1A. The most common types of myopathies and myositis

Myopathies

A myopathy is a disease of the muscle itself, not involving innervation or the NMJ. There are many different types.

Etiology

• Hereditary myopathies
  • Progressive muscular dystrophies
    • Duchenne muscular dystrophy
    • Becker muscular dystrophy
    • Myotonic dystrophy
    • Facioscapulohumeral muscular dystrophy (FSHD)
  • Congenital myopathies
  • Metabolic myopathies
    • Mitochondrial
    • Pompe disease – rare but treatable, so important to recognize
• Acquired myopathies
  • Hypothyroidism
  • Toxin myopathy (e.g. statins, steroids)
  • Infection
  • Illegal drugs
  • Critical illness myopathy (due to ICU treatment)

Duchenne muscular dystrophy is an X-linked recessive deletion of the dystrophin gene.

Clinical features

The typical feature of myopathy is symmetrical proximal muscle weakness. Muscle pain is not typical.

Progressive muscular dystrophies start in certain age and then progress. Duchenne has symptoms like progressive proximal muscle weakness and atrophy. It starts at 3 – 5 years. The Gower sign is characteristic, where patients stand up by supporting themselves on their thighs and then using their arms to “walk up” the body to a standing position. Patients with Duchenne are usually wheelchair-bound at 10 – 15 years, and their life expectancy is 30 years. Becker is a milder version of Duchenne with better prognosis.

Myotonic dystrophy is associated with myotonia (impaired muscle relaxation after voluntary contraction), muscle weakness, and other problems.

Congenital myopathies have good prognosis and slow development.

Diagnosis and evaluation

Diagnosis is based on phenotype, EMG findings, muscle biopsy findings, and genetic studies. On EMG, short motor unit potentials (MUPs) can be seen.

Muscle biopsy is usually required for diagnosis. On muscle biopsy, atrophy and necrosis can be seen, as well as specific findings like absent dystrophin in Duchenne. Genetic studies can sometimes eliminate the need for a muscle biopsy, in Duchenne and myotonic dystrophy, for example.

EMG can differentiate between neurogenic and myopathic problems.

In any person with myopathy, it’s important to rule out hypothyroidism.

Treatment

Unfortunately, there is not much to offer patients with myopathies except treating any underlying cause, if present. Physiotherapy is always important. Steroids delay the need for wheelchair in Duchenne by 2 years.

Myositis

Inflammatory myopathy, also called myositis, is a subtype of myopathy where there is inflammation of muscle.

Etiology

• Polymyositis (PM)
• Dermatomyositis (DM)
• Necrotising myositis
• Inclusion body myositis (IBM)
• Overlap myositis
• Paraneoplastic myositis

Dermatomyositis and necrotising myositis can either be primary (idiopathic) or secondary to adenocarcinoma.

Clinical features

In most types of myositis there is proximal weakness, atrophy, and muscle pain. The only exception is inclusion body myositis, in which there is distal muscle weakness. IBM affects elderly.

In dermatomyositis there are also skin symptoms, namely heliotrope rash (erythematous/purple rash of periorbital area) and Gottron papules (erythematous papules on knuckles) and shawl erythema (erythema of neck and upper trunk). Polymyositis is like dermatomyositis but without skin symptoms. Necrotising myositis is similar to them but worse.

Diagnosis and evaluation

The creatine kinase (CK) is elevated in most cases. MRI can show the distribution and extent of inflammation and can guide the location for biopsy.

Muscle biopsy is usually required for diagnosis. On muscle biopsy, inflammation, and MHC-I expression (which is usually not expressed on muscle) can be seen. In necrotising myositis, necrosis can be seen, and in IBM, vacuoles are present.

EMG can differentiate between neurogenic and myopathic problems.

In dermatomyositis and necrotising myositis it’s important to rule out adenocarcinoma.

Treatment

The treatment is usually immunosuppressants, usually steroids + azathioprine or methotrexate. Immunosuppressants is not effective for IBM, however.