Applied Evidence

Suspect myopathy? Take this approach to the work-up

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This practical guide, with handy reference tools, will help you to distinguish between myopathy and nonmyopathic disease and further refine your diagnosis.


 

References

PRACTICE RECOMMENDATIONS

› Categorize patients with muscle complaints into suspected myositic, intrinsic, or toxic myopathy to help guide subsequent work-up. C
› Look for diffusely painful, swollen, or boggy-feeling muscles—as well as weakness and pain with exertion—in patients you suspect may have viral myopathy. C
› Consider electromyography and muscle biopsy for patients you suspect may have dermatomyositis. C

Strength of recommendation (SOR)

A Good-quality patient-oriented evidence
B Inconsistent or limited-quality patient-oriented evidence
C Consensus, usual practice, opinion, disease-oriented evidence, case series

CASE › Marie C, a 75-year-old Asian woman, reports weakness in her legs and arms with unsteadiness when walking. She has a vague but persistent ache in her large muscles. Her symptoms have developed slowly over the past 3 months. She denies recent signs or symptoms of infection or other illness. Her medical history includes hypertension, hyperlipidemia, osteopenia, and obesity. Ms. C takes lisinopril 10 mg/d and atorvastatin, which was recently increased from 10 to 20 mg/d.

What would your next steps be in caring for this patient?

Patients who experience muscle-related symptoms such as pain, fatigue, or weakness often seek help from their family physician (FP). The list of possible causes of these complaints can be lengthy and vary greatly, from nonmyopathic conditions such as fibromyalgia to worrisome forms of myopathy such as inclusion body myositis or polymyositis. This article will help you to quickly identify which patients with muscle-related complaints should be evaluated for myopathy and what your work-up should include.

Myopathy or not?

Distinguishing between myopathy and nonmyopathic muscle pain or weakness is the first step in evaluating patients with muscle-related complaints. Many conditions share muscle-related symptoms, but actual muscle damage is not always present (eg, fibromyalgia, chronic pain, and chronic fatigue syndromes).1 While there is some overlap in presentation between patients with myopathy and nonmyopathic conditions, there are important differences in symptoms, physical exam findings, and lab test results (TABLE 11-4). Notably, in myopathic disease, patients’ symptoms are usually progressive, vital signs are abnormal, and weakness is common, whereas patients with nonmyopathic disease typically have remitting and relapsing symptoms, normal vital signs, and no weakness.

Myopathy itself is divided into 3 categories—myositic, intrinsic, and toxic—which reflect the condition, or medication, that brought on the muscle damage (TABLE 22,4-15). Placing patients into one of these categories based on their risk factors, history, and physical exam findings can help to focus the diagnostic work-up on areas most likely to provide useful information.

Myositic myopathy can be caused by infection or autoimmunity

Myositic myopathies result in inflammatory destruction of muscle tissue. Patients with myositic myopathy often exhibit fever, malaise, weight loss, and general fatigue. Though weakness and pain are common, both can be variable or even absent in myositic myopathy.2,5 Myositic myopathy can be caused by infectious agents or can develop from an autoimmune disease.

Infectious myositic myopathy is one of the more common types of myopathy that FPs will encounter.2 Viruses such as influenza, parainfluenza, coxsackievirus, human immunodeficiency virus, cytomegalovirus, echovirus, adenovirus, Epstein-Barr, and hepatitis C are common causes.2,4,16 Patients with 
a viral myositis often report prodromal symptoms such as fever, upper respiratory illness, or GI distress one to 2 weeks before the onset of muscle complaints. Bacterial and fungal myositides are relatively rare. Both most often occur as the result of penetrating trauma or immunocompromise, and are generally not subtle.2 Parasitic myopathy can occur from the invasion of skeletal muscle by trichinella after ingesting undercooked, infected meat.2 Although previously a more common problem, currently only 10 to 20 cases of trichinellosis are reported in the United States each year.17 Due to their rarity, bacterial, fungal, and parasitic myositides are not reviewed here.

Patients with a viral myositis often report prodromal symptoms such as fever, upper respiratory illness, or gastrointestinal distress one to 2 weeks before the onset of muscle complaints. Muscle pain is usually multifocal, involving larger, bilateral muscle groups, and may be associated with swelling.

Patients with viral myositis may exhibit diffusely painful, swollen, or boggy-feeling muscles as well as weakness and pain with exertion. Other signs of viral infection such as rash, fever, upper respiratory symptoms, or meningeal signs may be present. Severe signs include arrhythmia or respiratory failure due to cardiac muscle or diaphragm involvement, or signs of renal failure due to precipitation of myoglobin in the renal system (ie, rhabdomyolysis).2 If the infection affects the heart, patients may develop palpitations, pleuritic chest pain, or shortness of breath.2

Diagnosis of viral myositis relies heavily on clinical suspicion in patients with a fitting history and physical exam findings. Helpful lab tests include a complete blood count (CBC), erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), creatine kinase (CK), and liver function tests (LFTs), all of which can be abnormal in viral myositis. Viral polymerase chain reaction, culture, or antigen testing may be helpful in severe or confusing cases, but in most cases such testing is unnecessary. Muscle biopsy is not recommended except in persistent cases, where definitive identification of the causative agent might alter treatment or when nonviral infection is suspected.2

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