Necrotizing autoimmune myopathy

Alternate names

Immune-mediated necrotizing myopathy; Anti-HMG-CoA myopathy; Anti-SRP myopathy; Autoimmune necrotizing myositis; IMNM; NAM; Immune myopathy with myocyte necrosis

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.

Cause

• 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

Age of onset ranges from 30 to 70 years of age in reported cases.

Signs and symptoms

• 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

• 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.

NIH genetic and rare disease info

• NIH info on Necrotizing autoimmune myopathy

Necrotizing autoimmune myopathy is a rare disease.

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