Congenital myopathy

Classification

Congenital myopathies with inclusion bodies and protein accumulation involve muscle proteins aggregating in the sarcoplasm, leading to dysfunction; nemaline myopathy is a well-understood example. Core myopathies, such as multicore myopathy and central core disease, show sharply-demarcated areas lacking oxidative enzymes NADH, SDH, and COX in muscle fibers. Myopathies with central nuclei, like myotubular myopathy, stem from a gene error affecting vesicle movement, disrupting myoblast fusion and creating structural problems in the Z line. Myopathies with varying fiber size, such as congenital fiber type disproportion, occur when type 1 slow-twitch fibers are smaller than type 2 fast-twitch fibers, explaining patients' difficulty with extended activity.

Diagnosis

Specific tests for congenital myopathies are rare, with muscle biopsy being the primary diagnostic tool. Blood tests for creatine kinase often show normal or mildly elevated levels, and electromyography can assess muscle electrical activity, but both are unreliable and non-specific; diagnosis therefore relies heavily on muscle pathology visualized at the cellular level. Prenatal screenings have advanced due to the genetic nature of these disorders.

Conditions included under the term vary; one source lists nemaline myopathy, myotubular myopathy, central core myopathy, congenital fiber type disproportion, and multicore myopathy. Nemaline myopathy, the most common, is characterized by generalized weakness and low muscle tone; in severe cases, infants may die from respiratory failure. Histologically, nemaline rods stain red with Gomori's trichrome and appear mostly in the subsarcolemmal region, also in the intermyofibrillar region and within the nucleus.

It can be autosomal dominant, autosomal recessive, or sporadic. Myotubular myopathy, also called centronuclear myopathy, presents with pain during exercise, difficulty walking, weakness in eye muscles, nerve function disorders, and sometimes intellectual disability; it is very rare, affecting fewer than 50 families. Genetically, it can be autosomal dominant (DNM2 gene) or autosomal recessive (BIN1 gene), with recessive onset most common.

Treatment

No treatments exist for congenital myopathies. Therapies such as physical or occupational therapy aim to strengthen skeletal muscles, and orthopedic surgery may correct secondary deformities like scoliosis. Survival depends on the degree of respiratory muscle insufficiency.