A 52-year-old woman comes to the clinic with gradual-onset weakness of the leg muscles. For several months, she has had increasing difficulty climbing stairs and rising from a chair. The patient initially attributed her symptoms to being "out of shape." She has no problem chewing food, changes in bowel or bladder function, morning stiffness, or joint pain. Medications include lisinopril and amlodipine for hypertension. She does not use tobacco, alcohol, or illicit drugs. Vital signs are normal. Examination shows 4/5 strength in the thigh muscles of both legs, and distal muscle strength is 5/5. Repeated muscle contractions do not alter the weakness. Mild tenderness is noted in the proximal thigh muscles. Deep tendon reflexes and sensory examination are normal, and Babinski sign is absent. Which of the following would establish a diagnosis in this patient?
Distinguishing features of fibromyalgia, polymyositis & polymyalgia rheumatica | ||
Clinical features | Diagnosis | |
Fibromyalgia |
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Polymyositis |
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Polymyalgia rheumatica |
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ANA = antinuclear antibody; AST = aspartate aminotransferase; ESR = erythrocyte sedimentation rate. | ||
Polymyositis is an inflammatory muscle disease that presents with slowly progressive proximal weakness of the lower extremities (eg, difficulty negotiating stairs or rising from a seated position). Proximal arm weakness usually follows, leading to difficulty working with the arms overhead. Patients may develop dysphagia due to involvement of the striated muscles of the upper pharynx. Mild pain or tenderness can develop due to inflammation; however, significant pain or a lack of weakness should prompt consideration of other diagnoses. Polymyositis is similar to dermatomyositis but without skin findings.
Muscle enzymes (eg, creatine kinase) are invariably elevated, and autoantibodies (eg, antinuclear antibodies, anti-Jo-1 antibodies) are present in most cases. A muscle biopsy is the most definitive diagnostic test and shows a mononuclear infiltrate surrounding necrotic and regenerating muscle fibers.
(Choice A) Acetylcholine receptor antibodies are present in myasthenia gravis, which is characterized by muscle fatigability (weakness increases with repeated activity) and involves oculobulbar muscles in addition to proximal limb muscles.
(Choices B and G) ACE inhibitors (eg, lisinopril) can cause hyperkalemia, leading to flaccid paralysis. However, this is unlikely to evolve over months and is uncommon in patients without underlying renal insufficiency or additional medications that cause hyperkalemia. Hypercalcemia can also cause weakness, but significant hypercalcemia would likely be associated with additional symptoms (eg, nausea, constipation, polydipsia).
(Choices C and D) Erythrocyte sedimentation rate and C-reactive protein are inflammatory markers often used in the assessment of polymyalgia rheumatica, a syndrome characterized by proximal muscle pain and stiffness out of proportion to weakness. It most often affects the shoulders. In polymyositis, weakness is typically more prominent than pain. Erythrocyte sedimentation rate and C-reactive protein may be elevated but are nonspecific. Electromyography is useful for differentiating polymyositis from other causes of weakness (eg, amyotrophic lateral sclerosis) but is also nonspecific.
(Choice E) This patient has no findings to warrant an MRI, such as back/radicular pain (herniated disk), bowel/bladder dysfunction (cauda equina syndrome), abnormal sensory examination (cord compression), or fever or intravenous drug use (epidural abscess).
Educational objective:
Polymyositis is characterized by chronic proximal weakness. Muscle enzymes (eg, creatine kinase) are elevated, and autoantibodies (eg, antinuclear antibodies, anti-Jo-1 antibodies) are present in most cases. Muscle biopsy is the most definitive test and shows a mononuclear infiltrate surrounding necrotic and regenerating muscle fibers.