Myotonia congenita

Myotonia is muscle stiffness, in which muscle relaxation after voluntary contraction is impaired. Mutations in several ion channel genes (Cl, Na, Ca, K channels) can cause myotonias, which can sometimes be differentiated clinically (e.g. paramyotonia is cold-sensitive). C1C-1 mutations cause pure myotonia congenita which is not sensitive to temperature. Channel myotonia comes in a recessive (Becker type)... 

Myotonia congenita (MIM 160800) Chloride channel Skeletal muscle... 

Special risk Use with caution in the presence of cardiac disease, particularly in digitalized patients or in the presence of renal disease, metabolic acidosis, Addison disease, acute dehydration, prolonged or severe diarrhea, familial periodic paralysis, hypoadrenalism, hyperkalemia, hyponatremia, and myotonia congenita. 

It is used in the treatment of cerebral falciparum malaria and multidrug resistant strains of cerebral malaria. It is also used along with clindamycin in the treatment of babesiosis. It is also effective in myotonia congenita and nocturnal muscle cramps. 

Patients with myotonia congenita and myotonia dystrophica respond differently to succinylcholine, in that their muscles are contracted rather than relaxed. 

Because of these dystrophic possibilities, the occasional transition of myotonia congenita to dystrophia myotonica (W6), and the extreme clinical variability of the latter, Maas and Paterson (Ml) consider both conditions variants of the same disease, though Bell (Bll) re-... 

Thus in muscular dystrophy it is apparent that both the mean elevations of the serum enzyme values and the magnitudes of their variations upon physical activity are proportional to the mass of dystrophic muscle remaining and to the severity of the disease in it. Both are thus greater in early than in evident Duchenne-type dystrophy, less in limb-girdle dystrophy, and least in myotonia congenita. Further, though serum creatine kinase has been found to be an exceedingly delicate index of myopathy (A2, S14), for present purposes serum aldolase is suflBciently... 

Fig. 14. EfFect of alternate rest and physical activity in myotonia congenita. A and B, during heavy emplojnnent. Period X, freely ambulant in hospital for 17 days periods Y and Z, successive intervals of 6 and 7 days of bed rest. E, after 2 days out of bed F, on discharge C, at home H, after 10 days in light employment, so continuing thereafter.

Ml. Maas, O., and Paterson, A. S., Myotonia congenita, dystrophia myotonica and paramyotonia reaiSrmation of their identity. Brain 73, 318 (1950). 

Myotonia has been precipitated by decamethonium in patients with myotonia congenita and dystrophia myo-tonica (3). 

Paradoxical contracture is most often associated with myotonia dystrophica and myotonia congenita. A myotonic reaction has also been reported in a patient with hjq)erkalemic periodic paralysis (60). Suxamethonium is therefore contraindicated in these conditions, even though normal responses are sometimes seen. Contracture has also been reported as a result of denervation in Pancoast s syndrome and after plexus injuries and, rarely, in patients with amyotrophic lateral sclerosis or multiple sclerosis (61-63). 

Patients with muscle disorders (dystrophia myotonica, myotonia congenita, myasthenia gravis, and hyperkalemic periodic paralysis) tend to react unpredictably to suxamethonium. In myasthenia gravis, small doses of suxamethonium may be tried and the resulting effect monitored. In the other diseases listed non-depolarizers, cautiously used, are preferable. Cardiac arrest has been reported in patients with pseudohypertrophic muscular dystrophy (Duchenne tjrpe) and excessive muscle damage may be produced by suxamethonium in this condition. 

The fatty acid composition of muscle lipids may show quantitative alterations in diseased muscle. Thus lecithin isolated from human dystrophic muscle had an increased amount of oleic but diminished linoleic acid (Tl). Changes have been recorded also in the fatty acid composition of lecithin from denervated muscle (PI). Recently it has been reported (K16) that the fatty acid pattern of muscle phosphatides from patients with the autosomal dominant form of myotonia congenita differed markedly from that of the autosomal recessive form and from the normal. Tani and his co-workers (F7) have made a detailed study of the phospholipids of normal and dystrophic mouse tissues. In normal mice phosphatidylcholine and phosphatidylethanolamine from skeletal and heart muscles had a very high content of 20-22-carbon polyunsaturated acids, in comparison with those for other tissues the most abundant was docosahexaenoic acid. In dystrophic mice there was a sharp decrease in the proportion of docosahexaenoic acid in the phosphoglycerides from skeletal and heart muscles, suggesting the likelihood of important alterations in muscle membranes. Somewhat similar studies have been reported by Owens (05), who also observed a fall in the proportion of docosahexaenoic acid, mainly in the phosphatidylcholine -j- choline plasmalogen fraction. 

K16. Kuhn, E., and Seiler, D., Biochemisohe Besonderheiten und Unterschiede der autosomal dominant und autosomal recessive vererbten Myotonia congenita. Klin. Wochenschr. 48, 1134-1136 (1970). 

K -activated myotonia is a disorder where patients clinically appear to have myotonia congenita. But their myotonia fluctuates, worsens when K is administered, and improves with carbonic anhydrase inhibitors (Trudell et al 1987). Interestingly, these patients do not develop attacks of weakness. This disorder is transmitted as an autosomal dominant trait. 

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