Rare Infectious Disease News

Disease Profile

Necrotizing autoimmune myopathy

Prevalence
Prevalence estimates on Rare Medical Network websites are calculated based on data available from numerous sources, including US and European government statistics, the NIH, Orphanet, and published epidemiologic studies. Rare disease population data is recognized to be highly variable, and based on a wide variety of source data and methodologies, so the prevalence data on this site should be assumed to be estimated and cannot be considered to be absolutely correct.

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Europe Estimated

Age of onset

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ICD-10

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Inheritance

Autosomal dominant A pathogenic variant in only one gene copy in each cell is sufficient to cause an autosomal dominant disease.

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Autosomal recessive Pathogenic variants in both copies of each gene of the chromosome are needed to cause an autosomal recessive disease and observe the mutant phenotype.

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X-linked
dominant X-linked dominant inheritance, sometimes referred to as X-linked dominance, is a mode of genetic inheritance by which a dominant gene is carried on the X chromosome.

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X-linked
recessive Pathogenic variants in both copies of a gene on the X chromosome cause an X-linked recessive disorder.

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Mitochondrial or multigenic Mitochondrial genetic disorders can be caused by changes (mutations) in either the mitochondrial DNA or nuclear DNA that lead to dysfunction of the mitochondria and inadequate production of energy.

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Multigenic or multifactor Inheritance involving many factors, of which at least one is genetic but none is of overwhelming importance, as in the causation of a disease by multiple genetic and environmental factors.

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Not applicable

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Other names (AKA)

Immune-mediated necrotizing myopathy; Anti-HMG-CoA myopathy; Anti-SRP myopathy;

Categories

Nervous System Diseases

Summary

The following summary is from Orphanet, a European reference portal for information on rare diseases and orphan drugs.
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Orpha Number: 206569

Definition
Necrotizing autoimmune myopathy (NAM) is a rare form of idiopathic inflammatory myopathy characterized clinically by acute or subacute proximal muscle weakness, and histopathologically by myocyte necrosis and regeneration without significant inflammation.

Epidemiology
The prevalence and annual incidence of NAM are not known but the disorder is very rare. About 300 cases have been reported to date.

Clinical description
Age of onset ranges from 30 to 70 years of age in reported cases. The main presenting feature of NAM is subacute severe symmetrical proximal myopathy, with a markedly elevated creatine kinase (CK) level. Its presentation is similar to that of polymyositis (see this term) with upper and lower limb weakness causing difficulty in moving from a sitting position, climbing stairs, or lifting objects The neck flexor, pharyngeal, and respiratorymuscles may also be involved. Other manifestations include fatigue, weight loss dysphagia and dyspnea. Interstitial lung disease (see this term) and cardiac involvement have also been reported. The course is often severe but may be self-limiting and recovery may occur within weeks to months of discontinuing the causative agent, if identified.

Etiology
The disease is thought to be related to an immune response possibly triggered by drug therapy (statins), connective tissue diseases, or cancer. The exact mechanism underling the disorder is not known but some autoantibodies appear to be a likely cause. Malignancy may be involved.

Diagnostic methods
Diagnosis is based on the clinical picture and on muscle biopsy showing minimal or no inflammatory infiltrates and marked muscle necrosis, unlike other inflammatory myopathies. Electromyography (EMG) shows myopathic findings. Creatine kinase (CK) levels are often more than 10 times above the upper limit of normal at the time of onset of muscle weakness. Magnetic resonance imaging (MRI) may show diffuse or patchy edema within muscles. Anti-SRP and anti-HMGCoAR autoantibodies are frequently associated with this condition. Currently, seronegative NAM represents 20-30% of the cases.

Management and treatment
Treatment of the underlying cause, if identified, is essential (statin discontinuation, or malignancy). NAM patients generally respond well to multiple-agent, long-term immunosuppressive therapies starting by high dose corticosteroids. Intravenous immunoglobulin (IVIg) appears to be effective. Rituximab has also shown beneficial effects. Response to therapy should be assessed clinically on the basis of muscle strength and biologically on CK levels.

Visit the Orphanet disease page for more resources.

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    Where to Start

      In-Depth Information

      • Orphanet is a European reference portal for information on rare diseases and orphan drugs. Access to this database is free of charge.