Myopathies

Myopathy is a muscle disease unrelated to any disorder of innervation or neuromuscular junction. Etiologies vary widely. The common symptoms are muscle weakness, impaired function in activities of daily life, and, rarely, muscle pain and tenderness. Presence of discolored or dark urine suggests myoglobinuria.

For the emergency physician, it is important to distinguish neurologic from muscular dysfunction. However, in the face of profound weakness, establishing ABCs (airway, breathing, circulation) with attention to airway and aspiration precautions and providing supportive care are indicated while inpatient consultation and detailed studies are performed.

Most congenital myopathies or inherited myopathies are chronic slowly progressive diseases. The emergency physician rarely attends to a patient specifically to treat congenital myopathy unless acute deterioration occurs. Emergency physicians attend to patients with metabolic, ^[1] inflammatory, endocrine, and toxic causes of myopathy more often than those with congenital causes because of the acute or subacute onset of symptoms associated with noncongenital forms. ^[2]

Periodic paralyses are a group of diseases that cause patients to present with acute weakness due to potassium shifts, leading to muscle dysfunction. A genetic defect of the sodium ion channel in muscle cell membranes is responsible for the paralysis, which may last from hours to days.

Idiopathic myopathies are thought to result from immune-mediated phenomena including sarcoidosis with myopathy, polymyositis, and dermatomyositis. Some idiopathic myopathies are associated with connective tissue disease (eg, systemic lupus erythematosus (SLE), rheumatoid arthritis(RA), polyarteritis nodosa).

Acute alcoholic myopathy should be considered in patients who, after binging on alcohol, present with muscle pain that mostly involves limb weakness and myoglobinuria. Note the following:

• Significance of acute alcoholic myopathy is that the precipitation of myoglobin in the renal tubules can cause acute renal tubular necrosis
• Aggressive hydration and, occasionally, administration of mannitol and furosemide to increase diuresis, are essential to maintain renal function
• Alcohol, in addition to the acute syndrome of muscle necrosis, causes a more chronic myopathy associated with gradual progressive weakness and atrophy that usually involves the hip and shoulder girdle; this chronic myopathy does not result in myoglobinuria or elevated creatine kinase-MM (CK-MM) levels

Infectious causes include the following:

• Trichinosis
• Cysticercosis ( Taenia solium)
• Toxoplasmosis
• Human immunodeficiency virus (HIV) ^[3]
• Coxsackie A and B viruses
• Influenza
• Lyme disease
• Staphylococcus aureus muscle infection (frequent cause of pyomyositis)
• Severe acute respiratory syndrome coronavirus disease 2 (SARS-CoV-2) ^{[4, 5, 6]}

Endocrine causes of myopathy include the following:

• Addison disease, particularly when fluid and electrolyte problems are present
• Cushing disease
• Hypothyroidism (CK may be mildly elevated)
• Hyperthyroidism (CK may be normal)
• Hyperparathyroidism

Drug-induced or toxic causes of myopathy include use of the following ^[7] :

• Steroids (especially with prolonged high doses, divided doses over 25 mg/day, fluorinated steroid use): Implicated in critical illness myopathy
• Zidovudine (AZT)
• Lovastatin and other statins ^{[8, 9]}
• Cocaine
• Colchicine
• Amiodarone and other agents that inhibit CYP3A4 when combined with simvastatin

Acute periodic paralysis may be classified as hypokalemic, hyperkalemic, or normokalemic. Note the following:

• Normokalemic paralysis causes the most severe and prolonged attacks
• Patients usually feel well between attacks, but some have myotonia (ie, muscle stiffness) or residual weakness after repeated episodes
• A genetic defect has been linked to these diseases, but in some instances, hypokalemia may cause acute weakness in healthy individuals
• Acute hypokalemic periodic paralysis may be primary (ie, familial) or secondary to excessive renal or GI losses or endocrinopathy; in these cases, intracellular shift of potassium depolarizes the cell membrane, rendering it inexcitable and ensuring that no muscle contraction can occur, with the result that the patient experiences paralysis; this may occur independent of the sodium-potassium pump
• Familial periodic paralysis usually occurs in Caucasian males, is autosomal dominant, and may last as long as 36 hours
• Attacks usually occur at night or in early morning upon awakening and can be precipitated by a diet high in carbohydrates, rest following exercise, or glucose and insulin given intravenously

Thyrotoxic periodic paralysis and Conn syndrome (ie, primary hyperaldosteronism) occur in Asians and are considered to have low potassium as the mechanism for paralysis. Treatment of the underlying disease and electrolyte disorder are curative.

Excessive licorice ingestion, as well as a myriad of other causes of hypokalemia, can cause paralysis.

Muscular dystrophies are chronic, progressive, inherited myopathies that present from early childhood to adolescence, as follows:

• Duchenne muscular dystrophy (DMD), observed in boys younger than 5 years, causes the most severe disease; cardiomyopathy is common in affected children
• Weakness and muscle wasting in a child with elevated CK occurs with DMD, but other dystrophies (eg, fascioscapulohumeral, limb-girdle, myotonic) may occur in boys and girls with normal muscle-enzyme levels
• Patients with mild cases may lead fairly normal lives, but progressive weakness and scoliosis impairing pulmonary function often results in recurrent infections and exacerbation of weakness

The prognosis varies depending on the etiology. The morbidity and mortality of myopathies is related to the etiology of the condition, severity of disease, and the presence of comorbid conditions. Severe weakness may lead to respiratory failure and death.

Thyrotoxic hypokalemic periodic paralysis is known to occur in Asian men, and one study suggests that Polynesians are also at risk for this condition. ^[10]

Complications

Complications of myopathy include the following:

• Cardiac arrhythmias
• Hypertension
• Dysphagia
• Acute gastric dilation
• Respiratory failure
• Endocrinopathies
• Cataracts
• Sensorineural hearing loss
• Seizures and cerebral dysplasias
• Early death