Necrotizing autoimmune myopathy
Alternate names[edit | edit source]
Immune-mediated necrotizing myopathy; Anti-HMG-CoA myopathy; Anti-SRP myopathy; Autoimmune necrotizing myositis; IMNM; NAM; Immune myopathy with myocyte necrosis
Definition[edit | edit source]
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[edit | edit source]
The prevalence and annual incidence of NAM are not known but the disorder is very rare. About 300 cases have been reported to date.
Cause[edit | edit source]
- 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.
Onset[edit | edit source]
Age of onset ranges from 30 to 70 years of age in reported cases.
Signs and symptoms[edit | edit source]
- 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 with upper and lower limb weakness causing difficulty in moving from a sitting position, climbing stairs, or lifting objects .
- The neck flexor, pharyngeal, and respiratory muscles may also be involved.
- Other manifestations include fatigue, weight loss dysphagia and dyspnea.
- Interstitial lung disease 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.
Diagnosis[edit | edit source]
- 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[edit | edit source]
- 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.
NIH genetic and rare disease info[edit source]
Necrotizing autoimmune myopathy is a rare disease.
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