Suspect myopathy? Take this approach to the work-up

Autoimmune myositic myopathy. Unlike infectious myopathies, autoimmune myopathies are usually chronic, subtle, and relatively rare. The 3 most common autoimmune myopathies—polymyositis, dermatomyositis, and inclusion body myositis—have a combined prevalence of approximately 10:100,000.⁶ Although these types of myopathies are uncommon, FPs will likely be the first to evaluate a patient with one of them.

Patients with an autoimmune myopathy typically complain of weakness and mild to moderate muscle pain, although pain may be absent. Compared to infectious myopathies, autoimmune myopathies usually exhibit a more indolent course. Patients with advanced disease may report fever, weight loss, shortness of breath from cardiomyopathy, heartburn from a weakened lower esophageal sphincter, and/or a rash.⁵

A patient
 with mild
 to moderate electrolyte problems may complain of muscle fatigue, weakness,
 or pain. Physical examination may reveal symmetric, proximal muscle weakness. Atrophy is typically not seen until late in the disease. Skin exam usually is normal in patients with inclusion body myositis and polymyositis. The typical rash of dermatomyositis is a heliotrope (blue-purple) discoloration on the upper eyelids and a raised, violaceous, scaly eruption on the knuckles (Gottron’s papules).

Laboratory tests that can be helpful include CK, lactate dehydrogenase (LDH), aldolase, and LFTs (reflecting muscle injury, not liver involvement). For polymyositis and dermatomyositis, CK is the most sensitive lab test and often exhibits the highest elevation above normal.⁶ Conversely, CK is often normal or only mildly elevated in inclusion body myositis. Up to 80% of patients with autoimmune myopathy will have antinuclear antibodies.^3,5 ESR and CRP levels are also often elevated.

Both electromyography (EMG) and muscle biopsy may be required to diagnose autoimmune myopathy, but these are typically done under the direction of a rheumatologist after an FP’s initial work-up is inconclusive.

Intrinsic myopathy: Suspect electrolyte problems, other causes

Intrinsic myopathy occurs in patients with electrolyte disorders, diseases of the endocrine system, or underlying metabolic dysfunction.

Electrolyte disorders. Muscle-related symptoms are unlikely to be the chief complaint of patients with severe electrolyte imbalance. However, a patient with mild to moderate electrolyte problems may develop muscle fatigue, weakness, or pain. TABLE 3 reviews other signs and symptoms of electrolyte abnormalities that may be helpful in establishing a diagnosis in a patient with muscle complaints.

Ordering a complete metabolic panel (CMP), CK, and urinalysis (UA) can help rule out electrolyte disorders. If electrolyte disorders are detected, an electrocardiogram is useful to evaluate for cardiac dysfunction. Once an electrolyte disorder is identified, investigate its underlying cause. Correcting the electrolyte disorder should help improve symptoms of myopathy.

Endocrine myopathy can be associated with hypothyroidism, hyperthyroidism, parathyroid disease, vitamin D deficiency, or Cushing syndrome.^8-10,18,19 Although less common than some other causes, identifying endocrine myopathy is crucial because correcting the underlying disease will often improve multiple aspects of the patient’s health.

The presentation of endocrine myopathy may be subtle. Patients with hypothyroidism may experience muscle pain or weakness, fatigue, cold sensitivity, constipation, and dry skin.²⁰ Muscle-related symptoms may be the only sign of endocrine myopathy in a patient who would otherwise be considered to have subclinical hypothyroidism.^8,18 Hyperthyroidism can present with weight loss, heat intolerance, frequent bowel movements, tachycardia, and muscle weakness.²¹

Patients with parathyroid disease— especially patients with chronic renal failure—may report proximal muscle weakness, often in the lower extremities.¹⁹ Complaints of muscle weakness or pain can occur with severe vitamin D deficiency.¹⁰ Patients with Cushing syndrome often experience proximal weakness and weight gain.⁹

Patients with a personal or family history of endocrine disorders, previous thyroid surgery, or those taking medications that can impair thyroid function, such as lithium, amiodarone, or interferon, are at risk for endocrine myopathy.^18-20 Suspect hyperparathyroidism in patients with chronic kidney disease who complain of weakness.

Vitamin D deficiency is relatively common, with at minimum 20% of elderly adults estimated to be deficient.¹⁰ Patients at risk for Cushing disease are most likely receiving pharmacologic doses of glucocorticoids, which can increase their risk of myopathy, or to have ectopic adrenocorticotropic hormone secretion.

Metabolic myopathy results from a lack of sufficient energy production in the muscle. The 3 main groups of metabolic myopathy are impaired muscle glycogenoses, disorders of fatty acid oxidation, and mitochondrial myopathies.⁷

Because metabolic myopathy can occur at any age, a thorough history and physical is crucial for diagnosis. Proximal weakness in metabolic myopathy is often associated with exercise intolerance, stressful illness, or fasting. Patients often present with dynamic abnormalities such as fatigue, muscle cramping, and even rhabdomyolysis during exertion.⁷

When evaluating patients you suspect may have metabolic myopathy, a physical exam may reveal muscle contractures, muscle swelling, or proximal muscle weakness. Patients with certain types of fatty acid oxidation disorders or mitochondrial disorders may also exhibit cardiomyopathy, neuropathy, retinopathy, ataxia, hearing loss, or other systemic manifestations.⁷