Myotonia congenita is a rare muscle disease characterized by sarcolemma over-excitability inducing skeletal muscle stiffness. The case report is compatible with congenital disease since it started early in youth and several cases were presented through generations. Myotonia is acceptable regarded the symptoms reported by the family and the signs showed by the proband at electromyogram evaluation. These included muscle stiffness when stimulated, difficulty in relaxing contracted muscles and abnormal enlargement of the muscles and burst of myotonic electric manifestations .
The two main types of myotonia congenita are Thomsen disease, which begins in infancy, and Becker disease, which usually begins between the ages 4 and 12. Thomsen disease is inherited in an autosomal dominant manner and Becker disease, the more common form, is inherited in an autosomal recessive manner [4, 5]. Taking into account the onset age, the fact that parents were normal allows us to conclude in favor of Becker disease. The family bears at clinical and electro-testing stages myotonia congenita type Becker disease [1, 2]. In respect with clinical presentation, and myopathy epidemiology, we tried to exclude other dystrophynopathies such as Duchene muscular disorder, Becker muscular disorder and Limb girdle muscular disorders with creatine phosphokinase dosage, other canal disorders with blood electrolytes like Westphal disease [6, 7]. The reported condition could not be mistaken for other myotonia especially the dystrophic myotonia. This occurred in mature adult with autosomal inheritance pattern. Affected patients have myotonia, cataracts and cardiac conduction defects. In affected men, hormonal changes may lead to early balding and infertility. There are two major types of myotonic dystrophy (type 1 and type 2). Their signs and symptoms overlap, although type 2 tends to be milder than type 1. The muscle weakness associated with type 1 particularly affects the lower legs, hands, neck, and face. Muscle weakness in type 2 primarily involves the muscles of the neck, shoulders, elbows, and hips. Both types are autosomal dominant and genes that are involved are DMPK for the type 1 and CNBP in type 2 . Molecular confirmation was a very good help since manifestations were predominant in upper limbs instead of lower limbs as generally reported. Furthermore, molecular finding was consistent with an autosomal recessive inheritance profile which is reasonably expected [9, 10]. Our patient benefited from symptomatic treatment with mexiletine since this molecule showed good results in some patients that suffered from Becker myotonia congenita .
Studies on Becker's myotonia remain very rare. In sub-Saharan Africa, we had noted cases in Tanzania, Nigeria and South Africa [12,13,14]. In general, the symptomatology was globally that found in our case. Within 15 Norwegian families, about 8 different mutations were found, thus reflecting the genetic heterogeneity of the disease . Also, most of the mutations were compound heterozygous as in our case.