Creatine kinase (CK)-linked IgA in Isaacs' syndrome. An immune complex disease?

Fasciculations in Children

Fasciculations are the most common form of spontaneous muscle contraction. They frequently occur in healthy individuals. However, there are a minority of situations that fasciculations are observed in association with specific neurologic disorders. Publications concerning the evaluation of pediatric patients experiencing fasciculations are limited. These children may undergo invasive or expensive diagnostic investigations that are unnecessary. Moreover, without careful consideration of differential diagnoses, rare neuromuscular disorders that present with fasciculations in the pediatric age group can be under-recognized by pediatric neurologists. This review examines the most important pediatric disorders presenting with fasciculations and other spontaneous muscle contractions to guide pediatric neurologists in evaluating these children.

Increased sensitivity to a nondepolarizing muscle relaxant in a patient with acquired neuromyotonia

Neuromyotonia is a disorder of hyperexcitability of the peripheral nerve. It has electromyographic features of spontaneous, continuous, irregularly occurring doublets, or multiplets of motor unit potential discharges. Neuromyotonia is characterized by both myokymic and neuromyotonic discharges. To our knowledge, this is the first report in the literature to assess the sensitivity of skeletal muscle to a nondepolarizing muscle relaxant drug, rocuronium, in a woman with acquired neuromyotonia. She had a past medical history notable for prolonged postoperative paralysis following anesthesia. The patient showed increased sensitivity to the neuromuscular effects of rocuronium. This increase in sensitivity may be explained by downregulation of acetylcholine receptors in response to chronic high agonist (acetylcholine) concentrations. If patients with neuromyotonia receive anesthesia, we recommend that smaller doses of a nondepolarizing muscle relaxant be administered, accompanied by monitoring of neuromuscular function, so as to provide optimal muscle relaxation while avoiding overdose and prolonged postoperative recovery.

CK-MM autoantibodies: Prevalence, immune complexes, and effect on CK clearance

Although the blood level of creatine kinase (CK) is the most commonly used marker of muscle injury, there is marked interindividual variability in this measure. Part of this variability may be attributed to variability in the rate of CK clearance from the circulation. In this study, we assessed the possibility that CK-MM autoantibodies form immune complexes with CK following muscle injury and subsequently affect the CK clearance rate. Using an enzyme-linked immunosorbent assay, CK-MM autoantibodies were detected in all 25 human subjects studied but the levels varied greatly. Using protein A-sepharose, the percentage of the plasma CK activity found in immune complexes was determined to be correlated with the CK-MM autoantibody level at lower CK levels (<1,022 U/L). When CK-MM antibodies were administered to mice, plasma CK activity following a bolus CK injection was reduced by 11%-32%. We conclude that CK-MM autoantibodies can modulate the rate of CK clearance from the circulation. Thus, the relatively low blood CK levels seen in some individuals following injury may be attributed partly or entirely to an autoantibody-enhanced clearance of CK.

Acquired neuromyotonia: a new autoantibody-mediated neuronal potassium channelopathy

Neuromyotonia (Isaacs syndrome) is a rare and heterogenous syndrome of continuous motor unit activity of peripheral nerve origin that manifests as various combinations of muscle stiffness, cramps, twitching, weakness, and delayed muscle relaxation. Although neuromyotonia may accompany an assortment of inherited diseases, most cases are acquired. The observation that the acquired form is often associated with an autoimmune disorder, and the demonstration that some cases improve after plasma exchange, has led to a search for an immune-mediated etiology. This review summarizes the recent immunological and electrophysiological evidence that autoantibodies to voltage-gated potassium channels produce the peripheral motor nerve hyperexcitability that characterizes neuromyotonia and thus establishes acquired neuromyotonia as an autoantibody-mediated disorder. In the nervous system, ion channels and neurotransmitter receptors that function as ligand-gated ion channels seem to be favored targets for autoantibody attack, and neuromyotonia can now be added to the growing list of autoimmune channelopathies.

Serum of Isaacs' syndrome suppresses potassium channels in PC-12 cell lines

Blockage of K+ channels in nerve terminals by immunoglobulin is the speculated pathomechanism of Isaacs' syndrome. Using patch-clamp technique (whole-cell clamp), we investigated the effects on K+ current of serum taken from 2 patients with Isaacs' syndrome employing the clonal cell line PC-12. The addition of a patient's serum to the perfusion solution had little effect on the K+ current of P-12 cells. In contrast, K+ current was reduced by 25-80% when cells were cultured for 3-6 days with 2% serum as compared to control serum values. Suppression of the K+ current appears to develop gradually over the period of culture. Our results suggest that the pathomechanism of Isaacs' syndrome is caused by K+ channel suppression via a humoral factor(s) in the serum, which subsequently induces nerve hyperexcitability.

Neuromyotonia, peripheral neuropathy and myasthenia gravis

A patient with neuromyotonia, peripheral neuropathy and myasthenia gravis (MG) is described. Neurophysiological studies, at rest, showed continuous muscle discharges of motor unit action potentials (MUAPs) in duplets and triplets. Motor (MNCV) and sensory (SNCV) nerve conduction studies revealed mild axonal and demyelinating peripheral neuropathy. Plasma exchange was followed by disappearance of clinical and electrophysiological signs of neuromyotonia and MG, as well as peripheral neuropathy.

Clinical evaluation of plasma exchange and high dose intravenous immunoglobulin in a patient with Isaacs' syndrome

The clinical effects of plasma exchange and high dose intravenous immunoglobulin (IVIg) were evaluated in a 41 year old woman with Isaacs' syndrome. After double filtration plasma exchange, symptoms almost disappeared for two to three weeks and the recorded continuous muscle action potentials were considerably decreased. Symptoms recurred within a few months. On the other hand, IVIg worsened the symptoms of the disorder: during and after IVIg at a dose of 0.2 g/kg/day (total 50 g), widespread myokymia, pseudomyotonia, and muscle cramps gradually increased. Symptoms improved after another course of plasma exchange.

Idiopathic generalized myokymia

Idiopathic generalized myokymia (IGM) is a rare, heterogeneous, and poorly understood syndrome. We present analysis of 75 reported cases in the world literature. IGM affects men and women equally, with a mean age of onset 29 +/- 19 years. Patients' common presenting complaints are stiffness (60%), cramps (12%), weakness (12%), and muscle twitching (4%). Family history is positive in 30%. In addition to generalized clinical myokymia (92%), abnormal neurologic findings include: hyporeflexia (70%), weakness (45%), grip myotonia (39%), and calf hypertrophy (16%). Electrical activity consisting of spontaneous continuous motor unit activity and/or electrical myokymia was documented in all patients. When electrical myokymia was observed (66%), the grouped discharges where irregular and had an interburst frequency of 2-300 Hz. Both phenytoin and carbamazepine are effective treatments. We conclude that IGM has a wide spectrum of symptoms and severity and should be considered in all patients that present with stiffness, cramps, or muscle twitching. EMG greatly aids in diagnosis.

A case of Isaacs syndrome with high CSF protein and a large cisterna magna

An 11-year-old boy exhibited continuous muscle fiber activity. He had suffered from stiffness of his hands, difficulty in relaxing his hands after gripping, and making skilled movements with his fingers. His clinical symptoms improved after treatment with carbamazepine. Electromyography (EMG) showed that he had continuous electrical discharges both at rest and during sleep. These discharges completely disappeared after the peripheral nerve was blocked with Lidocaine. An evoked electromyogram showed suppression of abnormal discharges after the F response. These electrophysiological findings indicated that the disorder originated in the spinal anterior horn cells. CT scanning showed a large cisterna magna in the posterior cranial fossa. Protein in the cerebrospinal fluid was elevated.