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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
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
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
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