When is myokymia neuromyotonia?

Occult bowel cancer presenting as Morvan syndrome

A man in his mid-60s presented with a 3-month history of progressive muscle twitching, agitation, cognitive impairment, insomnia, hyperhidrosis and lower limb pain. He had fasciculations, myokymia, myoclonus, exaggerated startle response and significant postural hypotension. Electrophysiological studies showed evidence of peripheral nerve hyperexcitability with neuromyotonia. Contactin-associated protein-like 2 antibodies (CASPR2) were strongly positive. A diagnosis of Morvan syndrome was made. CT of the chest, abdomen and pelvis was undertaken to identify any occult malignancy, and a large bowel carcinoma in situ was identified and resected. His central nervous system and autonomic symptoms significantly improved following surgery, but neuromyotonia persisted, and this was treated with intravenous immunoglobulins and steroids. Early detection of bowel cancer in this patient enabled curative treatment.

Generalized myokymia, or neuromyotonia, or both in dogs with or without spinocerebellar ataxia

BACKGROUND

KCNJ10 and CAPN1 variants cause "spinocerebellar" ataxia in dogs, but their association with generalized myokymia and neuromyotonia remains unclear.

OBJECTIVE

To investigate the association between KCNJ10 and CAPN1 and myokymia or neuromyotonia, with or without concurrent spinocerebellar ataxia.

ANIMALS

Thirty-three client-owned dogs with spinocerebellar ataxia, myokymia neuromytonia, or a combination of these signs.

METHODS

Genetic analysis of a cohort of dogs clinically diagnosed with spinocerebellar ataxia, myokymia or neuromyotonia. KCNJ10 c.627C>G and CAPN1 c.344G>A variants and the coding sequence of KCNA1, KCNA2, KCNA6, KCNJ10 and HINT1 were sequenced using DNA extracted from blood samples.

RESULTS

Twenty-four Jack Russell terriers, 1 Jack Russell terrier cross, 1 Dachshund and 1 mixed breed with spinocerebellar ataxia were biallelic (homozygous) for the KCNJ10 c.627C>G variant. Twenty-one of those dogs had myokymia, neuromyotonia, or both. One Parson Russell terrier with spinocerebellar ataxia alone was biallelic for the CAPN1 c.344G>A variant. Neither variant was found in 1 Jack Russell terrier with ataxia alone, nor in 3 Jack Russell terriers and 1 Yorkshire terrier with myokymia and neuromyotonia alone. No other causal variants were found in the coding sequence of the investigated candidate genes in these latter 5 dogs.

CONCLUSION

The KCNJ10 c.627C>G variant, or rarely the CAPN1 c.344G>A variant, was confirmed to be the causal variant of spinocerebellar ataxia. We also report the presence of the KCNJ10 c.627C>G variant in the Dachshund breed. In dogs with myokymia and neuromyotonia alone the reported gene variants were not found. Other genetic or immune-mediated causes should be investigated to explain the clinical signs of these cases.

Clinical, neurophysiological and serological clues for the diagnosis of neuromyotonia and distinction from cramp-fasciculation syndrome

Neuromyotonia and cramp-fasciculation syndrome diagnosis currently relies on neurophysiological examination. In this study we investigated the clinical features and neural antibody profile of patients with neuromyotonia and cramp-fasciculation syndrome to assess the diagnostic value of serological testing. Available sera from adult patients with electromyography-defined neuromyotonia and cramp-fasciculation syndrome were tested for neural antibodies by indirect immunofluorescence on mouse brain sections and live cell-based assays. Forty patients were included, 14 with neuromyotonia and 26 with cramp-fasciculation syndrome. Neural antibodies were detected in 10/10 neuromyotonia sera, most commonly against contactin-associated protein 2 (7/10, 70%), and in 1/20 (5%) cramp-fasciculation syndrome sera. Clinical myokymia, hyperhidrosis, and paresthesia or neuropathic pain were more common in neuromyotonia and mostly associated with contactin-associated protein 2 antibodies. Central nervous system involvement was present in 4/14 (29%) neuromyotonia patients. A tumor was detected in 13/14 (93%) neuromyotonia patients (thymoma, 13), and in 4/26 (15%) with cramp-fasciculation syndrome (thymoma, 1; other neoplasms, 3). Twenty-one/27 (78%) patients achieved a significant improvement or complete remission. Our findings highlight clinical, neurophysiological and serological clues that can be useful in the diagnosis of neuromyotonia and cramp-fasciculation syndrome. Antibody testing is valuable for neuromyotonia diagnosis, while its usefulness in cramp-fasciculation syndrome confirmation is limited.

Function of KCNQ2 channels at nodes of Ranvier of lumbar spinal ventral nerves of rats

Previous immunohistochemical studies have shown the expression of KCNQ2 channels at nodes of Ranvier (NRs) of myelinated nerves. However, functions of these channels at NRs remain elusive. In the present study, we addressed this issue by directly applying whole-cell patch-clamp recordings at NRs of rat lumbar spinal ventral nerves in ex vivo preparations. We show that depolarizing voltages evoke large non-inactivating outward currents at NRs, which are partially inhibited by KCNQ channel blocker linopirdine and potentiated by KCNQ channel activator retigabine. Furthermore, linopirdine significantly alters intrinsic electrophysiological properties of NRs to depolarize resting membrane potential, increase input resistance, prolong AP width, reduce AP threshold, and decrease AP amplitude. On the other hand, retigabine significantly decreases input resistance and increases AP rheobase at NRs. Moreover, linopirdine increases excitability at NRs by converting single AP firing into multiple AP firing at many NRs. Saltatory conduction velocity is significantly reduced by retigabine, and AP success rate at high stimulation frequency is significantly increased by linopirdine. Collectively, KCNQ2 channels play a significant role in regulating intrinsic electrophysiological properties and saltatory conduction at NRs of motor nerve fibers of rats. These findings may provide insights into how the loss-of-function mutation in KCNQ2 channels can lead to neuromuscular disorders in human patients.

EMG-Phänomene peripherer motorisch axonaler Übererregbarkeit

Bei der Nadel-Elektromyographie (EMG) besitzen Phänomene der vermehrten Erregbarkeit von Muskelfasern und von motorischen Axonen Bedeutung für die Diagnostik neuromuskulärer Erkrankungen. Zur motorisch axonalen Übererregbarkeit gehören spontane Phänomene wie Faszikulationen, spontane kontinuierliche Einzelentladungen der motorischen Einheit (SKEME), Myokymien, neuromyotone Entladungsserien und Krampi. Ferner gehören dazu reizinduzierte Phänomene wie manche A-Wellen, reizinduzierte komplex repetitive Entladungen oder tetanischen Spasmen bei Elektrolytstörungen. In der vorliegenden Übersicht wird der Kenntnisstand zu den verschiedenen Phänomenen motorisch axonaler Übererregbarkeit referiert. Ein Schwerpunkt liegt dabei auf den SKEME als neuem Mitglied der Gruppe spontaner Potenziale aus dem motorischen Axon.

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.

From Collar to Coccyx: Truncal Movement Disorders: A Clinical Review

Background

Movement disorders affecting the trunk remain a diagnostic challenge even for experienced clinicians. However, despite being common and debilitating, truncal movement disorders are rarely discussed and poorly reviewed in the medical literature.

Objectives

To review common movement disorders affecting the trunk and provide an approach for clinicians based on the truncal region involved (shoulder, chest, diaphragm, abdomen, pelvis, and axial disorders). For each disorder, clinical presentation, etiologic differential diagnosis, and "clinical clues" are discussed.

Conclusion

This review provides a clinically focused, practical approach to truncal movement disorders, which will be helpful for physicians in everyday practice.

Electrodiagnostic Assessment of Hyperexcitable Nerve Disorders

Peripheral nerve hyperexcitability (PNH) typically presents with complaints of muscle twitching, cramps, and muscle stiffness. Symptoms and signs indicating central and/or autonomic nervous system dysfunction also may be reported. An electroclinical spectrum exists, spanning from the milder cramp-fasciculation syndrome to more severe syndromes characterized by continuous muscle fiber activity. It is important to recognize that PNH may be an autoimmune phenomenon associated with antibodies targeting proteins of the voltage-gated potassium channel-complex and, in some patients, a paraneoplastic phenomenon. Symptomatic therapies include medicines that reduce neuronal excitability and in severe disease immunomodulatory treatments may be indicated.

Demystifying the spontaneous phenomena of motor hyperexcitability

Possessing a discrete functional repertoire, the anterior horn cell can be in one of two electrophysiological states: on or off. Usually under tight regulatory control by the central nervous system, a hierarchical network of these specialist neurons ensures muscular strength is coordinated, gradated and adaptable. However, spontaneous activation of these cells and their axons can result in abnormal muscular twitching. The muscular twitch is the common building block of several distinct clinical patterns, namely fasciculation, myokymia and neuromyotonia. When attempting to distinguish these entities electromyographically, their unique temporal and morphological profiles must be appreciated. Detection and quantification of burst duration, firing frequency, multiplet patterns and amplitude are informative. A common feature is their persistence during sleep. In this review, we explain the accepted terminology used to describe the spontaneous phenomena of motor hyperexcitability, highlighting potential pitfalls amidst a bemusing and complex collection of overlapping terms. We outline the relevance of these findings within the context of disease, principally amyotrophic lateral sclerosis, Isaacs syndrome and Morvan syndrome. In addition, we highlight the use of high-density surface electromyography, suggesting that more widespread use of this non-invasive technique is likely to provide an enhanced understanding of these motor hyperexcitability syndromes.

International veterinary canine dyskinesia task force ECVN consensus statement: Terminology and classification

Movement disorders are a heterogeneous group of clinical syndromes in humans and animals characterized by involuntary movements without changes in consciousness. Canine movement disorders broadly include tremors, peripheral nerve hyperexcitability disorders, paroxysmal dyskinesia, and dystonia. Of these, canine paroxysmal dyskinesias remain one of the more difficult to identify and characterize in dogs. Canine paroxysmal dyskinesias include an array of movement disorders in which there is a recurrent episode of abnormal, involuntary, movement. In this consensus statement, we recommend standard terminology for describing the various movement disorders with an emphasis on paroxysmal dyskinesia, as well as a preliminary classification and clinical approach to reporting cases. In the clinical approach to movement disorders, we recommend categorizing movements into hyperkinetic vs hypokinetic, paroxysmal vs persistent, exercise‐induced vs not related to exercise, using a detailed description of movements using the recommended terminology presented here, differentiating movement disorders vs other differential diagnoses, and then finally, determining whether the paroxysmal dyskinesia is due to either inherited or acquired etiologies. This consensus statement represents a starting point for consistent reporting of clinical descriptions and terminology associated with canine movement disorders, with additional focus on paroxysmal dyskinesia. With consistent reporting and identification of additional genetic mutations responsible for these disorders, our understanding of the phenotype, genotype, and pathophysiology will continue to develop and inform further modification of these recommendations.

Sustained atypical myokymia of the abductor pollicis brevis with a focal slowing of the median nerve motor axons at the wrist

OBJECTIVE

We report a case of sustained atypical myokymia associated with short bursts of neuromyotonic discharges involving the abductor pollicis brevis (APB) muscle and describe a useful way of detecting a focal slowing involving a small number of median nerve motor fibers with a concentric needle using the filter setting for single fiber electromyography (EMG).

METHODS AND RESULTS

A 62-year-old woman developed right thumb twitches at regular interval of 1.7-3.3 s (0.6-0.3 Hz), which continued for more than four months. Muscle twitches remained the same during altered hand position, psychological stress, or sleep. A concentric needle inserted in the active zone of the APB muscle revealed myokymic bursts with a characteristic of neuromyotonic discharges. Inching study, stimulating at 5 mm increment along the median nerve and recording with a concentric needle using a filter setting for single fiber EMG, revealed a focal slowing of the motor fibers at a point 5-10 mm distal from the distal crease of the wrist, an entrapment site occasionally seen in the carpal tunnel syndrome. One injection of botulinum toxin type A eliminated the myokymia, which then recurred two and a half years later, showing less prominent muscle twitches.

CONCLUSIONS

Sustained atypical myokymia seen in our case represented bursts of neuromyotonic discharges originated from a focal demyelinating lesion involving a few median nerve motor fibers.

How can neurophysiological studies help with movement disorders characterization in clinical practice? A review

BACKGROUND

Neurophysiological studies are ancillary tools to better understand the features and nature of movement disorders. Electromyography (EMG), together with electroencephalography (EEG) and accelerometer, can be used to evaluate a hypo and hyperkinetic spectrum of movements. Specific techniques can be applied to better characterize the phenomenology, help distinguish functional from organic origin and assess the most probable site of the movement generator in the nervous system.

OBJECTIVE

We intend to provide an update for clinicians on helpful neurophysiological tools to assess movement disorders in clinical practice.

METHODS

Non-systematic review of the literature published up to June 2019.

RESULTS

A diversity of protocols was found and described. These include EMG analyses to define dystonia, myoclonus, myokymia, myorhythmia, and painful legs moving toes pattern; EMG in combination with accelerometer to study tremor; and EEG-EMG to study myoclonus. Also, indirect measures of cortical and brainstem excitability help to describe and diagnose abnormal physiology in Parkinson's disease, atypical parkinsonism, dystonia, and myoclonus.

CONCLUSION

These studies can be helpful for the diagnosis and are usually underutilized in neurological practice.

Peripheral nerve hyperexcitability

Neuromyotonic and myokymic discharges are abnormal electrical muscular discharges caused by ectopic discharges from motor axons and represent the hallmarks of peripheral nerve hyperexcitability. Neuromyotonic discharges are specific for peripheral nerve hyperexcitability syndromes, whereas myokymic discharges may occur either focally or in a more generalized fashion in many other peripheral nerve disorders. Isaacs syndrome and Morvan syndrome are rare acquired peripheral nerve hyperexcitability disorders that share common clinical features and are often associated with elevated voltage-gated potassium channel-complex antibodies. Central nervous system symptomatology is more common in Morvan syndrome, which also overlaps with limbic encephalitis. Cramp-fasciculation syndrome, a more common syndrome, may represent a milder form of peripheral nerve hyperexcitability. Peripheral nerve hyperexcitability syndromes should be distinguished from stiff person syndrome, myotonic disorders, and rippling muscle disease. When severe, Isaacs syndrome and Morvan syndrome may be disabling but often respond to membrane-stabilizing drugs and immunomodulatory treatments. The electrophysiologic features of these disorders are described.

Neuromyotonia in a horse

This article describes the clinical and electromyographic findings of neuromyotonia in a 19‐month‐old male crossbred Quarter Horse that presented with stiffness and muscle asymmetry in the hind limbs as well as sacrococcygeal, paravertebral, and gluteal myokymia. An electromyographic study showed spontaneous continuous muscle fiber activity with high‐frequency discharges, fibrillations, positive sharp waves, fasciculation potentials, and complex repetitive discharges. Histological examination of the gluteal muscle showed a mixed neurogenic and myopathic pattern. The findings are consistent with neuromyotonia.

Paroxysmal motor disorders: expanding phenotypes lead to coalescing genotypes

Paroxysmal movement disorders encompass varied motor phenomena. Less recognized features and wide phenotypic and genotypic heterogeneity are impediments to straightforward molecular diagnosis. We describe a family with episodic ataxia type 1, initially mis‐characterized as paroxysmal dystonia to illustrate this diagnostic challenge. We summarize clinical features in affected individuals to highlight underappreciated aspects and provide comprehensive phenotypic description of the rare familial KCNA1 mutation. Delayed diagnosis in this family is emblematic of the broader challenge of diagnosing other paroxysmal motor disorders. We summarize genotypic and phenotypic overlap and provide a suggested diagnostic algorithm for approaching patients with these conditions.

Voltage-gated potassium channel-complex autoimmunity and associated clinical syndromes

Voltage-gated potassium channel (VGKC)-complex antibodies are defined by the radioimmunoprecipitation of Kv1 potassium channel subunits from brain tissue extracts and were initially discovered in patients with peripheral nerve hyperexcitability (PNH). Subsequently, they were found in patients with PNH plus psychosis, insomnia, and dysautonomia, collectively termed Morvan's syndrome (MoS), and in a limbic encephalopathy (LE) with prominent amnesia and frequent seizures. Most recently, they have been described in patients with pure epilepsies, especially in patients with the novel and distinctive semiology termed faciobrachial dystonic seizures (FBDS). In each of these conditions, there is a close correlation between clinical measures and antibody levels. The VGKC-complex is a group of proteins that are strongly associated in situ and after extraction in mild detergent. Two major targets of the autoantibodies are leucine-rich glioma-inactivated 1 (LGI1) and contactin-associated protein 2 (CASPR2). The patients with PNH or MoS are most likely to have CASPR2 antibodies, whereas LGI1 antibodies are found characteristically in patients with FBDS and LE. Crucially, each of these conditions has a good response to immunotherapies, often corticosteroids and plasma exchange, although optimal regimes require further study. VGKC-complex antibodies have also been described in neuropathic pain syndromes, chronic epilepsies, a polyradiculopathy in porcine abattoir workers, and some children with status epilepticus. Increasingly, however, the antigenic targets in these patients are not defined and in some cases the antibodies may be secondary rather than the primary cause. Future serologic studies should define all the antigenic components of the VGKC-complex, and further inform mechanisms of antibody pathogenicity and related inflammation.

Peripheral nerve hyperexcitability syndromes

Peripheral nerve hyperexcitability (PNH) syndromes can be subclassified as primary and secondary. The main primary PNH syndromes are neuromyotonia, cramp-fasciculation syndrome (CFS), and Morvan's syndrome, which cause widespread symptoms and signs without the association of an evident peripheral nerve disease. Their major symptoms are muscle twitching and stiffness, which differ only in severity between neuromyotonia and CFS. Cramps, pseudomyotonia, hyperhidrosis, and some other autonomic abnormalities, as well as mild positive sensory phenomena, can be seen in several patients. Symptoms reflecting the involvement of the central nervous system occur in Morvan's syndrome. Secondary PNH syndromes are generally seen in patients with focal or diffuse diseases affecting the peripheral nervous system. The PNH-related symptoms and signs are generally found incidentally during clinical or electrodiagnostic examinations. The electrophysiological findings that are very useful in the diagnosis of PNH are myokymic and neuromyotonic discharges in needle electromyography along with some additional indicators of increased nerve fiber excitability. Based on clinicopathological and etiological associations, PNH syndromes can also be classified as immune mediated, genetic, and those caused by other miscellaneous factors. There has been an increasing awareness on the role of voltage-gated potassium channel complex autoimmunity in primary PNH pathogenesis. Then again, a long list of toxic compounds and genetic factors has also been implicated in development of PNH. The management of primary PNH syndromes comprises symptomatic treatment with anticonvulsant drugs, immune modulation if necessary, and treatment of possible associated dysimmune and/or malignant conditions.

Classification of involuntary movements in dogs: Tremors and twitches

This review focuses on important new findings in the field of involuntary movements (IM) in dogs and illustrates the importance of developing a clear classification tool for diagnosing tremor and twitches. Developments over the last decade have changed our understanding of IM and highlight several caveats in the current tremor classification. Given the ambiguous association between tremor phenomenology and tremor aetiology, a more cautious definition of tremors based on clinical assessment is required. An algorithm for the characterisation of tremors is presented herein. The classification of tremors is based on the distinction between tremors that occur at rest and tremors that are action-related; tremors associated with action are divided into postural or kinetic. Controversial issues are outlined and thus reflect the open questions that are yet to be answered from an evidence base of peer-reviewed published literature. Peripheral nerve hyper-excitability (PNH; cramps and twitches) may manifest as fasciculations, myokymia, neuromyotonia, cramps, tetany and tetanus. It is anticipated that as we learn more about the aetiology and pathogenesis of IMs, future revisions to the classification will be needed. It is therefore the intent of this work to stimulate discussions and thus contribute to the development of IM research.

Use of botulinum toxin type A for the treatment of radiation therapy-induced myokymia and neuromyotonia in a dog

CASE DESCRIPTION A 5-year-old castrated male Maltese was evaluated for intermittent clinical signs of muscle cramping and abnormal movements of the skin of the right pelvic limb at the site where an infiltrative lipoma had twice been resected. After the second surgery, the surgical field was treated with radiation therapy (RT). The clinical signs developed approximately 14 months after completion of RT. CLINICAL FINDINGS When clinical signs were present, the right biceps femoris and semitendinosus muscles in the area that received RT were firm and had frequently visible contractions, and the skin overlying those muscles had episodic vermiform movements. Electromyography of those muscles revealed abnormal spontaneous activity with characteristics consistent with myokymic discharges and neuromyotonia. Magnetic resonance imaging of the affected leg revealed no evidence of tumor regrowth. The myokymia and neuromyotonia were considered secondary to RT. TREATMENT AND OUTCOME 4 U of Clostridium botulinum toxin type A (BoNT-A) neurotoxin complex was injected into the affected muscles at each of 6 sites twice during a 24-hour period (ie, 48 U of BoNT-A were administered). The clinical signs were completely resolved 10 days after BoNT-A treatment and were controlled by repeated BoNT-A treatment every 3 to 4 months for > 1 year. CLINICAL RELEVANCE To our knowledge, this is the first report of myokymia and neuromyotonia secondary to RT in a dog. For the dog of this report, injection of BoNT-A into the affected muscles was safe, effective, and easy to perform.

The episodic ataxia type 1 mutation I262T alters voltage-dependent gating and disrupts protein biosynthesis of human Kv1.1 potassium channels

Voltage-gated potassium (Kv) channels are essential for setting neuronal membrane excitability. Mutations in human Kv1.1 channels are linked to episodic ataxia type 1 (EA1). The EA1-associated mutation I262T was identified from a patient with atypical phenotypes. Although a previous report has characterized its suppression effect, several key questions regarding the impact of the I262T mutation on Kv1.1 as well as other members of the Kv1 subfamily remain unanswered. Herein we show that the dominant-negative effect of I262T on Kv1.1 current expression is not reversed by co-expression with Kvβ1.1 or Kvβ2 subunits. Biochemical examinations indicate that I262T displays enhanced protein degradation and impedes membrane trafficking of Kv1.1 wild-type subunits. I262T appears to be the first EA1 mutation directly associated with impaired protein stability. Further functional analyses demonstrate that I262T changes the voltage-dependent activation and Kvβ1.1-mediated inactivation, uncouples inactivation from activation gating, and decelerates the kinetics of cumulative inactivation of Kv1.1 channels. I262T also exerts similar dominant effects on the gating of Kv1.2 and Kv1.4 channels. Together our data suggest that I262T confers altered channel gating and reduced functional expression of Kv1 channels, which may account for some of the phenotypes of the EA1 patient.

Isaacs syndrome: A review

Isaacs syndrome is a peripheral nerve hyperexcitability (PNH) syndrome that presents as continuous motor activity. Clinical findings include cramps, fasciculations, and myokymia. Electrodiagnosis plays a key role in diagnosis by demonstrating after-discharges on nerve conduction studies, and fasciculation potentials, myokymic discharges, neuromyotonic discharges, and other types of abnormal spontaneous activity on needle examination. Etiopathogenesis involves the interaction of genetic, autoimmune, and paraneoplastic factors, which requires a broad-ranging evaluation for underlying causes. Initial treatment is symptomatic, but immune therapy is often needed and can be effective. The purpose of this review is to describe the syndrome and its pathogenesis, assist the reader in evaluating patients with suspected Isaacs syndrome and distinguishing it from other disorders of PNH, and suggest an approach to management, including both symptomatic and immunomodulating therapy.

Myokymia and neuromyotonia in veterinary medicine: a comparison with peripheral nerve hyperexcitability syndrome in humans

Involuntary muscle hyperactivity can result from muscle or peripheral nerve hyperexcitability or central nervous system dysfunction. In humans, diseases causing hyperexcitability of peripheral nerves are grouped together under the term 'peripheral nerve hyperexcitability' (PNH). Hyperexcitability of the peripheral motor nerve can result into five different phenotypic main variants, i.e. fasciculations, myokymia, neuromyotonia, cramps and tetany, each with their own clinical and electromyographic characteristics. This review focuses on the most commonly described expressions of PNH in veterinary medicine, i.e. myokymia and neuromyotonia, in particular in young Jack Russell terriers. Data from 58 veterinary cases with generalized myokymia and neuromyotonia were analyzed, including unpublished treatment and follow-up data on eight Jack Russell terriers from a previous study and seven additional Jack Russell terriers. A dysfunction of the potassium channel or its associated proteins has been found in many human syndromes characterized by PNH, in particular in generalized myokymia and neuromyotonia, and is suspected to occur in veterinary medicine. Potential pathomechanisms of potassium channel dysfunction leading to signs of PNH are broad and include genetic mutations, antibody-mediated attack or ion channel maldistribution due to axonal degeneration or demyelination. A more accurate classification of the different PNH syndromes will facilitate a more rapid diagnosis and guide further research into natural occurring PNH in animals.

Paraneoplastic Isaacs' syndrome: a case series and review of the literature

OBJECTIVE

Isaacs' syndrome is a rare disease resulting from hyperexcitability of peripheral nerves causing continuous muscle fiber activity characterized by muscle twitching and stiffness at rest and delayed muscle relaxation after voluntary contraction. Our objective was to discuss the relationship of Isaacs' syndrome to paraneoplastic syndromes as reported in the available literature and in 3 patients evaluated at our academic medical center.

METHODS

We review the literature on Isaacs' syndrome and describe 3 patients in whom Isaacs' syndrome heralded underlying malignancy or benign neoplasm, including their presenting symptoms, electrophysiologic findings, and laboratory and pathology results.

RESULTS

In all 3 cases, clinical and electrodiagnostic testing was suggestive of Isaacs' syndrome. Two patients tested positive, and one was negative for voltage-gated potassium channel antibodies. Two of the patients developed malignant tumors, that is, one was diagnosed with metastatic thymoma and one with lymphoplasmacytic lymphoma, ranging from 6 months to 1 year after the diagnosis of Isaacs' syndrome. One patient was diagnosed with a spinal cord hemangioblastoma 5 years after he was diagnosed with Isaacs' syndrome.

CONCLUSIONS

Our case series highlights the association of Isaacs' syndrome with a variety of neoplasms both malignant and benign. Our report also underscores the fact that Isaacs' syndrome may be diagnosed several years before a neoplasm is discovered. In our cases, Isaacs' syndrome overlapped with other neuromuscular disorders, that is, myasthenia gravis in a patient with thymoma and chronic inflammatory demyelinating polyneuropathy in a patient with lymphoplasmacytic lymphoma with paraproteinemia. To our knowledge, this is the first report of an association between Isaacs' syndrome with lymphoplasmacytic lymphoma and a spinal cord hemangioblastoma.

Made for "anchorin": Kv7.2/7.3 (KCNQ2/KCNQ3) channels and the modulation of neuronal excitability in vertebrate axons

Kv7.2 and Kv7.3 (encoded by KCNQ2 and KCNQ3) are homologous subunits forming a widely expressed neuronal voltage-gated K(+) (Kv) channel. Hypomorphic mutations in either KCNQ2 or KCNQ3 cause a highly penetrant, though transient, human phenotype-epilepsy during the first months of life. Some KCNQ2 mutations also cause involuntary muscle rippling, or myokymia, which is indicative of motoneuron axon hyperexcitability. Kv7.2 and Kv7.3 are concentrated at axonal initial segments (AISs), and at nodes of Ranvier in the central and peripheral nervous system. Kv7.2 and Kv7.3 share a novel ∼80 residue C-terminal domain bearing an "anchor" motif, which interacts with ankyrin-G and is required for channel AIS (and likely, nodal) localization. This domain includes the sequence IAEGES/TDTD, which is analogous (not homologous) to the ankyrin-G interaction motif of voltage-gated Na(+) (Na(V)) channels. The KCNQ subfamily is evolutionarily ancient, with two genes (KCNQ1 and KCNQ5) persisting as orthologues in extant bilaterian animals from worm to man. However, KCNQ2 and KCNQ3 arose much more recently, in the interval between the divergence of extant jawless and jawed vertebrates. This is precisely the interval during which myelin and saltatory conduction evolved. The natural selection for KCNQ2 and KCNQ3 appears to hinge on these subunits' unique ability to be coordinately localized with Na(V) channels by ankyrin-G, and the resulting enhancement in the reliability of neuronal excitability.

Myokymia and neuromyotonia in 37 Jack Russell terriers

The clinical and clinicopathological characteristics, treatment and outcome of vermicular muscle contractions (myokymia) and generalized muscle stiffness (neuromyotonia) in 37 Jack Russell terriers were evaluated retrospectively. Thirty dogs were affected by both disorders, whereas seven were presented with myokymia and never developed neuromyotonia. Clinical signs started at the mean age of 8 months. Except for signs of myokymia and neuromyotonia, clinical and neurological examination was normal in all dogs. Thirty dogs demonstrated typical signs of hereditary ataxia. Changes in serum chemistry included increased creatine kinase, aspartate aminotransferase and alanine aminotransferase concentrations. Electromyographic abnormalities, especially in muscles showing macroscopically visible myokymia, consisted of semirhythmic bursts of doublet, triplet, or multiplet discharges of a single motor unit. The amplitudes varied between 80 μV and 1 mV and occurred with an interburst frequency between 10 and 40 Hz and an intraburst frequency between 150 and 280 Hz. Most dogs were treated with a sodium channel blocker with variable results. Seven dogs died (most likely because of hyperthermia) or were euthanased during a neuromyotonic attack; 15 dogs were euthanased due to worsening of clinical signs, or lack of or no long-lasting effect of medication, and three were euthanased for unknown or unrelated reasons. Nine dogs were lost to follow-up and three were still alive 5-10.5 years after the start of clinical signs. In conclusion, young Jack Russell terriers with myokymia and neuromyotonia should undergo a complete blood and electrophysiological examination. Long-term prognosis is not favourable.

Clinical and electrophysiological characterization of myokymia and neuromyotonia in Jack Russell Terriers

BACKGROUND

Generalized myokymia and neuromyotonia (M/NM) in Jack Russell Terriers (JRTs) is related to peripheral nerve hyperexcitability syndrome in humans, a symptom complex resulting from diverse etiologies.

OBJECTIVE

Clinical and electrodiagnostic evaluation is used to narrow the list of possible etiological diagnoses in JRTs with M/NM.

ANIMALS

Nine healthy JRTs and 8 affected JRTs.

METHODS

A prospective study was conducted comparing clinical and electrophysiological characteristics in 8 JRTs affected by M/NM with 9 healthy JRT controls.

RESULTS

All affected dogs except 1 had clinical signs typical of hereditary ataxia (HA). In 6 dogs, neuromyotonic discharges were recorded during electromyogram. Motor nerve conduction studies showed an axonal neuropathy in only 1 affected dog. Compared with controls, brainstem auditory-evoked potentials (BAEP) showed prolonged latencies (P<.05) accompanied by the disappearance of wave components in 3 dogs. Onset latencies of tibial sensory-evoked potentials (SEP) recorded at the lumbar intervertebral level were delayed in the affected group (P<.001). The BAEP and SEP results of the only neuromyotonic dog without ataxia were normal.

CONCLUSIONS AND CLINICAL IMPORTANCE

The BAEP and spinal SEP abnormalities observed in JRTs with M/NM were associated with the presence of HA. Therefore, these electrophysiological findings presumably arise from the neurodegenerative changes characterizing HA and do not directly elucidate the pathogenesis of M/NM. An underlying neuronal ion channel dysfunction is thought to be the cause of M/NM in JRTs.

Neuromuscular hyperexcitability associated with acetylcholine receptor antibodies in a child

This report describes the clinical and electrophysiological features of an 8-year-old boy with autoimmune neuromuscular hyperexcitability. He presented with features of lower extremity pain, stiffness, and rippling muscles. The diagnosis was made by electromyography and supported by the presence of an antibody directed against nicotinic acetylcholine receptors. His symptoms responded to immunomodulatory treatment with intravenous immunoglobulin. Response to immunomodulatory therapy has not previously been described in the pediatric population with autoimmune neuromuscular hyperexcitability. This case highlights that this is a treatable condition.

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.

Fatal encephalitis in a patient with refractory celiac disease presenting with myorhythmia and carpal spasm

We report the case of a woman with refractory celiac disease who developed abnormal spontaneous movements of the extremities and face consistent with myorhythmia. Investigation led to a diagnosis of encephalitis, confirmed by postmortem examination. The movements were likely caused by nonparaneoplastic encephalitis associated with refractory celiac disease. Etiologic and diagnostic considerations and treatment options are discussed.

Neuromyotonia

Neuromyotonia is a rare condition of spontaneous and continuous muscle fibre activity of peripheral nerve origin. It represents the more severe phenotype of peripheral nerve hyperexcitability, and when acquired is often associated with antibodies to voltage-gated potassium channels. There are no specific published electromyographic or clinical diagnostic criteria for this disorder. This review highlights the classical clinical, electrophysiological and immunological features of this disorder from what is currently known in the literature to date, and also from the author's own patients' studies. Neuromyotonia is best classified as a moderately severe disorder of peripheral nerve hyperexcitability, with electromyographic features of spontaneous, continuous, irregularly occurring doublet, or multiplet single motor unit (or partial motor unit) discharges, firing at a high intraburst frequency (30-300Hz). Invariably, patients develop persistent muscle contraction, often worse following exercise. About 40% of patients with acquired neuromyotonia will have detectable voltage-gated potassium-channel antibodies. Clinical, electrophysiological and immunological measurements are important in defining the phenotype of neuromyotonia, and other, milder forms of peripheral nerve hyperexcitability.

Myokymia in axonal disorders

Myokymic discharges are spontaneous bursts of individual motor units occurring rhythmically or semirhythmically. Myokymia is typically seen in demyelinating disorders. We present 5 patients with myokymia in axonal disorders, including a case of myokymia after Botox injection, which has not been reported previously.

Myokymia and neuromyotonia in a cat

A 6-year-old spayed female domestic shorthair cat was examined because of a 2-week history of rhythmic muscle movements. Physical examination revealed thoracic limb rigidity, contracture of the carpi, generalized muscle atrophy, and rhythmic rippling of the muscles of all 4 limbs. Results of a CBC and serum biochemistry profile were unremarkable other than high creatine kinase activity. Electromyography revealed unique high-frequency discharges, including rhythmic bursts of single motor unit potentials appearing as doublets (myokymia) and more prolonged bursts of nonrhythmic motor unit potentials with characteristic waning amplitudes (neuromyotonia). Histologic examination of muscle biopsy specimens revealed noninflammatory necrotizing myopathy with regeneration. The cat did not respond to treatment with carbamazepine or prednisone but improved rapidly after treatment with phenytoin was initiated. Six months after initial examination, electromyography revealed a substantial decrease in the amount of spontaneous activity in previously affected muscles. However, the myokymic and neuromyotonic discharges were still present, albeit with a substantial decrease in frequency.

KCNQ2 is a nodal K+ channel

Mutations in the gene encoding the K+ channel KCNQ2 cause neonatal epilepsy and myokymia, indicating that KCNQ2 regulates the excitability of CNS neurons and motor axons, respectively. We show here that KCNQ2 channels are functional components of axon initial segments and nodes of Ranvier, colocalizing with ankyrin-G and voltage-dependent Na+ channels throughout the CNS and PNS. Retigabine, which opens KCNQ channels, diminishes axonal excitability. Linopirdine, which blocks KCNQ channels, prolongs the repolarization of the action potential in neonatal nerves. The clustering of KCNQ2 at nodes and initial segments lags that of ankyrin-G during development, and both ankyrin-G and KCNQ2 can be coimmunoprecipitated in the brain. KCNQ3 is also a component of some initial segments and nodes in the brain. The diminished activity of mutant KCNQ2 channels accounts for neonatal epilepsy and myokymia; the cellular locus of these effects may be axonal initial segments and nodes.

Myokymia, neuromyotonia, dermatomyositis, and voltage-gated K+ channel antibodies

A young woman presented with facial myokymia in association with dermatomyositis. There was no evidence of peripheral neuropathy. Needle electromyography showed prominent myokymic discharges and brief neuromyotonic discharges in addition to many small-amplitude, short-duration motor unit potentials. Myokymia and dermatomyositis both responded to immunosuppressive treatment. The presence of antibodies to voltage-gated potassium channels and the association with dermatomyositis indicated an autoimmune cause for myokymia, which may have been due to reversible peripheral nerve hyperexcitability.

Isaacs' syndrome as a potassium channelopathy of the nerve

Isaacs' syndrome (acquired neuromyotonia) is an antibody-mediated potassium channel disorder (channelopathy). The target channel proteins of the antigens are voltage-gated potassium channels (VGKCs), especially dendrotoxin-sensitive fast potassium channels. The suppression of voltage-gated outward K(+) current by antibodies induces hyperexcitability of the peripheral nerve. Patch clamp studies show that antibodies may not directly block the kinetics of VGKCs but may decrease channel density. Electrophysiological, pharmacological, and immunological findings indicate that the site of origin of spontaneous discharges is principally in the distal portion of the motor nerve and/or within the terminal arborization. The spectrum of potassium channelopathies is expanding. The existence of antibodies against VGKCs should be considered in patients who present with generalized nerve hyperexcitability of undetermined etiology.