IgM-containing fraction suppressed voltage-gated potassium channels in acquired neuromyotonia

Acquired neuromyotonia in childhood: case report and review

Recently characterized as an immune-mediated channelopaty, Isaacs' syndrome (also known as acquired neuromyotonia) was first described in 1961 in two men with persistent, generalized muscle stiffness, in addition to spontaneous, rapid discharges of motor-unit potentials on electromyography. In the peripheral nervous system, antibodies targeted to voltage-gated potassium channels induce hyperexcitability of motor axons, resulting in signs of muscle stiffness or of pseudomyotonia. A spontaneous burst of single motor-unit activity, and myokymic and neuromyotonic discharges, are the most characteristic features found in electromyography studies. This report describes Isaacs' syndrome in a child, in whom the diagnosis was made by clinical features of acquired, spontaneous muscle overactivity and typical electromyographic findings.

Interspike interval analysis in a patient with peripheral nerve hyperexcitability and potassium channel antibodies

Neuromyotonia or Isaacs' syndrome is a rare peripheral nerve hyperexcitability disorder caused by antibodies against potassium channels of myelinated axons. We present the high-density surface electromyographic (EMG) recordings of a patient with fasciculations and cramps due to neuromyotonia. To characterize the time course of hyperexcitability, we analyzed the interspike intervals (ISIs) between fasciculation potentials, doublet, and multiplet discharges. ISI duration increased within each burst. The ISI histograms found can be explained by the recovery cycle of the myelinated axon and its dependency on the slow potassium conductance. We conclude that ISI analysis is a useful tool to understand the membrane dynamics underlying abnormal motor unit activity.

Autoimmune Channelopathies and Related Neurological Disorders

Ion channels are crucial elements in neuronal signaling and synaptic transmission, and defects in their function are known to underlie rare genetic disorders, including some forms of epilepsy. A second class of channelopathies, characterized by autoantibodies against ligand- and voltage-gated ion channels, cause a variety of defects in peripheral neuromuscular and ganglionic transmission. There is also emerging evidence for autoantibody-mediated mechanisms in subgroups of patients with central nervous system disorders, particularly those involving defects in cognition or sleep and often associated with epilepsy. In all autoimmune channelopathies, the relationship between autoantibody specificity and clinical phenotype is complex. But with this new information, autoimmune channelopathies are detected and treated with increasing success, and future research promises new insights into the mechanisms of dysfunction at neuronal synapses and the determinants of clinical phenotype.