Wednesday, June 12, 2024

Myopathies

Practice Essentials

A myopathy is a muscle disease unrelated to any disorder of innervation or neuromuscular junction. These conditions have widely varying etiologies, including congenital or inherited, idiopathic, infectious, metabolic, inflammatory, endocrine, and drug-induced or toxic.

History

Important information to obtain during the patient’s history includes the following:

Family history: Any periodic paralysis or muscular dystrophy?

Personal history: Presence of autoimmune disease, endocrinopathy, renal insufficiency, and/or alcoholism? Previous episodes of severe weakness (eg, postexercise, after exposure to cold [possibly one of periodic paralyses]; post high-carbohydrate meals [familial hypokalemic periodic paralysis])

Medications (eg, steroids, lipid-lowering agents, retroviral agents, alcohol, colchicine, pentachlorophenol [PCP], heroin)

Occupational and travel history (potential ingestion of barium chloride or carbonate [acute hypokalemic paralysis])

Signs and symptoms

The common symptoms of myopathy are muscle weakness, impaired function in activities of daily life, and, rarely, muscle pain and tenderness. Significant muscle pain and tenderness without weakness should prompt consideration of other causes.

General signs and symptoms of myopathy include the following:

Symmetric proximal muscle weakness

Malaise, fatigue

Dark-colored urine (suggests myoglobinuria) and/or fever

Absence of sensory complaints or paresthesias; however, deep tendon reflexes (DTRs) may be diminished/absent in hypokalemic paralysis

Very late findings: Atrophy and hyporeflexia (early presence usually implicates neuropathies)

Normal level of consciousness

Gottron papules in dermatomyositis: Pink-to-violaceous scaly areas over knuckles, elbows, and knees

The acuity of symptom onset may aid in the diagnosis, as follows:

Weakness progressing over hours: Possible toxic etiology or one of episodic paralyses

Weakness developing over days: May be an acute dermatomyositis or rhabdomyolysis

Symptom development over a period of weeks: May be polymyositis, steroid myopathy, or myopathy resulting from endocrine causes (eg, hyperthyroidism, hypothyroidism)

Indications of which muscle groups are involved include the following symptoms:

Proximal muscle weakness: Difficulty rising from chairs, getting out of the bathtub, climbing stairs, and/or shaving or combing the hair

Weakness of distal muscles: Weak grasp, handwriting problems, and walking difficulties, (eg, flapping gait)

See Clinical Presentation for more detail.

Diagnosis

Laboratory testing

The following laboratory tests may be used to evaluate patients with myopathies:

Creatine kinase (CK) levels with isoenzymes

Levels of electrolytes, calcium, and magnesium

Serum myoglobin levels

Serum creatinine and blood urea nitrogen levels

Urinalysis: Myoglobinuria indicated by positive urinalysis with few red blood cells on microscopic evaluation

Complete blood count

Erythrocyte sedimentation rate

Thyroid function tests

Aspartate aminotransferase levels

Other studies may include the following:

Electrocardiography

Antinuclear antibody levels

Genetic testing

Electromyography

Magnetic resonance imaging (to assess complications or rule out neurologic disease)

Muscle biopsy

See Workup for more detail.

Management

The treatment of a myopathy is dependent on its etiology and can range from supportive and symptomatic management to therapy for specific conditions. Such treatments may include the following:

Supportive: Management of airway, breathing, circulation; hydration; intensive care management may be needed in some cases

Drug therapy

Physical therapy

Bracing

Surgery

See Treatment for more detail.

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