Inclusion Body Myositis
Last updated: November 4, 2014
ICD-9 Code: 359.71.
ICD-10: M60.8
Definition: Inclusion body myositis is a form of idiopathic inflammatory myositis (IIM)of unknown etiology. Inclusion body myositis has distinct differences from the more common forms of IIM, PM, and DM.
Pathology: Histopathologic analysis is the key differentiating feature between inclusion body myositis and other forms of IIM. On light microscopic analysis, there is an endomysial accumulation of mononuclear cells, particularly CD8+ T cells (similar to the findings in PM). Other characteristic findings in muscle include vacuoles lined with basophilic granules, eosinophilic inclusions, abnormal microtubular filaments in nuclear and cytoplasmic inclusions, and ragged red fibers.
Demographics: Affected patients are usually older than 50 years old. The male:female ratio is 2:1 or higher. Inclusion body myositis represents 15% to 30% of IIM cases, which have a prevalence of five to 10 per million population.
Cardinal Findings: Symmetric proximal muscle weakness is typically present. Weakness is often slowly progressive and painless. In contrast to PM/DM, distal muscle weakness, and asymmetric involvement are more commonly seen in inclusion body myositis. In addition, 30% of patients may have an associated axonal neuropathy.
Diagnostic Tests: Muscle enzymes (e.g., creatine kinase) are elevated in inclusion body myositis as they are in IIM but usually less so in inclusion body myositis (as much as 10 times normal) than in PM/DM (as much as 50 times normal). Electromyography shows a myopathic pattern (similar to that of PM/DM). Nonetheless, muscle biopsy with electron microscopic analysis is required to make a diagnosis of inclusion body myositis.
Keys to Diagnosis: Often the diagnosis of inclusion body myositis is made when patients with presumed PM or DM either do not respond to steroid therapy or display atypical manifestations (e.g., asymmetry, distal muscle weakness, peripheral neuropathy). In such instances, inclusion body myositis should be suspected, and muscle biopsy repeated or reviewed with emphasis on electron microscopic findings.
Differential Diagnosis: The differential diagnosis of inclusion body myositis includes other types of IIM as well as other endocrinologic, metabolic, infectious, and toxic etiologies (see Myopathy).
Diagnostic Criteria: Diagnostic criteria for inclusion body myositis have been proposed but not validated (Table 19).
| Table 19: Proposed Diagnostic Criteria for Inclusion Body Myositis |
|---|
| Pathologic criteria |
| Electron microscopy |
| 1. Microtubular filaments in the inclusions |
| Light microscopy |
| 1. Lined vacuoles |
| 2. Intranuclear and intracytoplasmic inclusions |
| Clinical criteria |
| 1. Proximal muscle weakness (insidious onset) |
| 2. Distal muscle weakness |
| 3. Electromyographic evidence of generalized inflammatory myopathy |
| 4. Elevation of muscle enzyme levels (creatine kinase, aldolase) |
| 5. Failure of muscle weakness to improve with steroids |
| Definite IBM = pathologic electron microscopic criterion and clinical criterion 1 + 1 other clinical criterion |
| Probable IBM = pathologic light microscopic criterion 1 and clinical criterion 1 + 3 other clinical criteria |
| Possible IBM = pathologic light microscopic criterion 2 + any 3 clinical criteria |
Therapy: Patients are treated like patients with PM/DM. However, because of the poor response to corticosteroid or immunosuppressive therapies, many physicians do not pursue aggressive or alternative therapies in patients with inclusion body myositis with little or no response to corticosteroids.
Prognosis: Inclusion body myositis is a slowly progressive disorder, and patients are less likely to respond to therapy than those with other types of IIM. Approximately 50% of patients have no response to corticosteroids; rare patients have a complete response.
BIBLIOGRAPHY
Rose MB. A prospective natural history study of inclusion body myositis: implications for clinical trials. Neurology 2001;57:548–550.PMID:11502935
Sayers ME, Chou SM, Calabrese LH. Inclusion body myositis: analysis of 32 cases. J Rheumatol 1992;19:1385–1389.PMID:1331441
