Rheumatology · Inflammatory Myopathies
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Inclusion body myositis is the most common inflammatory myopathy in patients older than 50 years of age.
Clinical presentation features asymmetric weakness involving the quadriceps and deep finger flexors.
Patients frequently present with distal muscle atrophy and early-onset dysphagia due to involvement of the pharyngeal muscles.
Physical examination reveals atrophy of the forearm flexors and weakness of knee extension leading to frequent falls.
Serum creatine kinase (CK) levels are typically only mildly elevated or normal, distinguishing it from polymyositis.
Muscle biopsy demonstrates rimmed vacuoles and intracellular amyloid deposits, which are characteristic of the diagnosis.
Inclusion body myositis is characteristically refractory to corticosteroids and immunosuppressive therapy, unlike other inflammatory myopathies.
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A 68-year-old male presents to the clinic complaining of progressive difficulty climbing stairs and frequent tripping over the last 18 months. He also reports increasing difficulty with fine motor tasks, such as buttoning his shirt and gripping objects. Physical examination reveals asymmetric atrophy of the quadriceps and weakness of the deep finger flexors. His serum creatine kinase is 350 U/L. He has no history of skin rashes or joint pain.
What is the most likely diagnosis?
Inclusion body myositis
The patient's age, asymmetric weakness of the quadriceps and finger flexors, and mildly elevated CK are classic for inclusion body myositis, which is confirmed via muscle biopsy showing rimmed vacuoles.
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Etiology / Epidemiology
Most common inflammatory myopathy in patients >50 years old; male predominance.
Clinical Manifestations
Asymmetric weakness of quadriceps and finger flexors; early atrophy.
Diagnosis
Muscle biopsy showing rimmed vacuoles and endomysial inflammation.
Treatment
Refractory to standard therapy; physical therapy is the primary management.
Prognosis
Slowly progressive; most patients eventually require assistive devices.
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Epidemiology & Etiology
This is the most frequent idiopathic inflammatory myopathy in patients over age 50. It exhibits a distinct male-to-female ratio of 3:1. The etiology remains idiopathic, though it is characterized by a combination of inflammatory and degenerative processes.
Pertinent Anatomy
The disease selectively targets the quadriceps (knee extensors) and the deep finger flexors (flexor digitorum profundus). This specific distribution leads to the classic clinical presentation of frequent falls and loss of grip strength.
Pathophysiology
The process involves both autoimmune-mediated muscle fiber injury and protein aggregation. Pathological hallmarks include amyloid deposits and rimmed vacuoles within muscle fibers. Chronic inflammation leads to progressive muscle fiber loss and replacement by fibrosis.
Clinical Manifestations
Patients present with insidious, asymmetric weakness. Look for early atrophy of the quadriceps and forearm muscles. Dysphagia occurs in up to 50% of patients due to involvement of the cricopharyngeus muscle. Unlike polymyositis, the weakness is often distal and asymmetric.
Diagnosis
The muscle biopsy is the gold standard, revealing rimmed vacuoles and endomysial inflammation. Serum creatine kinase (CK) is typically only mildly elevated, usually <10-12 times the upper limit of normal. Electromyography (EMG) shows a mixed myopathic and neuropathic pattern.
Treatment
Inclusion body myositis is notoriously refractory to corticosteroids and immunosuppressants. Physical therapy and occupational therapy are the cornerstones of management to maintain function. Avoid aggressive immunosuppression as it provides no clinical benefit and increases infection risk.
Prognosis
The disease is slowly progressive with a high rate of morbidity. Most patients will eventually require a cane or wheelchair. Monitor for aspiration pneumonia secondary to progressive dysphagia.
Differential Diagnosis
Polymyositis: symmetric proximal weakness, usually responds to steroids
Dermatomyositis: associated with pathognomonic skin findings like Gottron's papules
ALS: upper and lower motor neuron signs without inflammatory biopsy findings
Myasthenia Gravis: fluctuating weakness and ocular involvement
Limb-Girdle Muscular Dystrophy: genetic etiology, usually earlier onset