Myopathies

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Inflammatory myopathies are diseases with an underlying abnormal immune response

The inflammatory myopathies are characterized by primary inflammation of muscle, with resulting fibre necrosis. The inflammatory infiltrate is mainly composed of T-cells and monocytes as part of an abnormal autoimmune response. There are three main types of inflammatory myopathy.

Polymyositis presents clinically with weakness of proximal limb muscles and facial muscles, ptosis, and dysphagia. Although the precise stimulus causing disease is unknown, there are well-recognized clinical associations, the most common being the presence of connective tissue diseases such as SLE, rheumatoid disease or scleroderma. Polymyositis may also be a non-metastatic manifestation of malignancy, in which case it may be part of the clinical syndrome of dermatomyositis in which muscle disease is accompanied by a characteristic skin rash.
Muscle biopsy shows lymphocytic infiltration of muscle with fibre necrosis. Disease may respond to immunosuppressive treatment with steroids or azathioprine.

Inclusion body myositis is clinically similar to polymyositis, but occurs mainly in elderly patients. Muscle biopsy shows inflammation of muscle, fibre necrosis and the presence of vacuoles and filamentous inclusions in fibres (seen by electron microscopy). Diagnosis of this disorder is important because it is slowly progressive and has a poor response to immunosuppressive treatment.

Sarcoidosis may affect muscle, but it is relatively uncommon compared to other types of inflammatory myopathy.

Secondary and metabolic myopathies are common in clinical practice

Diseases of muscle secondary to either systemic disease or metabolic abnormality are seen frequently in clinical practice. There are many types and only the clinically most common are presented.

Type 2 fibre atrophy myopathy is the most common pathological finding in biopsies from patients with muscle weakness. The Type 2 fibres undergo selective atrophy, which can be detected on histochemical staining.
This pattern of muscle abnormality is non-specific and may be secondary to several problems, the main ones being disuse, malignancy, steroid administration, Cushing's disease, thyroid disease and connective tissue diseases.
Endocrine myopathy describes the association of muscle weakness and wasting with endocrine disease. The main cause is corticosteroid excess (therapeutic or in Cushing's disease), which results in atrophy of Type 2 fibres. Similar changes can occur with thyroid disease.

Carcinomatous myopathy describes the association of muscle weakness and wasting with systemic neoplasia, The tumour may cause non-metastatic manifestations in muscle through Type 2 fibre atrophy myopathy, the development of polymyositis or dermatomyositis, or denervation due to a paraneoplastic neuropathy.

Mitochondrial myopathy is due to heritable genetic abnormalities affecting mitochondrial function. Diseases are due to either mitochondrial (maternal inheritance) or nuclear genetic abnormalities (Mendelian inheritance). Disease syndromes are characterized by muscle weakness, particularly extraocular muscles, with or without other neurological and metabolic disturbances. Muscle biopsy reveals abnormal pleomorphic mitochondria, often with crystalline inclusions on electron microscopy. Molecular genetic analysis may show specific abnormalities in the mitochondrial genome, but the nuclear genetic causes are as yet uncharacterized. This is now recognized to be an important and common cause of disease, which can have onset at all ages.

Glycogenoses may affect muscle. The two main types of glycogenosis affecting muscle are acid maltase deficiency (Type 2) and McArdle's disease (myophosphorylase deficiency, Type 5). Both are diagnosable on muscle biopsy.
Congenital myopathies are a cause of childhood hypotonia

The congenital myopathies usually present in childhood as hypotonia (floppy baby syndrome) or muscle weakness. The majority of congenital myopathies are named after the structural abnormalities seen on muscle biopsy, e.g. central core disease, nemaline body myopathy, myotubular myopathy, congenital fibre-type disproportion. They are generally non-progressive, which distinguishes them from muscular dystrophies. Inheritance is variable, with both sporadic and heritable types. Although many types are compatible with a long life expectancy, others may cause disability because of secondary skeletal deformities or respiratory muscle involvement.
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