A rare form of painful nondystrophic myotonia

OBJECTIVE

In this paper we report a painful nondystrophic myotonia which has not been previously described. Pain is a rare symptom in myotonia. We report a myotonic disorder in a 34-year-old woman and her 14-year-old daughter. Painful cramps occur during and after exercise in the mother, and both patients can demonstrate unusual contractions in the tongue. In the present study we try to evaluate the mechanisms behind the unique finding of trains of high amplitude of positive waves, not seen in the earlier known myotonic conditions.

METHODS

Clinical investigations and electromyography with single and dual channel recordings and muscle morphometry were performed.

RESULTS

The electromyographic recordings reveal positive waves, fibrillation potentials and myotonic discharges. In addition, extraordinary findings were made of trains of high frequency positive potentials with very high amplitudes and with conduction block along the muscle fibres.

CONCLUSIONS

In this new form of myotonia with likely dominant heredity, the specific finding of trains of high amplitude positive waves indicates ephaptic transmission within bundles of neighbouring muscle fibres.

Characterization of two new dominant ClC-1 channel mutations associated with myotonia

Voltage-gated ClC-1 chloride channels encoded by the CLCN1 gene have a major role in setting the membrane potential in skeletal muscle. More than 60 CLCN1 mutations have been associated with myotonia congenita. These mutations are traditionally classified as recessive (Becker's disease) or dominant (Thomsen's disease). In this study, we have electrophysiologically characterized two new dominant ClC-1 mutations, thereby elucidating the observed phenotype in patients. The two ClC-1 mutants M128V and E193K were identified, and the DNA was isolated from patients and subsequently expressed in Xenopus laevis oocytes for electrophysiological characterization. Both ClC-1 mutants, M128V and E193K, showed a large rightward shift in the current-voltage relationship. In addition, the activation kinetics were slowed in the ClC-1 M128V mutant, as compared to the wild-type ClC-1. Interestingly, ClC-1 E193K revealed a change in reversal potential compared to wild-type channels. This finding supports the notion that the E193 amino acid is an important determinant in the selectivity filter of the human ClC-1 channel. The electrophysiological behavior of both mutants demonstrates a severe reduction in ClC-1 channel conductance under physiologically relevant membrane potentials. These studies thereby explain the molecular background for the observed myotonia in patients.

A novel alteration of muscle chloride channel gating in myotonia levior

Mutations in the voltage-dependent skeletal muscle chloride channel, ClC-1, result in dominant or recessive myotonia congenita. The Q552R mutation causes a variant of dominant myotonia with a milder phenotype, myotonia levior. To characterise the functional properties of this mutation, homodimeric mutant and heterodimeric wild-type (WT) mutant channels were expressed in tsA201 cells and studied using the whole-cell recording technique. Q552R ClC-1 mutants formed functional channels with normal ion conduction but altered gating properties. Mutant channels were activated by membrane depolarisation, with a voltage dependence of activation that was shifted by more than +90 mV compared to WT channels. Q552R channels were also activated by hyperpolarisation, and this process was dependent upon the intracellular chloride concentration ([Cl(-)](i)). Together, these alterations resulted in a substantial reduction in the open probability at -85 mV at a physiological [Cl(-)](i). Heterodimeric WT-Q552R channels did not exhibit hyperpolarisation-activated gating transitions. As was the case for WT channels, activation occurred upon depolarisation, but the activation curve was shifted by 28 mV to more positive potentials. The functional properties of heterodimeric channels suggest a weakly dominant effect, a finding that correlates with the inheritance pattern and symptom profile of myotonia levior.

Skeletal muscle channelopathies

Ion channelopathies have common clinical features, recurrent patterns of mutations, and almost predictable mechanisms of pathogenesis. In skeletal muscle, disorders are associated with mutations in voltage-gated Na(+), K(+), Ca(2+), and Cl(-) channels leading to hypoexcitability, causing periodic paralysis and to hyperexcitabilty, resulting in myotonia or susceptibility to malignant hyperthermia.

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.

Increased rigidity of the chiral centre of tocainide favours stereoselectivity and use-dependent block of skeletal muscle Na(+) channels enhancing the antimyotonic activity in vivo

1. Searching for the structural requirements improving the potency and the stereoselectivity of Na(+) channel blockers as antimyotonic agents, new derivatives of tocainide, in which the chiral carbon atom is constrained in a rigid alpha-proline or pyrrolo-imidazolic cycle, were synthesized as pure enantiomers. 2. Their ability to block Na(+) currents, elicited from -100 to -20 mV at 0.3 Hz (tonic block) and 2-10 Hz (use-dependent block) frequencies, was investigated in vitro on single fibres of frog semitendinosus muscle using the vaseline-gap voltage-clamp method. 3. The alpha-proline derivative, To5, was 5 and 21 fold more potent than tocainide in producing tonic and 10 Hz-use-dependent block, respectively. Compared to To5, the presence of one methyl group on the aminic (To6) or amidic (To7) nitrogen atom decreased use-dependence by 2- and 6-times, respectively. When methylene moieties were present on both nitrogen atoms (To8), both tonic and use-dependent block were reduced. 4. Contrarily to tocainide, all proline derivatives were stereoselective in relation to an increased rigidity. A further increase in the molecular rigidity as in pyrrolo-imidazolic derivatives markedly decreased the drug potency with respect to tocainide. 5. Antimyotonic activity, evaluated as the shortening of the time of righting reflexes of myotonic adr/adr mice upon acute drug in vivo administration was 3 fold more effective for R-To5 than for R-Tocainide. 6. Thus, constraining the chiral centre of tocainide in alpha-proline cycle leads to more potent and stereoselective use-dependent Na(+) channel blockers with improved therapeutic potential.

Mice transgenic for the human myotonic dystrophy region with expanded CTG repeats display muscular and brain abnormalities

The autosomal dominant mutation causing myotonic dystrophy (DM1) is a CTG repeat expansion in the 3'-UTR of the DM protein kinase (DMPK) gene. This multisystemic disorder includes myotonia, progressive weakness and wasting of skeletal muscle and extramuscular symptoms such as cataracts, testicular atrophy, endocrine and cognitive dysfunction. The mechanisms underlying its pathogenesis are complex. Recent reports have revealed that DMPK gene haploinsufficiency may account for cardiac conduction defects whereas cataracts may be due to haploinsufficiency of the neighboring gene, the DM-associated homeobox protein (DMAHP or SIX5) gene. Furthermore, mice expressing the CUG expansion in an unrelated mRNA develop myotonia and myopathy, consistent with an RNA gain of function. We demonstrated that transgenic mice carrying the CTG expansion in its human DM1 context (>45 kb) and producing abnormal DMPK mRNA with at least 300 CUG repeats, displayed clinical, histological, molecular and electrophysiological abnormalities in skeletal muscle consistent with those observed in DM1 patients. Like DM1 patients, these transgenic mice show abnormal tau expression in the brain. These results provide further evidence for the RNA trans-dominant effect of the CUG expansion, not only in muscle, but also in brain.

Quantitative myotonia assessment: an experimental protocol

Severe clinical myotonia can be physically disabling and socially impairing but as yet there is no standardized treatment regimen. The aim of our study is to present a protocol to measure myotonia using quantitative muscle assessment measures. The proposed protocol addresses two main issues. Muscle strength is assessed in 8 muscles on the right and on the left using a myometer (QMA, quantitative muscle assessment) and by testing strength manually using the 5-point MRC scale (5 = normal) in 15 muscles on the right and on the left. Grip myotonia is assessed by: (a) measuring 1/2 and 3/4 relaxation times (RT) after maximum voluntary contraction (MVC) using QMA apparatus; (b) functional tests (time to open a fist 10 times, time to open and squeeze the eyes 10 times, time to climb 10 steps starting from a seated position, time to protrude the tongue 10 times, time to step onto a chair 10 times; (c) subjective measures of the severity of myotonia using an arbitrary 4-point scale (0 = absent, 4 = severe); and (d) electromyography (EMG) relaxation times after MVC. Although QMA seems to be a reliable tool to measure myotonia, there are still a number of unsolved issues. Further studies are needed to ensure the ability of QMA to quantify myotonia and to guarantee the reliability of the results for clinical research purposes.

Respiratory abdominal muscle recruitment and chest wall motion in myotonic muscular dystrophy

Abdominal muscles are selectively active in normal subjects during stress and may increase the potential energy for inspiration by reducing the end-expiratory lung volume (EELV). We hypothesized that a similar process would occur in subjects with myotonic muscular dystrophy (MMD), but would be less effective, because of to their weakness and altered chest wall mechanics. Fine-wire electromyography (EMG) of the transversus abdominis (TA), internal oblique (IO), external oblique, and rectus abdominis was recorded in 10 MMD and 10 control subjects. EMG activity, respiratory inductive plethysmography, and gastric pressure were recorded during static pressure measurement and at increasing levels of inspiratory resistance breathing. EELV was reduced and chest wall motion was synchronous only in controls. Although the TA and IO were selectively recruited in both groups, EMG activity of the MMD group was twice that of controls at the same inspiratory pressure. In MMD subjects with mildly reduced forced vital capacity, significant differences can be seen in abdominal muscle recruitment, wall motion, work of breathing, and ventilatory parameters.

A new mutation in a family with cold-aggravated myotonia disrupts Na(+) channel inactivation

OBJECTIVE

To identify the molecular and physiologic abnormality in familial myotonia with cold sensitivity, hypertrophy, and no weakness.

BACKGROUND

Sodium channel mutations were previously identified as the cause of several allelic disorders with varying combinations of myotonia and periodic paralysis. A three-generation family with dominant myotonia aggravated by cooling, but no weakness, was screened for mutations in the skeletal muscle sodium channel alpha-subunit gene (SCN4A).

METHODS

Single-strand conformation polymorphism was used to screen all 24 exons of SCN4A and abnormal conformers were sequenced to confirm the presence of mutations. The functional consequence of a SCN4A mutation was explored by recording sodium currents from human embryonic kidney cells transiently transfected with an expression construct that was mutated to reproduce the genetic defect.

RESULTS

A three-generation Italian family with myotonia is presented, in which a novel SCN4A mutation (leucine 266 substituted by valine, L266V) is identified. This change removes only a single methylene group from the 1,836-amino-acid protein, and is present in a region of the protein previously not known to be critical for channel function (domain I transmembrane segment 5). Electrophysiologic studies of the L266V mutation showed defects in fast inactivation, consistent with other disease-causing SCN4A mutations studied to date. Slow inactivation was not impaired.

CONCLUSIONS

This novel mutation of the sodium channel indicates that a single carbon change in a transmembrane alpha-helix of domain I can alter channel inactivation and cause cold-sensitive myotonia.

Equine muscular dystrophy with myotonia

OBJECTIVES

To describe a case of equine muscular dystrophy with myotonia.

METHODS

A 5-year-old horse presented with hypertrophy and delayed relaxation of the muscles of the hindlimbs from age 2 months. Testicular atrophy developed from 2 years of age. Action and percussion myotonia was associated with weakness in these muscles, and EMG showed diffuse myotonic discharges and myopathic features. Biopsy of the gluteal muscle showed adipose and connective tissue infiltration, marked variation in muscle fibre size, and moth-eaten, ring and whorled fibres.

RESULTS

Injection of apamin, a peptide blocker of calcium-activated potassium channels, which inhibits myotonia in human myotonic dystrophy, was ineffective in blocking myotonic discharges. Discharges promptly abated with 2% lidocaine injection.

CONCLUSIONS

Myotonia in this horse is associated with dystrophic changes similar to human myotonic dystrophy, though there are some pharmacological differences.

The expanding clinical and genetic spectrum of the myotonic dystrophies

Core features of the dominantly inherited myotonic dystrophies are myotonia, muscle weakness and cataract. Classic myotonic dystrophy (Steinert's disease) has been defined as a genetic entity by the underlying CTG repeat expansion on chromosome 19q13.3 (= DM1 locus). Later on, another disorder similar to but different from myotonic dystrophy was described as proximal myotonic myopathy (PROMM). The majority of PROMM families have been linked to a recently discovered locus on chromosome 3q21 (= DM2 locus).--This article analyses the clinical features of 70 patients from 14 German PROMM families linked to the 3q locus. In contrast to Steinert's disease, these patients did not reveal mental deficiency; no congenital type was found; weakness was mainly located in the proximal leg muscles; clinical myotonia was very mild and sometimes absent; episodes of pain occurred. In the majority of patients, the disorder seems to be more benign compared to Steinert's disease. However, life threatening cardiac involvement is possible; rarely, muscle weakness may progress until the patient is bedridden.--Some families with a PROMM-like phenotype do not link to the locus on 3q. The group of the myotonic dystrophies will get new members in the future.

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.

Spectrum of sodium channel disturbances in the nondystrophic myotonias and periodic paralyses

Several heritable forms of myotonia and periodic paralysis are caused by missense mutations in the voltage-gated sodium channel of skeletal muscle. Mutations produce gain-of-function defects, either disrupted inactivation or enhanced activation. Both defects result in too much inward Na current which may either initiate pathologic bursts of action potentials (myotonia) or cause flaccid paralysis by depolarizing fibers to a refractory inexcitable state. Myotonic stiffness and periodic paralysis occur as paroxysmal attacks often triggered by environmental factors such as serum K+, cold, or exercise. Many gaps remain in our understanding of the interactions between genetic predisposition and these environmental influences. Targeted gene manipulation in animals may provide the tools to fill in these gaps.

Homologation of mexiletine alkyl chain and stereoselective blockade of skeletal muscle sodium channels

The optical isomers (-)-(S)- and (+)-(R)-3-(2, 6-dimethylphenoxy)-2-methyl-1-propanamine (Me2), homologues of the antiarrhythmic and antimyotonic drug mexiletine (Mex), were synthesized and assayed as new potential antimyotonic agents. As observed with Mex, Me2 exhibits an enantioselective behaviour. Tests carried out on sodium currents of single muscle fibres of Rana esculenta demonstrated that (-)-(S)- and (+)-(R)-Me2 were less potent than Mex in producing tonic block, but showed a higher use-dependent block. (-)-(S)-Me2 and (-)-(R)-Mex were also used to study the excitability of muscle fibres of myotonic ADR mice, a phenotype of a recessive form of low G(Cl) myotonia. (-)-(S)-Me2 reduced spontaneous discharges and after discharges better than (-)-(R)-Mex in agreement with the use-dependent block of sodium currents.

Increased hindrance on the chiral carbon atom of mexiletine enhances the block of rat skeletal muscle Na+ channels in a model of myotonia induced by ATX

1 The antiarrhythmic drug mexiletine (Mex) is also used against myotonia. Searching for a more efficient drug, a new compound (Me5) was synthesized substituting the methyl group on the chiral carbon atom of Mex by an isopropyl group. Effects of Me5 on Na+ channels were compared to those of Mex in rat skeletal muscle fibres using the cell-attached patch clamp method. 2 Me5 (10 microM) reduced the maximal sodium current (INa) by 29.7+/-4.4 % (n=6) at a frequency of stimulation of 0.3 Hz and 65.7+/-4.4 % (n=6) at 1 Hz. At same concentration (10 microM), Mex was incapable of producing any effect (n=3). Me5 also shifted the steady-state inactivation curves by -7. 9+/-0.9 mV (n=6) at 0.3 Hz and -12.2+/-1.0 mV (n=6) at 1 Hz. 3 In the presence of sea anemone toxin II (ATX; 5 microM), INa decayed more slowly and no longer to zero, providing a model of sodium channel myotonia. The effects of Me5 on peak INa were similar whatever ATX was present or not. Interestingly, Me5 did not modify the INa decay time constant nor the steady-state INa to peak INa ratio. 4 Analysis of ATX-induced late Na+ channel activity shows that Me5 did not affect mean open times and single-channel conductance, thus excluding open channel block property. 5 These results indicate that increasing hindrance on the chiral atom of Mex increases drug potency on wild-type and ATX-induced noninactivating INa and that Me5 might improve the prophylaxis of myotonia.

Potassium current suppression in patients with peripheral nerve hyperexcitability

Acquired neuromyotonia (Isaac's syndrome) is considered to be an autoimmune disease, and the pathomechanism of nerve hyperexcitability in this syndrome is correlated with anti-voltage-gated K(+) channel (VGKC) antibodies. The patch-clamp technique was used to investigate the effects of immunoglobulins from acquired neuromyotonia patients on VGKCs and voltage-gated Na(+) channels in a human neuroblastoma cell line (NB-1). K(+) currents were suppressed in cells that had been co-cultured with acquired neuromyotonia patients' immunoglobulin for 3 days but not for 1 day. The activation and inactivation kinetics of the outward K(+) currents were not altered by these immunoglobulins, nor did the immunoglobulins significantly affect the Na(+) currents. Myokymia or myokymic discharges, with peripheral nerve hyperexcitability, also occur in various neurological disorders such as Guillain-Barré syndrome and idiopathic generalized myokymia without pseudomyotonia. Immuno-globulins from patients with these diseases suppressed K(+) but not Na(+) currents. In addition, in hKv 1.1- and 1.6-transfected CHO (Chinese hamster ovary)-K1 cells, the expressed VGKCs were suppressed by sera from acquired neuromyotonia patients without a change in gating kinetics. Our findings indicate that nerve hyperexcitability is mainly associated with the suppression of voltage-gated K(+) currents with no change in gating kinetics, and that this suppression occurs not only in acquired neuromyotonia but also in Guillain-Barré syndrome and idiopathic generalized myokymia without pseudomyotonia.

Strength-duration properties of peripheral nerve in acquired neuromyotonia

The strength-duration time constant (SDTC) of a myelinated axon is a property of the nodal membrane and is sensitive to changes in membrane potential. Strength-duration time constants for motor axons and cutaneous afferents of the median nerve were measured in 9 patients with acquired neuromyotonia (NMT), a condition of peripheral nerve hyperexcitability, and 15 control patients. Mean motor axon time constants were significantly prolonged (344 +/- 100 micros) in patients compared to healthy controls (264 +/- 34 micros; P = 0.038), but sensory axon time constants were not significantly different. Motor axon time constants were longer than sensory axon time constants in 4 of the patients with neuromyotonia, suggesting that the nodal membrane was depolarized by an ectopic focus at the site of nerve stimulation at the wrist, ionic conductances were altered at the node, or that the size of the node was increased, possibly as a result of immune-mediated damage. The anti-voltage-gated potassium channel antibodies thought to generate peripheral nerve hyperexcitability in acquired neuromyotonia may be indirectly responsible for changes in motor axon nodal membrane properties.