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Neurology India, Vol. 54, No. 4, October-December, 2006, pp. 350 Invited Commentaries Neuromyotonia: A diverse disorder Newsom-Davis John University of Oxford, Department of Clinical Neurology, Radcliffe Infirmary, Oxford OX2 6HE Code Number: ni06123 Peripheral nerve hyperexcitability at its least severe causes muscle cramps and fasciculations termed the Cramp-Fasciculation syndrome by Tahmoush et al.[1] In more severe cases myokymia (visible muscle twitching that can have an undulating characteristic), pseudomyotonia (failure to relax after voluntary contraction) and muscle hypertrophy can be observed and some patients experience painful sensory symptoms and feelings of anxiety. Several names have been applied to this latter syndrome including Isaacs' syndrome and neuromyotonia. The electromyographic changes, first described by Denny Brown, are characterised by myokymic discharges (doublet or multiplet motor unit discharges) and by spontaneous (neuromyotonic) motor unit discharges. There are several known conditions that cause neuromyotonia. These include axonal damage (e.g., from local nerve irradiation or toxins), timber rattle snake envenomation, certain idiopathic or inherited neuropathies and genetic mutations affecting potassium channels. In the remainder, who constitute the majority of patients, the disorder was regarded as idiopathic until studies dating from 1991 showed that many of these patients have strong immunological associations and respond to plasma exchange.[2] The neurophysiological abnormalities can be transferred to experimental animals by injection of neuromyotonic IgG,[3] unequivocally pointing to an antibody-mediated disorder and implicating voltage-gated potassium channels (VGKCs) as their target. It was later shown that antibodies to VGKCs could be detected in about 40% of patients by radioimmunoassay using 125a-dendrotoxin.[4] By down-regulating VGKCs, the antibodies would be expected to interfere with nerve repolarisation and lead to repetitive discharges. Neuromyotonia is now increasingly being recognized as a potentially treatable disorder and the study[5] is remarkable for the large number of patients in their series. The disorder was characterised by electromyography in all of them, but measurement of VGKC antibodies was only possible in one patient in whom the titre was markedly elevated. It cannot be assumed that the disorder in all the remaining 19 patients was due to VGKC antibodies. First, VGKC antibodies are only detected in about 40% of patients. Second, neuromyotonia can associate with inflammatory neuropathies presumably as a result of damage to juxta-nodal VGKCs by an unknown disease process that may not involve an autoantibody attack. Thirdly and importantly, 11 of the 20 patients had been receiving Ayurvedic medications that, as the authors point out, could have been the incriminating factor in these patients. This important observation needs to be widely known and the authors should be congratulated on bringing this to the attention of neurologists. References
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