Muscular dystrophies are a heterogeneous group of hereditary illnesses affecting both children and adults, with at least 30 different genes responsible for the disease development. These conditions are characterized by muscle wasting and weakness, and may be associated with cardiomyopathy with increased risk of sudden cardiac death. They have been divided into several groups.
Dystrophinopathies are muscle diseases caused by pathogenic variants in the gene DMD which encodes the protein dystrophin. The severe end of the spectrum includes two diseases – Duchenne and Becker muscular dystrophy – while the mild end of the spectrum can manifest only with increased serum concentration of creatine phosphokinase (CK), myoglobinuria and occasional muscle cramps.
Duchenne muscular dystrophy represents a severe, progressive disease that affects 1 in 3600–6000 live male births. It is one of the most severe muscle diseases to affect children. The majority of patients are unable to run and jump adequately due to proximal muscle weakness, which also results in the use of the classic Gowers’ maneuver when arising from the floor.
Muscle weakness in Duchenne muscular dystrophy can begin as early as in fourth year of life, initially affecting pelvic area, hip muscles, thighs and shoulders. By the early teenage years, the heart and respiratory muscles are also affected. The disease is rapidly progressive, with affected children being wheelchair dependent by the age of 13.
Becker muscular dystrophy is usually milder and more varied when compared to Duchenne muscular dystrophy. In most cases, muscle weakness becomes obvious later in childhood or in adolescence, and muscle deterioration occurs at a much slower rate. It occurs in about 3 to 6 out of every 100 thousand live male births.
Congenital muscular dystrophy
Congenital muscular dystrophies represent a mixed group of conditions with various degree of severity and progression rates. They are characterized by diminished muscle tone at birth (congenital hypotonia), delayed motor development, spinal rigidity, early onset of progressive muscle weakness and dystrophic pattern on muscle biopsy.
The main subtypes of congenital muscular dystrophy (grouped by involved protein function and mutated gene) are laminin alpha-2 (or merosin) deficiency, collagen VI-deficient congenital muscular dystrophy, the dystroglycanopathies, SEPN1-related congenital muscular dystrophy and LMNA-related congenital muscular dystrophy. Several less known subtypes have also been reported in a small number of individuals.
Cognitive impairment that range from intellectual disability to mild cognitive delay, structural eye and brain abnormalities, and seizures are almost exclusively found in the dystroglycanopathies, whereas white matter abnormalities without major cognitive involvement are usually seen in the laminin alpha-2-deficient subtype.
Myotonic dystrophy and other types
Myotonic dystrophy is considered the most common adult form of muscular dystrophy and it is characterized by autosomal dominant progressive myopathy, myotonia and the involvement of multiple organs. To date two distinct forms caused by similar mutations have been identified, which require different diagnostic and management strategies despite their clinical and genetic similarities.
Other types of muscular dystrophy that can occur later in life include limb-girdle muscular dystrophy, distal muscular dystrophy and opthalmoplegic muscular dystrophy. The severity, age of onset and features of these conditions vary significantly among the many subtypes, and may also be inconsistent even within the same family.
Facioscapulohumeral dystrophy is a form of muscular dystrophy that affects the face and shoulders. It is generally considered more benign (less serious) than other forms of muscular dystrophy. Most cases arise due to a deletion near the end of chromosome 4, and symptoms develop in the teenage years.