Neuromyotonia in mice with hereditary myelinopathies

A dystonia-like movement disorder with brain and spinal neuronal defects is caused by mutation of the mouse laminin β1 subunit, Lamb1

A new mutant mouse (lamb1t) exhibits intermittent dystonic hindlimb movements and postures when awake, and hyperextension when asleep. Experiments showed co-contraction of opposing muscle groups, and indicated that symptoms depended on the interaction of brain and spinal cord. SNP mapping and exome sequencing identified the dominant causative mutation in the Lamb1 gene. Laminins are extracellular matrix proteins, widely expressed but also known to be important in synapse structure and plasticity. In accordance, awake recording in the cerebellum detected abnormal output from a circuit of two Lamb1-expressing neurons, Purkinje cells and their deep cerebellar nucleus targets, during abnormal postures. We propose that dystonia-like symptoms result from lapses in descending inhibition, exposing excess activity in intrinsic spinal circuits that coordinate muscles. The mouse is a new model for testing how dysfunction in the CNS causes specific abnormal movements and postures.

Lack of neuropathy-related phenotypes in hint1 knockout mice

Mutations in HINT1, the gene encoding histidine triad nucleotide-binding protein 1 (HINT1), cause a recessively inherited peripheral neuropathy that primarily involves motor dysfunction and is usually associated with neuromyotonia (i.e. prolonged muscle contraction resulting from hyperexcitability of peripheral nerves). Because these mutations are hypothesized to cause loss of function, we analyzed Hint1 knockout mice for their relevance as a disease model. Mice lacking Hint1 appeared normal and yielded normal behavioral test results or motor performance, although they moved more slowly and for a smaller fraction of time in an open-field arena than wild-type mice. Muscles, neuromuscular junctions, and nodes of Ranvier were anatomically normal and did not show evidence of degeneration or regeneration. Axon numbers and myelination in peripheral nerves were normal at ages 4 and 13 months. Axons were slightly smaller than those in wild-type mice at age 4 months, but this did not cause a decrease in conduction velocity, and no differences in axon diameters were detected at 13 months. With electromyography, we were unable to detect neuromyotonia even after using supraphysiologic stimuli and stressors such as reduced temperature or 3,4-diaminopyridine to block potassium channels. Therefore, we conclude that Hint1 knockout mice may be useful for studying the biochemical activities of HINT1, but these mice do not provide a disease model or a means for investigating the basis of HINT1-associated neuropathy and neuromyotonia.

Na(v)1.8 channelopathy in mutant mice deficient for myelin protein zero is detrimental to motor axons

Myelin protein zero mutations were found to produce Charcot-Marie-Tooth disease phenotypes with various degrees of myelin impairment and axonal loss, ranging from the mild 'demyelinating' adult form to severe and early onset forms. Protein zero deficient homozygous mice ( ) show a severe and progressive dysmyelinating neuropathy from birth with compromised myelin compaction, hypomyelination and distal axonal degeneration. A previous study using immunofluorescence showed that motor nerves deficient of myelin protein zero upregulate the Na(V)1.8 voltage gated sodium channel isoform, which is normally present only in restricted populations of sensory axons. The aim of this study was to investigate the function of motor axons in protein zero-deficient mice with particular emphasis on ectopic Na(V)1.8 voltage gated sodium channel. We combined 'threshold tracking' excitability studies with conventional nerve conduction studies, behavioural studies using rotor-rod measurements, and histological measures to assess membrane dysfunction and its progression in protein zero deficient homozygous mutants as compared with age-matched wild-type controls. The involvement of Na(V)1.8 was investigated by pharmacologic block using the subtype-selective Na(V)1.8 blocker A-803467 and chronically in Na(V)1.8 knock-outs. We found that in the context of dysmyelination, abnormal potassium ion currents and membrane depolarization, the ectopic Na(V)1.8 channels further impair the motor axon excitability in protein zero deficient homozygous mutants to an extent that precipitates conduction failure in severely affected axons. Our data suggest that a Na(V)1.8 channelopathy contributed to the poor motor function of protein zero deficient homozygous mutants, and that the conduction failure was associated with partially reversible reduction of the electrically evoked muscle response and of the clinical function as indicated by the partial recovery of function at rotor-rod measurements. As a consequence of these findings of partially reversible dysfunction, we propose that the Na(V)1.8 voltage gated sodium channel should be considered as a novel therapeutic target for Charcot-Marie-Tooth disease.

Atypical presentation of acute neurotoxicity secondary to oxaliplatin

Neurotoxicity is the main and dose-limiting toxicity of oxaliplatin. It may produce two different syndromes, acute and chronic. We describe here a case of a patient with an acute syndrome with the particularity of affecting only contralateral hemibody to arm of infusion. A 62-year-old female diagnosed with stage IV colon cancer, underwent palliative treatment with combination of oxaliplatin (130 mg/m 2 on day 1), capecitabine (1.250 mg/m2 bid on days 1 to 14 every 3 weeks), and bevacizumab. Thirty minutes after cycle 1 oxaliplatin infusion, which was into the left arm, she experienced right hemibody paresthesia with muscle cramping of her right calf. She associated dysphonia and painful jamming sensation in her right upper limb with difficulty to release grip. She noted also undulating movements under the skin of her right lower extremity. She was unable to stand or walk. She was given intravenous magnesium sulfate and calcium gluconate and after 3 h all her symptoms were solved. Subsequent doses were reduced by 25% and the infusions were prolonged to 3 h and the patient tolerated well except minimal paresthesia in her right hand lasting few minutes.

Kinematic and electromyographic tools for characterizing movement disorders in mice

Increasing interest in rodent models for movement disorders has led to an increasing need for more accurate and precise methods for both delineating the nature of abnormal movements and measuring their severity. These studies describe application of simultaneous high-speed video kinematics with multichannel electromyography (EMG) to characterize the movement disorder exhibited by tottering mutant mice. These mice provide a uniquely valuable model, because they exhibit paroxysmal dystonia superimposed on mild baseline ataxia, permitting the examination of these two different problems within the same animals. At baseline with mild ataxia, the mutants exhibited poorly coordinated movements with increased variation of stance and swing times, and slower spontaneous walking velocities. The corresponding EMG showed reduced mean amplitudes of biceps femoris and vastus lateralis, and poorly modulated EMG activities during the step cycle. Attacks of paroxysmal dystonia were preceded by trains of EMG bursts with doublets and triplets simultaneously in the biceps femoris and vastus lateralis followed by more sustained coactivation. These EMG characteristics are consistent with the clinical phenomenology of the motor phenotype of tottering mice as a baseline of mild ataxia with intermittent attacks of paroxysmal dystonia. The EMG characteristics of ataxia and dystonia in the tottering mice also are consistent with EMG studies of other ataxic or dystonic animals and humans. These studies provide insights into how these methods can be used for delineating movement disorders in mice and for how they may be compared with similar disorders of humans.

Axonal ion channels from bench to bedside: a translational neuroscience perspective

Over recent decades, the development of specialised techniques such as patch clamping and site-directed mutagenesis have established the contribution of neuronal ion channel dysfunction to the pathophysiology of common neurological conditions including epilepsy, multiple sclerosis, spinal cord injury, peripheral neuropathy, episodic ataxia, amyotrophic lateral sclerosis and neuropathic pain. Recently, these insights from in vitro studies have been translated into the clinical realm. In keeping with this progress, novel clinical axonal excitability techniques have been developed to provide information related to the activity of a variety of ion channels, energy-dependent pumps and ion exchange processes activated during impulse conduction in peripheral axons. These non-invasive techniques have been extensively applied to the study of the biophysical properties of human peripheral nerves in vivo and have provided important insights into axonal ion channel function in health and disease. This review will provide a translational perspective, focusing on an overview of the investigational method, the clinical utility in assessing the biophysical basis of ectopic symptom generation in peripheral nerve disease and a review of the major findings of excitability studies in acquired and inherited neurological disease states.

Successful amelioration of oxaliplatin-induced hyperexcitability syndrome with the antiepileptic pregabalin in a patient with pancreatic cancer

BACKGROUND

Oxaliplatin, a platinum derivative used in the treatment of gastrointestinal cancers, has been associated with sensory neuropathies and, more infrequently, a neuromyotonia-like hyperexcitability syndrome. We present a case of hyperexcitability syndrome that developed during the treatment of pancreatic cancer with oxaliplatin and gemcitabine (GEMOX) that was successfully treated with pregabalin.

CASE PRESENTATION

A 54-year-old woman was undergoing chemotherapy with gemcitabine and oxaliplatin (GEMOX) for stage II-B pancreatic adenocarcinoma. On the third day of her fourth cycle, she presented with twitching of eyelids and tremors of hands. This twitching started bilaterally on the eyelids, followed by teeth jittering, hand shaking, and slurring of speech. A thorough neurological exam revealed no abnormalities except increased tone of both hands-she had difficulty opening her hand after closing it for a hand-grip. She was given a dose of 1 g of IV magnesium sulfate and 1 g of IV calcium gluconate, and 50 mg of IV diphenhydramine. In addition to reassurance, pregabalin was prescribed for these myotonic symptoms at a dosage of 50 mg by mouth three times daily. Improvement occurred in these symptoms within 12 h and she was almost asymptomatic within 72 h.

CONCLUSION

Oxaliplatin causes a unique spectrum of acute neurological toxicities that have not been observed in patients receiving either cisplatin or carboplatin. Clinically, sensory alterations are most prominent, particularly cold-induced and perioral paresthesias. Other symptoms, such as cramps, jaw stiffness, voice changes, ptosis, and visual field changes suggest that motor nerves or muscles may also be involved (hyperexcitability). Hyperexcitability syndrome, distinct from cold-induced paresthesias and sensory neuropathy, is a rare complication of oxaliplatin chemotherapy; and up to date no pharmacotherapy has been successful in treating these symptoms. This is the first report of the successful amelioration of this syndrome with the antiepileptic pregabalin.

Treadmill locomotion in normal mice-Step related multi-channel EMG profiles of thigh muscles

Mouse models are increasingly used in current research on motor disorders. In mice, the myoelectrical activation of thigh muscles during locomotion has not yet, however, been investigated in depth. Especially intramuscular coordination has hardly been clarified. Therefore, the aims of this study were to characterize myoelectrical activity in the vastus lateralis (VL) and the biceps femoris (BF) muscle of the healthy mouse for reference purposes. The VL and the BF muscles of 12 healthy mice performing a total of 1985 steps during treadmill locomotion were investigated with two subcutaneous arrays each incorporating four electrodes. Eight-channel EMG was recorded simultaneously with high-speed videography. The EMG curves of each step were rectified and smoothed by calculating root mean square (RMS) profiles and then time-normalized for comparisons within and between animals. The EMG-activity of both muscles increased during late swing phase. The VL activity rose steeply and peaked during mid-stance phase, while the biceps activity reached a plateau during early stance phase. With increasing gait velocity, stance time decreased. The increase in gait velocity was also associated with greater EMG amplitudes. The results suggest that the BF lifts the lower hind leg during swing phase and stabilizes the leg during stance, while the VL bears the weight of the body during the stance phase.

A surface EMG multi-electrode technique for characterizing muscle activation patterns in mice during treadmill locomotion

In mice a new method for 2x4-channel surface electromyography (EMG) recordings of the vastus lateralis and biceps femoris muscles during locomotion on a treadmill with varying speed is presented. The approach involves high-speed-videography (sampling interval 2.5 ms) in concert with the application of chronically implanted surface EMG multi-electrodes (EMG sampling rate 4000 Hz, frequency range 10-700 Hz). The recordings are started 2 days after surgery and finished 2 weeks after surgery. During the whole investigation period EMG recordings of both muscles have been possible. The monopolar EMG activities recorded by the electrode-arrays and the bipolar EMG signals derived from the monopolar activities permit an evaluation of the extent of myo-electrical activation in larger regions of both muscles and co-ordination between the flexor and extensor muscles. Bipolar EMG signals indicate propagation of activities along the muscle fibers and a slight effect of non-propagating signal components. Thus, the cross talk between these muscles is small and does not influence the evaluation of the EMG results. The resolution of the simultaneously recorded synchronized data allows a precise temporal correlation of kinematic and EMG parameters.

[Isaacs' syndrome. Diagnosis and differential diagnosis of neuromyotonia]

Neuromyotonia is a clinical and electrophysiological syndrome of spontaneous muscle fiber activity due to hyperexcitability of peripheral nerve origin causing generalised, visible myokymia and muscular cramps. Electromyography shows abnormal doublet and triplet discharges of high intraburst frequency as well as myokymic and neuromyotonic discharges. Fasciculations and fibrillation potentials are common. Most commonly, neuromyotonia is an acquired immune-mediated disorder (Isaacs' syndrome) showing elevated antibody levels against presynaptic, voltage-gated, potassium channels. Some of these patients have additional autonomic (hyperhidrosis) and/or CNS symptoms similar to those from limbic encephalitis (referred to then as Morvan's syndrome). We report on a patient with Isaacs' syndrome and discuss the clinical and electrophysiological features, pathophysiology, diagnosis, and differential diagnosis of diseases with peripheral nerve hyperexcitability.

Altered expression of ion channel isoforms at the node of Ranvier in P0-deficient myelin mutants

To elucidate the impact of myelinating Schwann cells on the molecular architecture of the node of Ranvier, we investigated the nodal expression of voltage-gated sodium channel (VGSC) isoforms and the localization of paranodal and juxtaparanodal membrane proteins in a severely affected Schwann cell mutant, the mouse deficient in myelin protein zero (P0). The abnormal myelin formation and compaction was associated with immature nodal cluster types of VGSC. Most strikingly, P0-deficient motor nerves displayed an ectopic nodal expression of the Na(v)1.8 isoform, where it is coexpressed with the ubiquitous Na(v)1.6 channel. Furthermore, Caspr was distributed asymmetrically or was even absent in the mutant nerve fibers. The potassium channel K(v)1.2 and Caspr2 were not confined to juxtaparanodes, but often protruding into the paranodes. Thus, deficiency of P0 leads to dysregulation of nodal VGSC isoforms and to altered localization of paranodal and juxtaparanodal components of the nodal complex.

Acute oxaliplatin-induced peripheral nerve hyperexcitability

PURPOSE

Oxaliplatin is a novel platinum compound with clinical activity in several malignancies. Neurotoxicity is dose-limiting and occurs in two distinct forms, an acute neurologic symptom complex that occurs within hours or days of therapy and a chronic, cumulative sensory neuropathy.

PATIENTS AND METHODS

Patients were treated in a phase I study designed to establish the maximum-tolerated dose of capecitabine given with oxaliplatin. Because of the unusual neurosensory toxicity of oxaliplatin, detailed neurologic examination, needle electromyography (EMG), and nerve conduction studies (NCS) were performed before and the day after oxaliplatin in a subset of 13 patients. Carbamazepine therapy was tried in 12 additional patients to determine whether the neurologic effects might be relieved.

RESULTS

All patients experienced acute, reversible neurotoxicities with oxaliplatin. Symptoms included paresthesias, dysesthesias, cold hypersensitivity, jaw pain, eye pain, pain in the arm used for drug infusion, ptosis, leg cramps, and visual and voice changes. Serial EMG and NCS revealed striking signs of hyperexcitability in motor nerves after oxaliplatin. In patients who achieved therapeutic levels, carbamazepine did not alter the clinical or electromyographic abnormalities.

CONCLUSION

The acute neurotoxicity seen with oxaliplatin is characterized by peripheral-nerve hyperexcitability, and the findings are similar to the clinical manifestations of neuromyotonia. Carbamezepine, which provides symptomatic relief in acquired neuromytonia, did not seem to be beneficial. Efforts to identify a successful neuroprotectant strategy would have a major impact on improving patient quality of life and the ability to deliver full doses of oxaliplatin.

The spectrum of ectopic motor nerve behavior: from fasciculations to neuromyotonia

BACKGROUND

Ectopic impulses in motor nerves generate clinically and electromyographically detectable activity in muscle. These discharges occur in the form of isolated fasciculations, as persistent muscle activity in the form of myokymia and neuromyotonia, or in fulminant contractions of individual muscles in the form of a cramp. In the last 20 years, new studies have helped establish the relationship of fasciculations with cramps and neuromyotonia and have identified common pathophysiologic mechanisms.

REVIEW SUMMARY

Current evidence suggests that both cramps and fasciculations originate primarily in the most distal motor nerve terminals. In this portion of the axon, the overlying Schwann cells do not form a myelin sheath and the blood-nerve barrier is relatively porous. The terminal axon is studded with receptors to monitor the release of neurotransmitters. Under certain stresses, reinnervation, ionic imbalances, motor nerve disease, or pharmacologic challenge, ectopic impulses arise and create visible, but sporadic fasciculations. Other circumstances, including muscle shortening and dehydration, give rise to more frequent and localized fasciculations that can erupt into a painful muscle cramp. The most unusual motor ectopic phenomena involves rapidly recurrent discharges in multiple motor nerves, giving rise to grouped fasciculations, including myokymia and neuromyotonia. Recent studies have implicated toxins, and autoimmune and genetic mechanisms in the generation of neuromyotonic and myokymic syndromes. Plasma exchange, carbamazepine, and phenytoin have proven helpful in select cases.

CONCLUSION

These findings indicate that motor nerve hyperexcitability is a fruitful subject of clinical and laboratory investigation and that treatment based on underlying mechanisms proves beneficial.

Neuromyotonia: autoimmune pathogenesis and response to immune modulating therapy

BACKGROUND

Neuromyotonia (NMT) has been postulated to be an autoimmune channelopathy, probably by affecting voltage gated potassium channels (VGKC) leading to excitation and abnormal discharges [Sinha et al., Lancet 338 (1991) 75].

OBJECTIVE

To report three patients with NMT who had other associated immune-mediated conditions, i.e., myasthenia gravis, thymoma and various types of peripheral neuropathies. One patient had peripheral neuropathy and involvement of pre- and post-synaptic neuromuscular junction.

RESULTS

All three patients had evidence of polyneuropathy and neuromyotonic discharges on electrodiagnostic studies. Elevated acetylcholine receptor antibodies were noted in all patients and malignant thymoma was found in two patients with metastasis. All three patients showed moderate to marked response to plasma exchange.

CONCLUSIONS

These findings strongly suggest a humoral autoimmune pathogenesis of NMT, probably by K(+) channel involvement, affecting acetylcholine quantal release and postsynaptic membrane. Clinicians should be aware of this association of immune-mediated conditions in NMT patients and marked improvement with plasma exchange.