THE PERIODIC PARALYSES: DIFFERENTIAL FEATURES AND PATHOLOGICAL OBSERVATIONS IN PERMANENT MYOPATHIC WEAKNESS

Lower Ca2+ enhances the K+-induced force depression in normal and HyperKPP mouse muscles

Hyperkalemic periodic paralysis (HyperKPP) manifests as stiffness or subclinical myotonic discharges before or during periods of episodic muscle weakness or paralysis. Ingestion of Ca²⁺ alleviates HyperKPP symptoms, but the mechanism is unknown because lowering extracellular [Ca²⁺] ([Ca²⁺]_e) has no effect on force development in normal muscles under normal conditions. Lowering [Ca²⁺]_e, however, is known to increase the inactivation of voltage-gated cation channels, especially when the membrane is depolarized. Two hypotheses were tested: (1) lowering [Ca²⁺]_e depresses force in normal muscles under conditions that depolarize the cell membrane; and (2) HyperKPP muscles have a greater sensitivity to low Ca²⁺-induced force depression because many fibers are depolarized, even at a normal [K⁺]_e. In wild type muscles, lowering [Ca²⁺]_e from 2.4 to 0.3 mM had little effect on tetanic force and membrane excitability at a normal K⁺ concentration of 4.7 mM, whereas it significantly enhanced K⁺-induced depression of force and membrane excitability. In HyperKPP muscles, lowering [Ca²⁺]_e enhanced the K⁺-induced loss of force and membrane excitability not only at elevated [K⁺]_e but also at 4.7 mM K⁺. Lowering [Ca²⁺]_e increased the incidence of generating fast and transient contractures and gave rise to a slower increase in unstimulated force, especially in HyperKPP muscles. Lowering [Ca²⁺]_e reduced the efficacy of salbutamol, a β2 adrenergic receptor agonist and a treatment for HyperKPP, to increase force at elevated [K⁺]_e. Replacing Ca²⁺ by an equivalent concentration of Mg²⁺ neither fully nor consistently reverses the effects of lowering [Ca²⁺]_e. These results suggest that the greater Ca²⁺ sensitivity of HyperKPP muscles primarily relates to (1) a greater effect of Ca²⁺ in depolarized fibers and (2) an increased proportion of depolarized HyperKPP muscle fibers compared with control muscle fibers, even at normal [K⁺]_e.

The Mary Walker effect: Mary Broadfoot Walker

Mary Broadfoot Walker (1888-1974) was the first to demonstrate the 'Mary Walker effect' describing the weakness of other muscle groups following release of the arteriovenous occlusion of an unrelated exercising muscle group in patients with myasthenia gravis, which led to the search for a circulating causative agent for myasthenia gravis. She was the first to clearly demonstrate that strength temporarily improved in patients with myasthenia gravis with physostigmine or Prostigmin (neostigmine). This dramatic treatment response has been erroneously termed the 'Mary Walker effect'. Further, she noted hypokalaemia during attacks of weakness in familial periodic paralysis, pioneering treatment with potassium chloride. Although Mary Walker practiced in a nonacademic setting and trained at a time when women were not allowed to train alongside men, she was the first to convincingly demonstrate three life-changing treatments in the field of neuromuscular medicine, a feat that few physicians of any era can claim.

Channelopathies of skeletal muscle excitability

Familial disorders of skeletal muscle excitability were initially described early in the last century and are now known to be caused by mutations of voltage-gated ion channels. The clinical manifestations are often striking, with an inability to relax after voluntary contraction (myotonia) or transient attacks of severe weakness (periodic paralysis). An essential feature of these disorders is fluctuation of symptoms that are strongly impacted by environmental triggers such as exercise, temperature, or serum K(+) levels. These phenomena have intrigued physiologists for decades, and in the past 25 years the molecular lesions underlying these disorders have been identified and mechanistic studies are providing insights for therapeutic strategies of disease modification. These familial disorders of muscle fiber excitability are "channelopathies" caused by mutations of a chloride channel (ClC-1), sodium channel (NaV1.4), calcium channel (CaV1.1), and several potassium channels (Kir2.1, Kir2.6, and Kir3.4). This review provides a synthesis of the mechanistic connections between functional defects of mutant ion channels, their impact on muscle excitability, how these changes cause clinical phenotypes, and approaches toward therapeutics.

Physiological basis for muscle stiffness and weakness in a knock-in M1592V mouse model of hyperkalemic periodic paralysis

The mechanisms responsible for the onset and progressive worsening of episodic muscle stiffness and weakness in hyperkalemic periodic paralysis (HyperKPP) are not fully understood. Using a knock‐in HyperKPP mouse model harboring the M1592V Na_V1.4 channel mutant, we interrogated changes in physiological defects during the first year, including tetrodotoxin‐sensitive Na⁺ influx, hindlimb electromyographic (EMG) activity and immobility, muscle weakness induced by elevated [K⁺]e, myofiber‐type composition, and myofiber damage. In situ EMG activity was greater in HyperKPP than wild‐type gastrocnemius, whereas spontaneous muscle contractions were observed in vitro. We suggest that both the greater EMG activity and spontaneous contractions are related to periods of hyperexcitability during which fibers generate action potentials by themselves in the absence of any stimulation and that these periods are the cause of the muscle stiffness reported by patients. HyperKPP muscles had a greater sensitivity to the K⁺‐induced force depression than wild‐type muscles. So, an increased interstitial K⁺ concentration locally near subsets of myofibers as a result of the hyperexcitability likely produced partial loss of force rather than complete paralysis. Na_V1.4 channel protein content reached adult level by 3 weeks postnatal in both wild type and HyperKPP and apparent symptoms did not worsen after the first month of age suggesting (i) that the phenotypic behavior of M1592V HyperKPP muscles results from defective function of mutant Na_V1.4 channels rather than other changes in protein expression after the first month and (ii) that the lag in onset during the first decade and the progression of human HyperKPP symptoms during adolescence are a function of Na_V1.4 channel content.

Extracellular Ca2+-induced force restoration in K+-depressed skeletal muscle of the mouse involves an elevation of [K+]i: implications for fatigue

We examined whether a Ca(2+)-K(+) interaction was a potential mechanism operating during fatigue with repeated tetani in isolated mouse muscles. Raising the extracellular Ca(2+) concentration ([Ca(2+)]o) from 1.3 to 10 mM in K(+)-depressed slow-twitch soleus and/or fast-twitch extensor digitorum longus muscles caused the following: 1) increase of intracellular K(+) activity by 20-60 mM (raised intracellular K(+) content, unchanged intracellular fluid volume), so that the K(+)-equilibrium potential increased by ∼10 mV and resting membrane potential repolarized by 5-10 mV; 2) large restoration of action potential amplitude (16-54 mV); 3) considerable recovery of excitable fibers (∼50% total); and 4) restoration of peak force with the peak tetanic force-extracellular K(+) concentration ([K(+)]o) relationship shifting rightward toward higher [K(+)]o. Double-sigmoid curve-fitting to fatigue profiles (125 Hz for 500 ms, every second for 100 s) showed that prior exposure to raised [K(+)]o (7 mM) increased, whereas lowered [K(+)]o (2 mM) decreased, the rate and extent of force loss during the late phase of fatigue (second sigmoid) in soleus, hence implying a K(+) dependence for late fatigue. Prior exposure to 10 mM [Ca(2+)]o slowed late fatigue in both muscle types, but was without effect on the extent of fatigue. These combined findings support our notion that a Ca(2+)-K(+) interaction is plausible during severe fatigue in both muscle types. We speculate that a diminished transsarcolemmal K(+) gradient and lowered [Ca(2+)]o contribute to late fatigue through reduced action potential amplitude and excitability. The raised [Ca(2+)]o-induced slowing of fatigue is likely to be mediated by a higher intracellular K(+) activity, which prolongs the time before stimulation-induced K(+) efflux depolarizes the sarcolemma sufficiently to interfere with action potentials.

Targeted mutation of mouse skeletal muscle sodium channel produces myotonia and potassium-sensitive weakness

Hyperkalemic periodic paralysis (HyperKPP) produces myotonia and attacks of muscle weakness triggered by rest after exercise or by K+ ingestion. We introduced a missense substitution corresponding to a human familial HyperKPP mutation (Met1592Val) into the mouse gene encoding the skeletal muscle voltage-gated Na+ channel NaV1.4. Mice heterozygous for this mutation exhibited prominent myotonia at rest and muscle fiber-type switching to a more oxidative phenotype compared with controls. Isolated mutant extensor digitorum longus muscles were abnormally sensitive to the Na+/K+ pump inhibitor ouabain and exhibited age-dependent changes, including delayed relaxation and altered generation of tetanic force. Moreover, rapid and sustained weakness of isolated mutant muscles was induced when the extracellular K+ concentration was increased from 4 mM to 10 mM, a level observed in the muscle interstitium of humans during exercise. Mutant muscle recovered from stimulation-induced fatigue more slowly than did control muscle, and the extent of recovery was decreased in the presence of high extracellular K+ levels. These findings demonstrate that expression of the Met1592ValNa+ channel in mouse muscle is sufficient to produce important features of HyperKPP, including myotonia, K+-sensitive paralysis, and susceptibility to delayed weakness during recovery from fatigue.

Mutations of sodium channel alpha-subunit genes in Chinese patients with normokalemic periodic paralysis

OBJECTIVE

In this study, we aim to investigate the clinical features and Mutations of sodium channel alpha-subunit (SCN4A) genes in Chinese patients with normokalemic periodic paralysis (normoKPP).

METHODS

Six unrelated Chinese families with normoKPP were analyzed in clinical features. Genomic DNA was extracted from peripheral blood leukocytes and amplified with PCR. We screened all 24 exons of SCN4A gene with denaturing high performance liquid chromatography (DHPLC) technology, and then sequence analysis was performed in those who showed heteroduplex as compared with unaffected controls.

RESULTS

The laboratory tests were within normal ranges. Electromyograms and electrocardiograms were normal. One muscle biopsy was performed with the patient in family 4 after a brief attack of normoKPP. Examination of light microscopy showed no changes, but electronic microscopy showed occasionally degenerating myofibers. The mutations of SCN4A genes were as follows: (1) Met1592Val occurred in family 1. (2) Val-781-Ile occurred with the patient and her father in family 4. (3) Both the patients had a novel mutation g2101a predicting the amino acid exchange Arg675Gln in family 5, which may be a disease-causing mutation.

CONCLUSIONS

In addition to Val-781-Ile and Met1592Val, the mutation g2101a (Arg675Gln) may be the novel mutation of SCN4A genes in Chinese patients with normoKPP.

Genetic disorders of neuromuscular ion channels

Ion channels are complex proteins that span the lipid bilayer of the cell membrane, where they orchestrate the electrical signals necessary for normal function of the central nervous system, peripheral nerve, and both skeletal and cardiac muscle. The role of ion channel defects in the pathogenesis of numerous disorders, many of them neuromuscular, has become increasingly apparent over the last decade. Progress in molecular biology has allowed cloning and expression of genes that encode channel proteins, while comparable advances in biophysics, including patch-clamp electrophysiology and related techniques, have made the study of expressed proteins at the level of single channel molecules possible. Understanding the molecular basis of ion channel function and dysfunction will facilitate both the accurate classification of these disorders and the rational development of specific therapeutic interventions. This review encompasses clinical, genetic, and pathophysiological aspects of ion channels disorders, focusing mainly on those with neuromuscular manifestations.

Thyrotoxic periodic paralysis: two case studies

Thyrotoxic periodic paralysis (TPP) is a disorder that affects a small percentage of patients with hyperthyroidism; the patients are predominantly Asian men. Two typical cases are presented here; both patients came to the emergency room with complaints of sudden onset of weakness. Their symptoms were initially resolved through treatment for acute hypokalemia. Additional tests identified the patients as being thyrotoxic for which condition they underwent thyroid ablation with radioiodine. No further episodes were reported for either patient. Often the underlying cause of the weakness or paralysis is undetected because of the absence of clinical symptoms of hyperthyroidism and because clinicians are unfamiliar with the disorder. It is imperative that TPP be recognized in the differential diagnosis of weakness and that the underlying thyrotoxicity is treated to permanently alleviate the condition. The authors discuss the epidemiology, etiology, clinical manifestations, and treatments of TPP.

[Periodic paralysis. Clinical analysis in 20 patients]

Twenty patients with periodic paralysis were evaluated and the aspects studied included epidemiological data, clinical manifestations, ancillary tests, treatment and evolution. Sixteen patients had the hypokalemic form (5 familiar, 5 sporadic, 5 thyrotoxic and 1 secondary). No patient with the normokalemic form was detected. Predominance of men was found (14 patients), especially in the cases with hyperthyroidism (5 patients). No thyrotoxic patient was of oriental origin. Only 4 patients had the hyperkalemic form (3 familiar, 1 sporadic). Attacks of paralysis began during the first decade in the hyperkalemic form and up to the third decade in the hypokalemic. In both forms the attacks occurred preferentially in the morning with rest after exercise being the most important precipitating factor. Seventy five percent of the hyperkalemic patients referred brief attacks (< 12 hours). Longer attacks were referred by 43% of the hypokalemic patients. The majority of the attacks manifested with a generalized weakness mainly in legs, and its frequency was variable. Creatinokinase was evaluated in 10 patients and 8 of them had levels that varied from 1.1 to 5 times normal. Electromyography was done in 6 patients and myotonic phenomenon was the only abnormality detected in 2 patients. Carbonic anhydrase inhibitors, especially acetazolamide, were used for prophylactic treatment in 9 patients with good results in all. Although periodic paralysis may be considered a benign disease we found respiratory distress in 5 patients, permanent myopathy in 1, electrocardiographic abnormalities during crises in 4; death during paralysis occurred in 2. Therefore correct diagnosis and immediate treatment are crucial. This study shows that hyperthyroidism is an important cause of periodic paralysis in our country, even in non oriental patients. Hence endocrine investigation is mandatory since this kind of periodic paralysis will only be abated after return to the euthyroid state.

Muscle pathology correlates with permanent weakness in hypokalemic periodic paralysis: a case report

We present a morphological follow-up in a case of familial hypokalemic periodic paralysis with progressive weakness. At age 12 years muscle biopsy revealed mild vacuolar changes. Seventeen years later, after the patient had developed permanent weakness, light and electron microscopy disclosed tubular aggregates in about 15% of the fibers and medium-grade myopathic alterations. We describe correlation of muscle pathology with permanent weakness in hypokalemic periodic paralysis.

Improvement of muscle strength in familial hypokalaemic periodic paralysis with acetazolamide

A double blind cross-over study of eight patients with familial hypokalaemic periodic paralysis was made to assess the influence of acetazolamide on muscle strength. All patients had a reduced interictal muscle fibre conduction velocity. Five patients had no attacks at the time of the study. One patient withdrew from the study because of an adverse reaction. The muscle strength of 11 muscle groups was measured with a hand-held dynamometer. The sum of force improved significantly in the seven patients (mean increase: 17%, p less than 0.05; 95% confidence interval: 7.2-26.8%). Endurance tests showed an improvement in the performance of 30 full kneebends. Surface EMG measurement showed no change in the muscle fibre conduction velocity or power spectra during treatment. The integrated EMG showed a (non significant) mean increase of 21%.

A new syndrome: angiotensin-converting enzyme dysfunction syndrome: differential diagnosis and pathogenesis--case reports

The authors previously reported a new syndrome, angiotensin-converting enzyme dysfunction syndrome (ACEDS), which is clinically characterized by mild systemic hypertension, a hypokalemic alkalosis, and hyperreninism with a high concentration of angiotensin-I (ANG-I), a normal angiotensin-II (ANG-II) value, and a normal aldosterone level. In the present study, they investigated the diagnosis and differentiation of diseases concomitant with hyperreninism, such as ACEDS, Bartter's syndrome, familial periodic paralysis, and renovascular hypertension treated with captopril for two months, and discussed the pathogenesis of ACEDS.

Hypokalemic periodic paralysis with unusual responses to acetazolamide and sympathomimetics

Five members in three generations of a family were affected by an illness that had many clinical features of the hypokalemic form of periodic paralysis (HPP). The serum potassium was either moderately reduced or normal during attacks, and there was no evidence of myotonia or cold-intolerance. All of the patients improved to a variable degree with oral potassium supplements, and 3 responded favorably to triamterene. The usually beneficial drug acetazolamide, however, invariably caused weakness in these patients, an effect previously described in only one other family with HPP. In addition, amphetamine-like sympathomimetic drugs effectively aborted or prevented paralysis in several members. Muscle biopsy in two patients revealed some unusual features, and electromyography showed myopathic potentials. There was no evidence of diabetes. The urine electrolyte concentrations during glucose tolerance tests, however, were different from those previously reported in HPP. This family may represent a variant form of HPP.

Paramyotonia congenita: successful treatment with tocainide. Clinical and electrophysiologic findings in seven patients

Seven patients with paramyotonia congenita (PC) from two families were studied. Voluntary exercise of the hand muscles was performed at different hand temperatures, both before and after treatment with tocainide. All patients developed stiffness, prolonged weakness, and small compound muscle action potentials (CMAPs) following exercise; the temperature at which this occurred was individually different. Two patients with PC and associated episodes of generalized weakness underwent potassium loading. A prolonged exercise test was performed both immediately before and 90 minutes after K-loading. Exercise-induced weakness and CMAP-decline occurred only with high serum K levels. Thiazide treatment in these two patients was ineffective. All seven patients responded well to tocainide. Treatment response and side effects were dose-dependent. Good clinical improvement has been maintained in all patients for more than 6 months, with relatively small doses of tocainide (400-1200 mg/day).

Electromyography in infants and children

Electromyography (EMG) is of proven value in the diagnosis of acute and chronic neuromuscular diseases in infants and children. When this technique is combined with nerve conduction studies, including repetitive nerve stimulation studies, it is often possible upon completion of the studies to identify the disorder as one of nerve, neuromuscular junction, or muscle. The purpose of this article is to review the principles and techniques of EMG in infants and children and to describe the EMG findings in several neuromuscular disorders.

Hypokalemic periodic paralysis: in vitro investigation of muscle fiber membrane parameters

To study the mechanism of attacks in familial hypokalemic paralysis, we recorded resting membrane potentials, action potentials, current-voltage relationships, and isometric forces in intercostal muscle fibers from three patients. In normal extracellular medium, the resting potential was reduced, but membrane conductance was not different from control. Excitability was reduced and the action potentials had no overshoot. On exposure to a 1-mM potassium solution, with or without insulin, the cells depolarized to about -50 mV, and became inexcitable. Over the tested membrane potential range from -120 to -40 mV, the slope conductance in the 1-mM potassium solution was not different from that of control fibers in a 1-mM potassium solution. In particular, the potassium component conductance was not reduced. Depolarized fibers could not be completely repolarized by returning to a 3.5-mM potassium solution. An experimentally induced transient shift of the chloride equilibrium potential to a highly negative value caused stable repolarization. Paralysis could also be induced by replacement of extracellular chloride with an impermanent anion, a treatment which causes myotonia in healthy fibers. It was concluded that the basic defects are a reduced excitability and an increased sodium conductance, and that these defects are aggravated on reduction of the extracellular potassium concentration.

Periodic paralysis with cardiac arrhythmia

In 1963, Klein et al. first described two girls with normokalemic periodic paralysis and cardiac arrhythmia. We have observed a 15-year-old girl with cardiac arrhythmia and normokalemia but with some features of hyperkalemic, periodic paralysis. The patient showed a waddling gait and Gower's sign. She had atrophy of the proximal muscles and deep tendon reflexes were reduced. Her ECG showed bigeminy with multifocal premature ventricular contractions. At the start of an episode of muscle weakness, the serum potassium concentration rose from 3.1 to 4.4 mEq/l. Muscle weakness was not provoked by intravenous administration of 45 g glucose or of 72 g glucose followed by 10 units of regular insulin. Muscle weakness was evoked by the ingestion of 4 g potassium chloride. During the provoked muscle weakness, the ECG showed normal sinus rhythm temporarily.

Treatment of "permanent" muscle weakness in familial Hypokalemic Periodic Paralysis

Three patients with Hypokalemic Periodic Paralysis (HOPP)-associated progressive interattack muscle weakness, who became unresponsive or worsened by acetazolamide, responded favorably to dichlorophenamide, a more potent carbonic anhydrase inhibitor. Dichlorophenamide in single-blind placebo-controlled trials, considerably improved functional strength in two of the patients and had a moderate but definite effect in the third. Muscle groups graded 4/5 (MRC scale)returned to normal; very weak (0-3/5) atrophic muscles, improved to a minor degree. In one patient with acetazolamide-resistant paralytic attacks, dichlorophenamide also diminished the frequency and severity of the acute attacks. Dichlorophenamide had, in the present study, less effect than acetazolamide in reducing serum HCO3(-) and elevating Cl-. Its effectiveness may be related to the degree of sensitivity of certain HOPP patients to alterations of Cl- and/or HCO3(-) serum levels or to a different action of the drug unrelated to carbonic anhydrase inhibition or acidosis. Dichlorophenamide should be considered as an alternate to acetazolamide in the treatment of patients with HOPP-associated interattack muscle weakness who have become unresponsive or worsened by acetazolamide.

Subsequent morphological changes in periodic paralysis. A study of seven cases

Muscle biopsies were studied in seven patients with various types of periodic paralysis. A sequence for the histopathological feature is suggested: 1. Histology may be normal early in the disease. 2. Vacuolation is found during the active phase characterized by frequent attacks. 3. Tubular aggregates appear later when the frequency of the attacks decreases. 4. Degenerative changes are correlated with the presence of permanent myopathic weakness.

[Thyrotoxic periodic paralysis. Report of a case]

A case of thyrotoxic periodic paralysis based on clinical grounds, laboratory data and therapeutic response as well is reported. The authors comment on the differential diagnosis with the most frequent types of periodic paralysis. The importance of a correct diagnosis and treatment as early as possible is stressed as to prevent further development of permanent paralysis due to irreversible degenerative myofibril changes as stated in literature.

Cardiac dysfunction in a patient with familial hypokalemic periodic paralysis

A 19-year-old white man with familial hypokalemic periodic paralysis developed evidence of cardiac dysfunction during a episode of flaccid paralysis. This consisted of elevated total creatine phosphokinase (CPK), an increased myocardial fraction of CPK (myocardial band), alteration in the lactic dehydrogenase isoenzyme pattern, severe bradycardia, and evidence of left ventricular dysfunction. These findings, in conjunction with selected cases from the literature, suggest the possibility that cardiomyopathy may be a heretofore unrecognized complication of this disorder.

Hypokalemic myopathy due to chronic alcoholism

A patient with hypokalemic myopathy occurring in the context of chronic alcoholism was reported. A 56-year-old male patient, heavy drinker for 20 years, complained of marked weakness and acutely developing pains in his limbs. The principal clinical findings were weakness and tenderness of the proximal limbs and girdle muscle. He was unable to lift his head or any extremities from the bed. Deep tendon reflexes were diminished, but not absent. There was no sensory disturbance except for muscle tenderness. These clinical manifestations disappeared gradually by abstinence from drinking, and potassium administration therapy, and the patient recovered completely on the 26th day after onset. On the day after admission (8th day), serum potassium value was 2.2 mEq/L, and serum CPK activity was 4270 IU. The ECG pattern was consistent with a diagnosis of low potassium content in serum, and the EMG pattern was consistent with a diagnosis of myopathy. These electrophysiological findings had a tendency to recover from this pattern to normal range correspondingly with clinical improvement. The repeated muscle biopsies showed that vacuolation, hyaline degeneration and significant phagocyte infiltration were observed in the muscle on the 9th day after the onset of muscle weakness, and that these pathological findings disappeared almost completely three weeks later. The frequently repeated examinations of potassium content and CPK activity in sera showed that there was a close correlation between these biochemical abnormalities and clinical improvement. The pathogenesis of alcoholic myopathy and significance of CPK abnormality in chronic alcoholism were discussed.

Carcinoma with multiple ectopic hormone secretion and associated myopathy

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