Acetazolamide-induced weakness in paramyotonia congenita

The diagnosis and treatment of myotonic disorders

Myotonia is a defining clinical symptom and sign common to a relatively small group of muscle diseases, including the myotonic dystrophies and the nondystrophic myotonic disorders. Myotonia can be observed on clinical examination, as can its electrical correlate, myotonic discharges, on electrodiagnostic testing. Research interest in the myotonic disorders continues to expand rapidly, which justifies a review of the scientific bases, clinical manifestations, and numerous therapeutic approaches associated with these disorders. We review the pathomechanisms of myotonia, the clinical features of the dystrophic and nondystrophic myotonic disorders, and the diagnostic approach and treatment options for patients with symptomatic myotonia.

Primary periodic paralyses

OBJECTIVE

To review the current knowledge about primary periodic paralyses (PPs).

RESULTS

Periodic paralyses are a heterogeneous group of disorders, clinically characterized by episodes of flaccid muscle weakness, occurring at irregular intervals. PPs are divided into primary (hereditary) and secondary (acquired) forms of which the secondary PPs are much more common than the primary PPs. Primary PPs are due to mutations in genes encoding for subunits of channel proteins of the skeletal muscle membrane, such as the muscular sodium, potassium or calcium channels, or the SCL4A1 protein. Primary PPs include entities such as hyperkalemic PP, hypokalemic PP, paramyotonia congenita von Eulenburg, Andersen's syndrome, thyrotoxic PP, distal renal tubular acidosis, X-linked episodic muscle weakness syndrome and congenital myasthenic syndromes. Attacks of weakness or myotonia may be triggered or enhanced by vigorous exercise, cold, potassium-rich food, emotional stress, drugs such as glucocorticosteroids, insulin or diuretics, or pregnancy. Depending on the pathomechanism, episodes of weakness may respond to mild exercise, ingestion of potassium, carbohydrates, salbutamol, calcium gluconate, thiazide diuretics, carboanhydrase inhibitors, such as acetazolamide or dichlorphenamine, and episodes may be prevented by avoidance of potassium-rich food, or drugs, which increase serum potassium.

CONCLUSION

This review presents and discusses current knowledge and recent advances in the etiology, molecular genetics, genotype-phenotype correlations, pathogenesis, diagnosis and treatment of primary PPs.

[Eulenburg's paramyotonia congenita]

INTRODUCTION

Paramyotonia congenita is an autosomal dominant sodium channelopathy, caused by mutations in gene coding for muscle voltage-gated sodium channel alpha subunit.

CASE REPORT

We report the case of a 38-year-old man who described since childhood muscle stiffness with attacks ok weakness induced by two provocative stimuli: cold exposure and exercise. It primarily concerned eyelids and hands, occasionally limbs. Family history suggested an autosomal dominant mode of transmission. Clinical examination revealed myotonia at the thenar eminence percussion. Generalized myotonic discharges were observed on electromyography. Molecular diagnosis reported an Arg1448Cys mutation in exon 24 in gene coding for muscle voltage-gated sodium channel alpha subunit (SCN4A) in chromosome 17.

CONCLUSION

Paramyotonia congenita is not evolutive. Treatment is essentially preventive. Some medications could be proposed: membrane stabilizing agents like antiarrhythmic drugs (mexiletine, tocainide), or the carbonic anhydrase inhibitor (acetazolamide). Precautions may be taken during general anaesthesia because of diaphragm myotonia risk.

Temperature-sensitive sodium channelopathy with heat-induced myotonia and cold-induced paralysis

The authors report a Japanese family with dominantly inherited heat-induced myotonia and cold-induced paralysis with hypokalemia. This phenotype is associated with a novel mutation in the voltage-dependent skeletal muscle sodium channel alpha subunit (SCN4A). This Pro1158Ser mutation is localized between the fourth and fifth transmembrane segments of domain III in SCN4A and may give rise to a new function; that is, thermosensitive permeability changes of the sodium channel.

Molecular basis of genetic heterogeneity: role of the clinical neurologist

Advances in molecular genetics have disclosed many different explanations for allelic heterogeneity, how different clinical syndromes arise from mutations in the same gene. The converse, how similar clinical syndromes arise from mutations of different genes on different chromosomes is called locus heterogeneity. Both, however, give rise to some disease-defining mutations, as in childhood spinal muscular atrophy or Duchenne muscular dystrophy. Nevertheless, new problems have been created, including what might be called "diagnosis by the number," diverse syndromes from mutations in the same gene without current explanation, or siblings with different clinical syndromes. These discoveries have transformed the clinical neurology of heritable diseases. They also provide clinicians with new responsibilities and opportunities in defining clinical syndromes and influencing the evolution of our clinical language.

Mutations in an S4 segment of the adult skeletal muscle sodium channel cause paramyotonia congenita

The periodic paralyses are a group of autosomal dominant muscle diseases sharing a common feature of episodic paralysis. In one form, paramyotonia congenita (PC), the paralysis usually occurs with muscle cooling. Electrophysiologic studies of muscle from PC patients have revealed temperature-dependent alterations in sodium channel (NaCh) function. This observation led to demonstration of genetic linkage of a skeletal muscle NaCh gene to a PC disease allele. We now report the use of the single-strand conformation polymorphism technique to define alleles specific to PC patients from three families. Sequencing of these alleles defined base pair changes within the same codon, which resulted in two distinct amino acid substitutions for a highly conserved arginine residue in the S4 helix of domain 4 in the adult skeletal muscle NaCh. These data establish the chromosome 17q NaCh locus as the PC gene and represent two mutations causing the distinctive, temperature-sensitive PC phenotype.

Paramyotonia congenita or hyperkalemic periodic paralysis? Clinical and electrophysiological features of each entity in one family

The nosological distinction between paramyotonia congenita (PC) and hyperkalemic periodic paralysis (HPP) continues to generate debate. Recently, electrophysiological signs thought to be specific for each entity have been described and have been used to bolster the argument that the two disorders are distinct. We report a particularly instructive family wherein individual members had clinical features of either PC or HPP and electrophysiological features of both. We suggest that PC and HPP represent part of the spectrum of a single genetic disorder. Evoked response testing, with exercise and cold provocation, may be useful in determining the physiologic pattern that predominates in any one individual.

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.

Treatment of paramyotonia congenita with acetazolamide

Treatment of paramyotonia congenita with acetazolamide has been shown to reduce myotonic symptoms but severe weakness has developed in some patients leading to a recommendation not to use the drug in this disorder. We studied a patient with the characteristic clinical and electrophysiological profile of paramyotonia congenita. Myotonia was effectively treated with a very low dose of acetazolamide and no weakness developed. We conclude that acetazolamide can be a safe and effective medication in paramyotonia congenita.

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).

Lack of cold sensitivity in hyperkalemic periodic paralysis

The nosologic distinction between paramyotonia congenita and hyperkalemic periodic paralysis is somewhat blurred. Muscle membrane inexcitability induced by cooling seems to be characteristic of paramyotonia congenita. The effect of cooling on the maximal compound muscle action potential (CMAP) in patients with paramyotonia congenita was compared to that in patients with hyperkalemic periodic paralysis. Diminution in CMAP amplitude and area, which was observed in paramyotonia congenita, did not occur in hyperkalemic periodic paralysis. We suggest that this effect of cooling on the CMAP can be utilized in the differentiation of these two syndromes.

Quantitative testing of handgrip strength, myotonia, and fatigue in myotonic dystrophy

An electrically calibrated, isometric hand dynamometer was used to quantitate maximum voluntary contraction (MVC(, relaxation time and fatigue time (at 50% MVC) in 18 normal and 10 myotonic dystrophy subjects. Precise measurements of MVC and fatigue time were obtained in normals and myotonic dystrophy patients. Relaxation times were markedly prolonged and were quite variable in all patients with myotonic dystrophy in contrast to the highly reproducible times in normals. Neither "warm-up" activity nor acetazolamide treatment altered relaxation time.

Decreased insulin sensitivity of forearm muscle in myotonic dystrophy

Previous studies of patients with myotonic dystrophy have demonstrated hyperinsulinism after glucose loading. This hyperinsulinism has been attributed by some investigators to tissue insulin resistance. We have directly studied insulin sensitivity of forearm muscle in patients having such hyperinsulinism. The effect of an intrabrachial arterial insulin infusion (100 mu U/kg per min) on glucose uptake was determined in six cases of myotonic dystrophy, six normal subjects, and in seven disease control subjects with myotonia or wasting from other disorders. There was no significant difference in insulin tolerance comparing myotonic dystrophy patients to the normal and disease control groups. Glucose tolerance and basal insulin levels were normal in the myotonic dystrophy patients, but hyperinsulinism occurred after glucose ingestion. After 25 min of intra-arterial insulin, the mean peak muscle glucose uptake in myotonic dystrophy was 2.54 +/- 0.54 mu mol/min per 100 ml forearm compared to 5.24 +/- 0.86 mu mol/min per 100 ml for disease controls (P is less than 0.05). Myotonic dystrophy patients showed a peak glucose uptake increment of only 2.6 +/- 0.2-fold over basal contrasted with the disease control value of 6.5 +/- 1.0-fold (P is less than 0.02) and the normal control value of 8.8 +/- 1.1-fold (P is less than 0.01). Thus, there was an absolute as well as a relative decrease in muscle insulin sensitivity in myotonic dystrophy patients compared to both control groups. The peak increments in arterio-superficial venous glucose concentration differences after insulin infusion were not significantly different comparing myotonic dystrophy and control groups. These data suggest that in myotonic dystrophy, there is insulin insensitivity of skeletal muscle.

Effects of acetazolamide on myotonia

Myotonia can occur in the periodic paralyses, particularly the hyperkalemic form. The beneficial response to acetazolamide in hypokalemic and hyperkalemic periodic paralysis has led us to study the effect of acetazolamide in 9 patients with disorders having myotonia as the major problem, 7 with myotonia congenita and 2 with paramyotonia congenita. Patients were studied before acetazolamide administration with glucose and potassium loading tests. All patients had an increase in myotonia with potassium, but no weakness occurred with either test. Acetazolamide treatment decreased myotonia in all patients and in 3 proved the most satisfactory therapy. Side-effects during acetazolamide therapy included paresthesias in 5 patients and renal calculus in 1. Flaccid weakness occurred in a patient with paramyotonia congenita. Acetazolamide treatment was associated in all patients with partially compensated metabolic acidosis and lowering of serum potassium within the normal range. Kaliuresis was also noted during introduction of therapy. Acetazolamide appears to be an acceptable treatment for occasional patients with myotonia who are unresponsive to or intolerant of other therapies.

Effect upon serum insulin, glucose and potassium concentrations of acetazolamide during attacks of familial periodic hypokalemic paralysis

In four patients with periodic hypokalemic paralysis paralytic attacks were induced in the untreated state and later, after the patients had been treated with acetazolamide. There was a distinct, clinically favourable effect of acetazolamide upon the length as well as the severity of paralysis. The maximum fall in serum potassium was less marked during acetazolamide therapy. After treatment all four patients showed significantly reduced serum levels of glucose and insulin during induced attacks of paralysis as compared with the levels obtained during paretic attacks in the untreated state. These findings indicate that the prophylactic effect of this drug does not relate merely to metabolic acidosis. The hypothesis is advanced that the lower serum insulin and glucose levels might represent reduced absorption which would amount to an indirect prophylactic action.

Weakness in myotonic syndromes

Muscle weakness occurs in a number of myotonic syndromes, including those in which muscle bulk is preserved. The possible causes of muscle weakness may be considered in terms of the different stages of the electromechanical activation process and defects in the contractile machinery. In individual myotonic disorders the ability to identify the origin of the muscle weakness (whether neurogenic or myogenic) and the functional level at which the defect occurs would assist the choice of rational treatment.