Among 22 patients with PMC, 14 with sodium channel myotonia, and 18 myotonia patients with mutations in the CLCN1 gene (118425), Fournier et al. (2006) found that cold temperature was able to exaggerate EMG findings in a way ... Among 22 patients with PMC, 14 with sodium channel myotonia, and 18 myotonia patients with mutations in the CLCN1 gene (118425), Fournier et al. (2006) found that cold temperature was able to exaggerate EMG findings in a way that enabled a clear correlation between EMG findings and genetic defects. Those with PMC showed a clear worsening of compound muscle action potential with cold temperature. Those with sodium channel myotonia tended not to show a decline in compound action muscle potentials, whereas those with myotonia due to CLCN1 mutations tended to show improvement of the muscle potential with exercise, concomitant with the clinical warm-up phenomenon.
The characteristics of paramyotonia congenita, first described by von Eulenburg (1886), are (1) inheritance as a dominant with high penetrance; (2) myotonia, increased by exposure to cold; (3) intermittent flaccid paresis, not necessarily dependent on cold or myotonia; ... The characteristics of paramyotonia congenita, first described by von Eulenburg (1886), are (1) inheritance as a dominant with high penetrance; (2) myotonia, increased by exposure to cold; (3) intermittent flaccid paresis, not necessarily dependent on cold or myotonia; (4) lability of serum potassium; (5) nonprogressive nature; and (6) lack of atrophy or hypertrophy of muscles. Lajoie (1961) described a family with many affected members. The condition, which was already evident in infancy, was manifested mainly by paralysis of muscles exposed to cold; it was not progressive, did not interfere with a reasonably normal social and economic life, and did not affect longevity. Drager et al. (1958) found 30 affected members in 6 generations of a family. Hudson (1963), who reported 17 affected persons in 5 generations of a family, commented on the phenotypic overlap of this condition with hypokalemic, eukalemic, and hyperkalemic periodic paralysis, with myotonia congenita, and with myotonic dystrophy. Six and possibly 9 generations had affected members in the French-Canadian family reported by Samaha (1964). Eating ice cream or swimming in cold water was dangerous to affected members. Serum potassium levels were moderately increased and the patients were sensitive to administered potassium. Chlorothiazide was remarkably beneficial. Becker (1970) gave an extensive review of the subject and described studies in 18 kindreds. In electrophysiologic studies of 5 unrelated patients with PMC, Ricker et al. (1986) concluded that slowed muscle relaxation precipitated by cooling the muscle contributed to paramyotonic stiffness. Borg et al. (1993) described a large Swedish family with paramyotonia congenita. Clinical features included cold-induced myotonia, attacks of weakness, persistent weakness, and some signs of muscle affection. Muscle action potentials decreased significantly after cooling. An ancestor of the family, who had myotonia, lived in the same town at the same time as Albert Eulenburg. Borg et al. (1993) suggested that their family was part of the originally described kindred (Eulenburg, 1886). Sasaki et al. (1999) reported a 4-generation Japanese family in which 9 members were affected with PMC. The proband was a 16-year-old girl who began to experience muscle stiffness in her face, hands, and limbs on exposure to cold at the age of 4 years. She showed grip and percussion myotonia in her hands which was aggravated by exercise (paradoxical myotonia), and hand muscle weakness after cold exposure. Serum potassium was normal. There were no episodes of generalized weakness. Other family members had the same symptoms and signs, and only 3 reported episodes of generalized paralysis, always after exposure to cold. Miller et al. (2004) identified mutations in the SCN4A gene in 49 of 56 patients with PMC. The average age at onset in all patients, with or without mutations, was 3 to 4 years. Clinical myotonia was present in all patients, and all of those tested showed increased myotonia in response to cold temperature. Approximately half of patients had mild muscle weakness, and most patients had increased serum creatine kinase. In a diagnostic flow chart for the periodic paralyses, Miller et al. (2004) indicated that PMC shows early age at onset and is characterized by frequent attacks lasting less than 24 hours with normal serum potassium levels. Petitprez et al. (2008) reported a large family with autosomal dominant myotonia associated with a heterozygous mutation in the SCN4A gene (603967.0029). Age at onset ranged from 4 to 22 years. All patients reported muscle stiffness, which occurred upon starting movement after a time of motor rest, especially when the rest period followed physical activity. The duration of the stiffness was usually very short, sometimes only seconds, and improved with repetitive movement. Most reported worsening of symptoms during exposure to cold, and approximately half reported worsening after fasting. Many patients reported worsening of symptoms with mental stress and pregnancy, worsening in puberty for those with childhood onset, and improvement in older age. Muscle weakness was not reported. Dupre et al. (2009) reported 14 French Canadian patients from 9 unrelated families with myotonia caused by SCN4A mutations (see, e.g., 603967.0007). The mean age at onset was 17.7 years (range, 3 to 56). The most common clinical features included lip myotonia (57%), percussion myotonia (86%), handgrip myotonia (71%), warm-up phenomenon (79%), generalized stiffness (64%), muscle pain (64%), and exacerbation with cold temperatures (79%). Less common features included lid lag (21%), tongue myotonia (28%), and generalized hypertrophy (21%). None had weakness. Among female patients, 38% reported aggravation of symptoms during menstruation or pregnancy, and alleviation after menopause. One patient reported improvement with alcohol. Although many patients (43%) tried medication, only a few (28%) had relief. The most effective medications were mexiletine and carbamazepine, followed by phenytoin. Electrophysiologic studies showed similar myotonia and decremental CMAP responses as patients with heterozygous mutations in the CLCN1 (118425) gene. - Paramyotonia without Cold Paralysis Brungger and Kaeser (1977) described an extensive kindred in which at least 26 members had paramyotonia congenita. Cold paralysis never occurred spontaneously and could not be induced by immersion in cold water or by potassium load. The findings supported the conclusion by DeJong (1955) of a separate form of paramyotonia without cold paralysis. Koch et al. (1995) reported 3 unrelated 3-generation pedigrees of German extraction which segregated paramyotonia congenita without paralysis on exposure to cold. The probands were only mildly affected. There was muscle stiffness in the cold, but never any weakness, even in a muscle contraction measurement registered by a standardized forearm cooling test. The authors stated that the absence of weakness after exposure to cold or exercise distinguished the phenotype from classic paramyotonia congenita of Eulenburg. Davies et al. (2000) reported an Irish family in which 5 members were affected with paramyotonia without periodic paralysis ('pure paramyotonia.') The proband developed symptoms in childhood consisting of spasms of muscle stiffness that preferentially affected the hands and face. Symptoms tended to worsen with exertion (paradoxical myotonia), but the most marked exacerbation occurred in cold weather. There was no history of periodic paralysis or progressive muscle weakness. Physical exam showed myotonia, and electromyography (EMG) showed profuse myotonic discharges. On cooling, there was a 78% drop in compound muscle action potential amplitude. Four other family members had a similar clinical history. Davies et al. (2000) identified a point mutation in the SCN4A gene (603967.0019) that segregated with the disease phenotype in this family. - Clinical Variability McClatchey et al. (1992) described a 3-generation Finnish family in which affected members had a disorder with features of both PMC and HYPP. The proband and her brother had infrequent attacks of muscle weakness while resting after exercise, myotonia, and hyperkalemia, typical of HYPP. Their father complained of nocturnal and cold-induced weakness consistent with PMC. Some other members of the family had myotonic features, limb stiffness, and myotonia precipitated by cold. McClatchey et al. (1992) also reported an Italian family with PMC and suggested that some members had a phenotype consistent with myotonia congenita (widespread spontaneous myotonia even without cooling). Both families were found to have mutations in the SCN4A gene (603967.0005-603967.0006). Kelly et al. (1997) described a large kindred in which affected members were phenotypically heterogeneous with episodic potassium-sensitive paralysis as well as stiffness and weakness induced by exercise and cold. A mutation in the SCN4A gene (603967.0002) was identified. The authors noted that the same mutation had previously been described in families with only HYPP (see Rojas et al., 1991). Brancati et al. (2003) reported an Italian kindred with 9 individuals affected with a severe form of HYPP and mild features of paramyotonia congenita. Onset of paralytic episodes was in the first 6 to 12 months in all patients. The episodes were frequent, 2 to 3 times per week, lasting 10 minutes to 2 hours, and were usually accompanied by muscle stiffness, usually of the lower limbs. During adolescence, episodes were precipitated by rest after exercise, cold, alcohol intake, and fasting. The frequency and severity of attacks worsened over the years, occurring daily and spontaneously. Five of 6 patients had normal serum potassium during attacks. Diffuse interictal weakness, primarily in the proximal muscles, occurred around the fourth to fifth decade. Gay et al. (2008) described a female infant with severe fatal neonatal nondystrophic myotonia who presented with facial dysmorphism, muscle hypertrophy, severe constipation, psychomotor delay, and frequent cold-induced episodes of myotonia and muscle weakness, leading to severe hypoxia and loss of consciousness; she died at 20 months of age following a bronchopulmonary infection. Matthews et al. (2008) reported a woman with classic PMC who had 2 affected children who presented with transient neonatal hypotonia. The older child recovery fully from the neonatal hypotonia and later developed classic PMC at age 3 years. The authors identified 4 additional cases from 3 additional families with transient neonatal hypotonia. Some of the patients required feeding or respiratory assistance, and all later developed classic PMC by age 5 years. All of these patients had the same missense mutation in the SCN4A gene (603967.0028). Earlier generations of 3 of the families reported a history of PMC without neonatal hypotonia. The findings expanded the phenotypic spectrum of PMC to include neonatal hypotonia. Matthews et al. (2011) reported a boy with PMC who presented with neonatal inspiratory stridor and poor feeding. Laryngoscopy showed findings consistent with laryngomalacia. He continued to have stridor for the first 6 months of life, and later motor milestones were mildly delayed. In early childhood, he was noted to have frequent episodic muscle weakness and stiffness associated with cold weather. At age 4 years, he continued to have episodes of inspiratory stridor exacerbated by viral illness, cold weather, and prolonged laughing or crying. His mother, grandfather, and great-uncle reported similar episodes of muscle stiffness and weakness exacerbated by cold and exercise. All were found to carry a heterozygous mutation in the SCN4A gene (T1313M; 603967.0008).
In patients with PMC from 3 families, Ptacek et al. (1992) identified 2 mutations in the SCN4A gene (603967.0003-603967.0004).
In 2 families with PMC, McClatchey et al. (1992) identified 2 different mutations in the SCN4A gene ... In patients with PMC from 3 families, Ptacek et al. (1992) identified 2 mutations in the SCN4A gene (603967.0003-603967.0004). In 2 families with PMC, McClatchey et al. (1992) identified 2 different mutations in the SCN4A gene (603967.0007-603967.0008). :McClatchey et al. (1992) suggested that these are examples of molecular definition of temperature-sensitive mutations in the human. At normal temperatures, this mutation may have minimal clinical significance; however, even a minor drop in temperature may impede movement of the channel loop enough to allow an abnormal sodium flux. In 3 families with paramyotonia without cold paralysis, Koch et al. (1995) found the same novel missense mutation in the SCN4A gene (603967.0013). In a family affected with PMC without periodic paralysis, Davies et al. (2000) identified a mutation in the SCN4A gene (603967.0019), and Sasaki et al. (1999) identified the same mutation in a family with PMC in which only 3 members showed periodic paralysis. In a female infant with severe fatal neonatal nondystrophic myotonia and overlapping features of PMC and HYPP, Gay et al. (2008) identified a heterozygous mutation (N1297K; 603967.0027) in the SCN4A gene.
Rossignol et al. (2007) identified a heterozygous founder mutation in the SCN4A gene (M1476I; 603967.0026) in 44 patients from 11 French Canadian families with a myotonia phenotype most consistent with paramyotonia congenita. The patients originated from the Saguenay-Lac-Saint-Jean ... Rossignol et al. (2007) identified a heterozygous founder mutation in the SCN4A gene (M1476I; 603967.0026) in 44 patients from 11 French Canadian families with a myotonia phenotype most consistent with paramyotonia congenita. The patients originated from the Saguenay-Lac-Saint-Jean region with notable clustering around Saint-Felicien, and haplotype analysis indicated a founder effect. The phenotype was quite variable, with age at onset ranging from 5 to 67 years (mean, 21 years) and patients showing mild (61%) to severe (14%) symptoms. Eleven (25%) patients were asymptomatic despite myotonic discharges on EMG. The most consistent features were cold-induced myotonia (41%) and painful myotonia (18%). Other variable features included aggravation of symptoms with pregnancy, myotonic reactions to anesthesia, food-induced paralysis, and rhabdomyolysis. Potassium challenge was not conducted.