Myopathies ?>

Myopathies

Myopathies

Introduction
Background

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 with attention to airway and aspiration precautions and providing supportive care are indicated while inpatient consultation and detailed studies are performed.
Pathophysiology

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, 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.

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.

Mortality/Morbidity
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.
Clinical
History
Obtain the family history to determine presence of periodic paralysis or muscular dystrophy. Personal history of autoimmune disease, endocrinopathy, renal insufficiency, and/or alcoholism should be noted.
Discuss any previous episodes of severe weakness, particularly any that occurred after exercise or exposure to cold temperatures, which may indicate one of the periodic paralyses. Some patients with familial hypokalemic periodic paralysis may note that the symptoms occur after eating high-carbohydrate meals.
History of medication use is very important. Steroids, lipid lowering agents, retroviral agents, alcohol, colchicine, pentachlorophenol (PCP), heroin, and a myriad of other medications may cause myopathies. In some cases, the combination of multiple myopathic agents is responsible for the acute deterioration.
Occupational and travel history may lead a physician to consider ingestion of barium chloride or carbonate as a cause for acute hypokalemic paralysis.
These are absorbable salts (in contrast to nonabsorbable, safe, widely used barium sulfate) that may contaminate table salt or flour. Absorbable salts may be used industrially for glazing pottery.
Paralysis results when passive efflux of potassium is blocked at the cell membrane and elevated intracellular potassium decreases the resting membrane potential.
Symptoms noted generally include the following:
Symmetric proximal muscle weakness
Malaise
Fatigue
Patient may note dark colored urine and/or fever.
No sensory complaints or paresthesias are noted with myopathies.
Atrophy and hyporeflexia are very late findings in most patients with myopathy. The early presence of these findings usually implicates neuropathies.
Significant muscle pain and tenderness without weakness should prompt physicians to consider other causes.
Acuity of symptom onset aids in diagnosis.
Weakness progressing over hours suggests a toxic etiology or one of episodic paralyses.
Weakness developing over days suggests acute dermatomyositis or rhabdomyolysis.
Symptom development over a period of weeks suggests polymyositis, steroid myopathy, or myopathy resulting from endocrine causes (eg, hyperthyroidism, hypothyroidism).
Symptoms of the patient indicate which muscle groups are involved.
Difficulty rising from chairs, getting out of the bathtub, climbing stairs, and/or shaving or combing the hair suggests proximal muscle weakness.
Weakness of distal muscles will present with symptoms of weak grasp, handwriting problems, and walking difficulties, (eg, flapping gait).
Physical
Objective weakness, usually in a symmetric distribution of proximal muscle groups is observed.
Muscle tenderness is rare.
Fever, particularly with pyomyositis or polymyositis may occur.
Muscle mass should be normal. Atrophy is a very late sign with muscle disorders.
Normal level of consciousness should be preserved.
Deep tendon reflexes (DTRs) and sensory perception should be normal. DTRs may be diminished or absent in hypokalemic paralysis.
Skin examination may reveal Gottron papules, which are pink-to-violaceous scaly areas over knuckles, elbows, and knees in dermatomyositis.
Causes
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), and 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.
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)
Coxsackie A and B viruses
Influenza
Lyme disease
Staphylococcus aureus muscle infection (frequent cause of pyomyositis)
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:
Steroids (especially with prolonged high doses, divided doses over 25 mg/d, fluorinated steroid use)
AZT
Lovastatin and other statins
Cocaine
Colchicine
Amiodarone and others that inhibit CYP3A4 when combined with simvastatin
Acute periodic paralysis may be classified as hypokalemic, hyperkalemic, or normokalemic.
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 no muscle contraction can occur; hence, 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.
Duchenne dystrophy, 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 Duchenne dystrophy, 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.

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