Approach

Clinical evaluation

The diagnosis of rhabdomyolysis is based on presenting complaints and a high index of suspicion if risk factors are present. Obvious presentations include trauma victims with an extremity crush injury or a physically unconditioned person who presents with limb pain after jogging or weightlifting. Patients with medical causes of rhabdomyolysis may be initially asymptomatic and/or present more insidiously.[2] Complaints may be nonspecific such as generalized malaise or dark urine.[2] 

Examination is usually unremarkable, but muscle weakness, pain/tenderness, or swelling of the affected extremity or body region may be present.[2] Commonly implicated muscle groups are the extremities and the lower back.[2] Patients with limb trauma or prolonged limb immobilization should be evaluated for signs of compartment syndrome (a cool extremity with diminished circulation).[2] 

Investigations

In the appropriate clinical setting, a measurement of the patient's serum creatine kinase (CK) level greater than 5 times the normal level, or more than 1000 IU/L, will confirm the diagnosis.[2] Patients with a urine dipstick positive for blood, but no red blood cells on urine microscopy, should have their CK measured.

Full metabolic and endocrine panels should be included as part of the investigative workup for rhabdomyolysis. Baseline evaluation of complete blood count, electrolytes, renal function, uric acid, and coagulation profile is important as patients are at risk of hyperkalemia, acute kidney injury (AKI), hyperphosphatemia, hypocalcemia, and disseminated intravascular coagulation. Serum and/or urine myoglobin levels may be elevated.[2]

Elevated liver aminotransferases, particularly aspartate aminotransferase (AST), are common in the setting of rhabdomyolysis in the absence of significant liver injury.[2] AST concentrations decrease in parallel to serum CK level in hospitalized patients with rhabdomyolysis, suggesting skeletal muscle may be a significant source of AST elevation in these patients.[26]

Other investigations to rule out precipitating causes include thyroid-stimulating hormone, ESR (to screen for inflammatory myopathies), and antinuclear antibodies (to screen for autoimmune diseases).

A toxicology screen to rule out recreational/illicit drug use, including opiate use, should be considered.

Usual medications should be reviewed to assess for medication related to rhabdomyolysis.

In the event of recurrent rhabdomyolysis, details regarding familial history of neuromuscular disorders and prior episodes of exercise intolerance should be obtained and further diagnostic testing pursued where appropriate.[27][28]

Muscle biopsy is only indicated if genetic etiology is suspected.

Risk prediction scores are unlikely to directly influence treatment, but may be useful in estimating prognosis and setting expectations.[2] The McMahon score uses demographic, clinical, and laboratory variables to estimate risk of mortality or severe AKI requiring renal replacement therapy.[29]

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