor denervation compensated from axonal "sprouting". More recently, NMT has increasingly been considered to have an autoimmune aetiology: Halbach et al [7] described a patient with NMT, thymoma and anti-AChR Abs and hypothesized that the binding of the antibody to the receptor could act by modifying the conformation of the same receptor, prolonging the opening time of the ion channels, particularly of the K+ channels, and sensitizing the same receptor to the action of ACh; Sinha et al [21], describing the improvement of a patient with NMT after plasmapheresis, suggest that NMT could be mediated by IgG Abs.…”