Muscle Channelopathies

PURPOSE OF REVIEW

This article describes the clinical features, diagnosis, pathophysiology, and management of nondystrophic myotonia and periodic paralysis.

RECENT FINDINGS

An increasing awareness exists about the genotype-phenotype overlap in skeletal muscle channelopathies, and thus genetic testing is needed to make a definitive diagnosis. Electrodiagnostic testing in channelopathies is highly specialized with significant overlap in various mutation subtypes. Randomized clinical trials have now been conducted in these disorders with expanded treatment options for patients with muscle channelopathies.

SUMMARY

Skeletal muscle channelopathies are rare heterogeneous conditions characterized by lifelong symptoms that require a comprehensive management plan that includes pharmacologic and nonpharmacologic interventions. The significant variability in biophysical features of various mutations, coupled with the difficulties of performing clinical trials in rare diseases, makes it challenging to design and implement treatment trials for muscle channelopathies.

Episodic Muscle Disorders

PURPOSE OF REVIEW

This article reviews the episodic muscle disorders, including benign cramp-fasciculation syndrome, the periodic paralyses, and the nondystrophic myotonias. The core diagnostic criteria for a diagnosis of primary periodic paralysis, including clues to distinguish between the hypokalemic and hyperkalemic forms, and the distinctive elements that characterize Andersen-Tawil syndrome are discussed. Management of patients with these disorders is also discussed.

RECENT FINDINGS

Childhood presentations of periodic paralysis have recently been described, including atypical findings. Carbonic anhydrase inhibitors, such as dichlorphenamide, have recently been approved by the US Food and Drug Administration (FDA) for the treatment of both hypokalemic and hyperkalemic forms of periodic paralysis. Muscle MRI may be a useful outcome measure in pharmacologic trials in periodic paralysis. Genetic research continues to identify additional gene mutations responsible for periodic paralysis.

SUMMARY

This article will help neurologists diagnose and manage episodic muscle disorders and, in particular, the periodic paralyses and the nondystrophic myotonias.

Diagnostics and therapy of muscle channelopathies--Guidelines of the Ulm Muscle Centre

This article is dedicated to our teacher, Prof. Erich Kuhn, Heidelberg, on the occasion of his 88th birthday on 23rd November 2008. In contrast to muscular dystrophies, the muscle channelopathies, a group of diseases characterised by impaired muscle excitation or excitation-contraction coupling, can fairly well be treated with a whole series of pharmacological drugs. However, for a proper treatment proper diagnostics are essential. This article lists state-of-the-art diagnostics and therapies for the two types of myotonic dystrophies, for recessive and dominant myotonia congenita, for the sodium channel myotonias, for the primary dyskalemic periodic paralyses, for central core disease and for malignant hyperthermia susceptibility in detail. In addition, for each disorder a short summary of aetiology, symptomatology, and pathogenesis is provided.

Reversal of RNA missplicing and myotonia after muscleblind overexpression in a mouse poly(CUG) model for myotonic dystrophy

RNA-mediated pathogenesis is a recently developed disease model that proposes that certain types of mutant genes produce toxic transcripts that inhibit the activities of specific proteins. This pathogenesis model was proposed first for the neuromuscular disease myotonic dystrophy (DM), which is associated with the expansion of structurally related (CTG)(n) and (CCTG)(n) microsatellites in two unrelated genes. At the RNA level, these expansions form stable hairpins that alter the pre-mRNA splicing activities of two antagonistic factor families, the MBNL and CELF proteins. It is unclear which altered activity is primarily responsible for disease pathogenesis and whether other factors and biochemical pathways are involved. Here, we show that overexpression of Mbnl1 in vivo mediated by transduction of skeletal muscle with a recombinant adeno-associated viral vector rescues disease-associated muscle hyperexcitability, or myotonia, in the HSA(LR) poly(CUG) mouse model for DM. Myotonia reversal occurs concurrently with restoration of the normal adult-splicing patterns of four pre-mRNAs that are misspliced during postnatal development in DM muscle. Our results support the hypothesis that the loss of MBNL1 activity is a primary pathogenic event in the development of RNA missplicing and myotonia in DM and provide a rationale for therapeutic strategies designed either to overexpress MBNL1 or inhibit MBNL1 interactions with CUG and CCUG repeat expansions.

Effects of acute and chronic denervation on human myotonia

A 32-year-old man with myotonia congenita (Becker type) sustained multiple gunshot wounds. These produced a partial thoracic spinal cord injury and a severe sciatic nerve injury. Six days following the incident, clinical (percussion) and electrophysiologic (EMG) myotonia could be elicited in paretic leg muscles resulting both from the myelopathy and peripheral nerve disruption. Eight months later, the myotonia was no longer present in denervated muscles from the sciatic nerve injury, but was still noted in muscles with upper-motor neuron weakness from the myelopathy. Although myotonia is related principally to abnormalities in the muscle fiber itself, it appears that it is also dependent upon the structural integrity of the peripheral nerve supply to the muscle for myotonia to continue to occur. The findings in this patient suggest that myotonia may well have diminished and disappeared in muscles shortly after the nerves had undergone Wallerian degeneration. Myotonia does not recur if there is no significant reinnervation.

Malignant hyperthermia and myotonic disorders

Advances in physiology and molecular genetics have promoted greater understanding of the various clinical manifestations of muscle disorders. For example, myotonia or profound weakness may be observed in sodium channel disease (e.g., paramyotonia congenita or hyperkalemic periodic paralysis), depending on the specific channel defect or with slight changes in membrane potential. Observed effects of anesthetic techniques have been essential to elucidating the primary muscular nature of myotonia. Commonly used anesthetic medications have potentially lethal (e.g., MH) or serious (e.g., myotonic dystrophy) adverse effects. Conversely, lidocaine or propofol may have therapeutic benefit for patients with skeletal muscle sodium channel disorders. Additional investigation is required to improve our understanding of how age, gender, or other factors determine the phenotypic expression of malignant hyperthermia. Future research holds the promise for more accurate pre-anesthetic identification of persons with heritable myopathies, especially those who are asymptomatic. Enhanced awareness of multiple organ system involvement in myotonic dystrophy is essential for planning perioperative care. Patients with periodic paralysis require that we know factors that incite or inhibit the development of their attacks. Advances in bench research and detailed clinical studies will further improve our ability to provide optimal care for patients with muscle disorders.

[Rehabilitation of children with diffuse muscular hypotonia and neurophysiologic criteria of its effectiveness]

Neuromapping, neuromyography, cerebrovascular mapping, cardiointervalography were conducted in children with the diagnosis "natal trauma of the cervical spine and vertebral arteries with ischemia of the reticular formation of the cerebral trunk in the form of myatonic syndrome". Adaptation reserves in the children were also studied. In addition to conventional methods, the treatment included kinesitherapy. The efficacy of the kinesitherapy was assessed.

Channelopathies: potassium-related periodic paralyses and similar disorders

Channelopathy is a term used to describe clinical problems caused by disorders of membrane ion channels. Included in this disease category are certain types of periodic paralyses, ataxia, myotonia, migraine headache, epilepsy, nephrolithiasis, and long QT syndrome. This article briefly summarizes membrane ion channel structure and function and details several relatively common channelopathies. In hyperkalemic periodic paralysis, mutant skeletal muscle sodium channels fail to close completely after an action potential. This evokes two apparently opposite symptoms: myotonia (caused by a small depolarization and repetitive excitation) or paralysis (caused by larger depolarization and inexcitability). In hypokalemic periodic paralysis, mutation affects the closing of skeletal muscle calcium channels, causing transient paresis or paralysis. The task of the advanced practice nurse is to recognize these disorders, institute appropriate prophylactic measures and treatments, monitor symptom progression, and avoid complications. Understanding of channelopathies is advancing rapidly. On the horizon are therapies tailored to counter specific membrane ion channel defects.

Myotonic dystrophy and proximal myotonic myophathy

Myotonic dystrophy (DM) is a well-known multisystem disorder with dominant inheritance. Proximal myotonic myopathy (PROMM) has been defined only recently, it is rather similar to but distinct from DM. Molecular genetic testing of the CTG trinucleotide repeat expansion is a reliable diagnostic method in DM. In PROMM these CTG repeats are normal, and no genetic test is so far available. Comparing the phenotypes of DM and PROMM, an important point seems to be that PROMM is a more benign disorder. There are almost no obvious mental changes in PROMM patients; premature death is extremely rare; anticipation appears to be present but to a milder degree; a severe congenital type of PROMM apparently is very rare if it occurs at all. On the other hand, at least in the German population, the frequency of PROMM may be almost equal to that of DM.

Terminating attacks of ocular neuromyotonia

We examined a 30-year-old woman who, for 6 months, had suffered from ocular neuromyotonia, which consisted of episodic ocular depression. Apart from the ocular complaint, her medical history and the clinical findings were unremarkable. The patient discovered that she could terminate each episode of tonic ocular depression instantly by forcefully directing her gaze upward. Stretching the affected muscle might also prove to be an effective way of ending attacks of neuromyotonia in other patients suffering from this condition.

Effect of temperature reduction on myotonia in rat skeletal muscles in vitro

1. The objective of the study was to determine the effect of temperature reduction on the response of rat skeletal muscles to myotonia-inducing agents. 2. A model myotonia was induced in the muscles in vitro, using either the chloride channel blocker anthracene-9-carboxylic acid or chloride-free Krebs solution. This model is similar in its characteristics to the myotonia which occurs in autosomal recessive generalized myotonia congenita in humans. 3. Isometric twitch contractions were recorded in the muscles in Krebs solution before and after the addition of the myotonia-inducing agent. The presence of myotonia was confirmed when the half-relaxation time of the twitch contraction after the addition of the agent was significantly greater than that before its addition. 4. Recordings were made at 37 degrees C, 30 degrees C, 25 degrees C and 15 degrees C. Myotonia developed at 37 degrees C, 30 degrees C and 25 degrees C, but not at 15 degrees C, indicating that at a temperature between 25 degrees C and 15 degrees C, anthracene-9-carboxylic acid-induced myotonia failed to develop. This supports the results obtained in humans suffering from myotonia congenita where myotonic contractions in the adductor pollicis muscle disappeared when the muscle temperature was cooled to 20 degrees C. 5. The myotonia which developed at 37 degrees C could be significantly reduced by exposure to 1 x 10(-4) mol/l ouabain or by elevation of the K+ concentration of the Krebs solution to 7.5 mmol/l. 6. Measurements made using microelectrodes showed that the conditions under which myotonia either did not develop or was significantly reduced, i.e. a temperature of 15 degrees C, exposure to 7.5 mmol/l K+ at 37 degrees C or exposure to 1 x 10(-4) mol/l ouabain at 37 degrees C were each associated with membrane depolarization. The results are discussed in terms of a possible role for depolarization in preventing/reducing the myotonic response.

Ocular neuromyotonia in Graves dysthyroid orbitopathy

OBJECTIVES

To describe 2 patients with ocular neuromyotonia in association with Graves orbitopathy and to consider the possible underlying mechanisms.

DESIGN

Description of the clinical findings in 2 patients with these conditions.

SETTING

Neuro-ophthalmology referral centers.

PATIENTS

Two patients, aged 55 and 52 years, had episodic, involuntary periods of vertical diplopia and dysthyroid orbitopathy.

INTERVENTION

Treatment with carbamazepine in one patient and external beam radiation therapy in the second patient.

MAIN OUTCOME MEASURES

Frequency and duration of episodic spasms of the extraocular muscles.

RESULTS

Although radiation therapy is the most common association with ocular neuromyotonia, it cannot explain the involuntary contractions of extraocular muscles in all affected patients. Other mechanisms must be involved, such as those discussed in this article.

CONCLUSION

Ocular neuromyotonia is described in 2 patients with dysthyroid orbitopathy, confirming previous findings. Possible mechanisms are given.

Carrell-Krusen Symposium Invited Lecture-1997. Myotonic disorders in childhood: diagnosis and treatment

The recent discoveries that mutations in the genes for the skeletal muscle sodium and chloride channels are responsible, respectively, for paramyotonia/hyperkalemic periodic paralysis and for myotonia congenita of Thomsen have made the classification, diagnosis, and treatment of these disorders much easier. The discovery that myotonic dystrophy results from an unstable [CTG]n trinucleotide expansion has permitted the accurate diagnosis of both symptomatic and asymptomatic individuals, and has led to major advances in preventive treatment, including prenatal and genetic counseling. Diseases that resemble the inherited myotonic disorders are now easier to identify, and as a result of genetic testing a new clinical disorder that is similar to but distinct from myotonic dystrophy has emerged. This new disorder, proximal myotonic myopathy, does not appear to be linked to the genes for the sodium or chloride channels, and has cataracts, myotonia, weakness, and no abnormal expansion of the [CTG]n repeat in the gene for myotonic dystrophy. This review discusses the diagnosis and treatment of these myotonic disorders.

Suspected herbicide toxicosis in a dog

An 8-year-old 38-kg spayed female Golden Retriever was admitted for vomiting, signs of abdominal pain on palpation, ataxia, anorexia, and generalized weakness of 2 days' duration. Ten hours prior to onset of clinical signs, the dog was found standing in and drinking from large pools of an accidentally spilled herbicide that contained an octanoic acid ester of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) and an isooctyl ester of (2-methyl-4-chloro) phenoxyacetic acid (MCPA). Appendicular muscles were firm on palpation and persistent muscle contraction (myotonia > 1 minute duration) was found on muscle percussion, using a reflex hammer. Electrical activity indicative of myotonia was identified on electromyographic evaluation. With supportive treatment, the dog eventually recovered from suspected MCPA toxicosis. Although rare, MCPA toxicosis should be considered as a cause of acquired myotonia in dogs.

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.

Acquired neuromyotonia: evidence for autoantibodies directed against K+ channels of peripheral nerves

Acquired neuromyotonia is characterized by hyperexcitability of motor nerves leading to muscle twitching, cramps, and weakness. The symptoms may improve following plasma exchange, and injection of immunoglobulin G (IgG) from 1 neuromyotonia patient into mice increased the resistance of neuromuscular transmission to d-tubocurarine. Here we examine nerves and muscle in vitro from mice injected with plasma or purified IgG from 6 neuromyotonia patients or pooled control subjects, and cultured dorsal root ganglion cells after treatment with IgG. Three of the patients had antibodies against human voltage-gated potassium channels labeled with 125I-alpha-dendrotoxin. The quantal release of acetylcholine (quantal content) at end-plates in diaphragms from mice treated with neuromyotonia IgG preparations was increased by 21% relative to control values (p = 0.0053). With one IgG preparation, the duration of the superficial peroneal nerve compound action currents was increased by 93%. The dorsal root ganglion cells treated with this IgG showed a marked increase in repetitive firing of action potentials. All effects were similar to those obtained with aminopyridines. We conclude that at least some patients with acquired neuromyotonia have antibodies directed against aminopyridine- or alpha-dendrotoxin-sensitive K+ channels in motor and sensory neurons, and they are likely to be implicated in the disease process.