Myopathies, or muscle diseases, are diseases of skeletal muscle. They are either hereditary or acquired and they are characterized by permanent (muscle weakness, blepharoptosis, diplopia) or transient (exercise intolerance, cramps, episodes of rhabdomyolysis) symptomatology, and/or high creatine kinase levels. Myopathies can present at any age, their progress is highly variable and they are often characterized by multi-organ involvement (cardiovascular, respiratory, central nervous system). Patients are thus in need of a multidisciplinary follow-up organized by a trained neurologist-myologist.
Spinal muscular atrophy
Spinal muscular atrophy (SMA) is a hereditary autosomal recessive disease characterized by progressive muscle weakness due to degeneration of lower motor neurons in the spinal cord. Depending on the age of presentation and the severity symptoms the disease is classified as follows: type 0 (newborn presentation, survival <6 months), type I (acute infantile or Werdnig Hoffman, onset is from birth to 6 months), type II (chronic infantile, onset is between 6 and 18 months), type III (chronic juvenile, onset is after 18 months), type IV (adult onset, onset is in adulthood, mean onset, mid 30s). In December 2016, nusinersen became the first drug approved by the FDA to treat children and adults with SMA.
Rare hereditary muscle diseases
In the European Union, a disease is defined as rare when it affects fewer than 1 in 2,000 people. Hereditary muscle diseases are considered rare disorders that are inherited either by the autosomal dominant trait (i.e. facio-scapulo-humeral muscular dystrophy, myotonic dystrophy type 1 and 2), the autosomal recessive trait (i.e. Pompe disease, recessive forms of limb girdle muscular dystrophy) or the X-linked trait (Duchenne and Becker muscular dystrophy).
Acquired (inflammatory) muscle diseases
Acquired (inflammatory) muscle diseases are rare autoimmune disorders affecting skeletal muscle and characterized by progressive muscle weakness. They are classified depending clinical picture in 5 main subtypes: dermatomyositis, polymyositis, overlap myositis, inclusion body myositis and necrotizing myopathies. They can be associated with arthritis, dermatological manifestations, heart or lung involvement or with an underlying malignancy.
Myotonic dystrophy
Myotonic dystrophies are rare, autosomal dominant hereditary disorders. They are characterized by muscle weakness and by a delayed relaxation of skeletal muscle following voluntary contraction (myotonia). Theyr are multisystemic diseases affecting the heart, respiratory system, nervous system, digest system and endocrine glands. There are 2 types of mytonic dystrophies: type 1 (Steinert disease) that can manifest at any age and type 2 (proximal myotonic myopathy-PROM), a disease of adult life.
Myofibrillar myopathies
Myofibrillar myopathies are rare diseases characterized by a disorganization of the Myofibrillar apparatus of skeletal muscle. They are characterized by progressive muscular weakness, usually in the distal parts of limbs and sometimes there is a concomitant heart involvement.
Pompe disease
Pompe disease (type II glycogenosis) is an autosomal recessive metabolic disorder characterized by abnormal accumulation of glycogen due to deficiency of the lysosomal enzyme acid alpha-glucosidase. There are 2 forms of the disease, a severe infantile form with muscle and heart involvement and a late onset form with muscle weakness and/or respiratory insufficiency. Since 2006 an enzyme replacement therapy using biologically active recombinant human alglucosidase alfa is approved.