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Myotonia Congenita in a Young Active Man

Symptoms and Physique Tip the Diagnosis

Kelvin T.L. Chew, MB, BCh, MSpMed; Yue-Shuen Wong, MBBS; Hock-Luen Teoh, MB, ChB; Erle C. H. Lim, MBBS, MMed (Intern Med)


In Brief: A 19-year-old man had a 7-year history of leg cramps and a tendency for his legs to "seize up" when he participated in sports. The condition was initially mild, and clinical examination revealed an extremely muscular physique and percussion and action myotonia. Electromyography was consistent with myotonic discharges, and myotonia congenita was the diagnosis. Carbamazepine therapy relieved all symptoms, except for a short time when the patient stopped taking medication. Physicians should carefully document family history and symptom triggers to facilitate making the correct diagnosis.

Muscle cramps and "seizing up" of the limbs are common complaints of patients seeking treatment at sports medicine facilities, but the myriad disorders exhibiting these symptoms can make diagnosis a challenge. The myotonias are a group of neuromuscular disorders with sustained involuntary contraction arising from hyperexcitability of the muscle membrane. This symptom manifests clinically as action or percussion myotonia. We report the case of a young man whose myotonic symptoms were initially attributed to a biomechanical cause and whose diagnosis was tipped by the presence of muscular hypertrophy and symptom exacerbation during sporting activities. Clinical demonstration of myotonia and electromyography (EMG) confirmed the diagnosis.

Case Report

History. A 19-year-old Chinese man was referred to our institution with complaints of lower-limb pain of 7 years' duration. Initially mild, the pain progressed, becoming problematic over the last 2 years to the extent that pain interfered with his ability to participate in sports. His symptoms were bilateral calf and thigh cramping and tightness that the patient described as "seizing up." Cramping occurred when he ran, ascended stairs, or participated in sports such as distance running and basketball. These episodes sometimes caused him to fall. He noticed that sudden bursts of movements triggered the attacks, which lessened with activity and were not followed by muscle weakness. Symptoms were not exacerbated in the cold. The patient had no history of trauma and had no other neurologic symptoms.

Family history was significant: His mother had also experienced intermittent muscle cramping during sudden movements. The patient had been referred to us after a course of physical therapy and stretching, ostensibly for addressing biomechanical muscle imbalance, failed to ameliorate symptoms.

Physical exam. Clinical examination was essentially unremarkable, with no apparent biomechanical causes to explain his symptoms. The patient had no obvious coordination deficits, and the pyramidal and cerebellar examinations were normal.

One conspicuous characteristic was the patient's pronounced muscle development despite a lack of intensive resistance training. This appearance prompted us to provoke action and percussion myotonia (figure 1), both of which were positive. The patient had no difficulty releasing his grip after shaking hands or in opening his eyes after forcefully closing them and had no temporal wasting, cataracts, testicular atrophy, or alopecia.

Diagnostic tests. Radiologic exam was normal. Needle EMG examination was significant for myotonic discharges with the characteristic "dive bomber" sound in the abductor pollicis brevis and in the vastus medialis. Recruitment and motor unit potentials were otherwise unremarkable. The patient was offered genetic testing for mutations in muscle chloride-channel protein genes, but he declined.

Diagnosis. Our diagnosis was myotonia congenita based on the young age of onset, muscular hypertrophy rather than dystrophy, and EMG findings. The characteristic EMG findings and myotonic potentials confirm the diagnosis of myotonia, and the lack of typical features of the dystrophic form (eg, muscle wasting, myopathic facies, cataracts, and frontal balding) support the diagnosis. Distinction of the type of myotonia congenita is based on inheritance. Without additional family members available, distinguishing type was not possible.

Treatment. The patient was prescribed a course of carbamazepine (200 mg, tid), which resolved his symptoms. The patient experienced symptoms briefly when he stopped taking medication, but, once he resumed the regimen, symptoms have not recurred. His mother consented to an EMG, but it was normal, even on percussion of the muscle being investigated.


Muscle cramps and stiffness are the bane of the athlete. The many disorders in the differential diagnosis make careful clinical examination important. Disorders include electrolyte imbalances, inborn errors of metabolism (such as glycogenoses that cause lactic acid buildup during exertion), drugs, heat illness, diabetic polyneuropathy, akathisia, and neuromuscular diseases such as dyskinesias, fasciculations, myokymia, dystonia, and myotonia. Biomechanical causes should also be considered, and these were excluded after clinical and radiologic examination. Our patient's complaints were actually attributable to activity-induced episodes of myotonia. Additional disorders to consider, as figured in this case, were paroxysmal kinesogenic dyskinesias, myotonia, and paramyotonia.

Myotonias. The myotonias encompass several neuromuscular disorders characterized by sustained involuntary contraction of a group of muscles. Myotonias are traditionally classified into dystrophic and nondystrophic types. Dystrophic myotonias are characterized by the prominence of muscle weakness and wasting, both of which were absent in our patient. Thus, he had a nondystrophic form of myotonia. These disorders include the paramyotonias and myotonia congenita, ie, the Thomsen type (autosomal dominant form) and Becker type (autosomal recessive form) diseases. Both diseases are caused by mutations in the chloride channel gene (CLCN1), and the criterion for differentiating Thomsen's from Becker's myotonia congenita lies in the mode of inheritance. However, because more than 60 mutations have been identified in families with the disorders, it has not been established that any particular mutation causes either type per se.1

Paramyotonia is characterized by cold-induced myotonic stiffness that is increased by sustained muscle activity (paradoxical myotonia) and may be followed by a variable degree of weakness.2 These characteristics arise from a sodium channel abnormality, and our patient's clinical picture was not consistent with a paramyotonia.

Myotonia congenita is nonprogressive and nondystrophic. The clinical features usually begin during the first decade and remain constant into adulthood. Worldwide prevalence is between 0.2 to 7.3 per 100,000. Finland has an unusually high occurrence, at 7.3 per 100,000.3 The incidence of the disease among Asians is not well established. A literature search produced only a single case report4 of the disease in a Chinese family in Singapore. Both dominant and recessive forms are associated with muscle hypertrophy and produce patients who have an extremely muscular appearance, classically described in the literature as "Herculean."

Myotonia congenita is due to a chloride channel abnormality (channelopathy) in skeletal muscle. Both forms of myotonia congenita are channelopathies caused by several nonsense and missense mutations in the skeletal muscle CLCN1 gene5 that has been localized to human chromosome 7q35.6,7

Chloride channels account for most (70% to 80%) of skeletal muscle's resting membrane conductance,8 and the hyperexcitability and repetitive firing of action potentials in myotonia congenita are caused by a low chloride conductance of the sarcolemma.9 The abnormal chloride conductance maps to a region that contains the skeletal muscle voltage-gated chloride channel gene CLCN1.9

Treatment and outcome. Therapy targets electrical stabilization of the muscle membrane. Successful therapies include anticonvulsants (such as carbamazepine and phenytoin), acetazolamide, and drugs such as quinine, procainamide hydrochloride, mexiletine hydrochloride, tocainide hydrochloride, thiazides, and beta-adrenergic agents.10,11 Our patient responded well to carbamazepine and did not have any episodes while complying with the drug regimen.


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Dr Chew is deputy director of the Sports Medicine Center, and Dr Wong is a consultant in the department of orthopedic surgery at Alexandra Hospital in Singapore. Dr Teoh is the neurology registrar in the department of medicine, and Dr Lim is a consultant neurologist in the division of neurology at the National University Hospital in Singapore. Address correspondence to Erle C. H. Lim, MBBS, MMed (Intern Med), Division of Neurology, National University Hospital, 5 Lower Kent Ridge Rd, Singapore 119074; e-mail to [email protected].

Disclosure information: Drs Chew, Wong, Teoh, and Lim disclose no significant relationship with any manufacturer of any commercial product mentioned in this article. Quinine, procainamide hydrochloride, mexiletine hydrochloride, and tocainide hydrochloride are mentioned in this article for an unlabeled use.