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A 36-year-old woman presents to the emergency department with left-sided flank pain. The pain is severe, located just below her left ribs, and has been constant for the past 12 hours. It does not radiate or change with position. She also reports a fever to 101°F and general malaise.

  • What additional questions will further characterize her flank pain and help narrow the differential diagnosis?
  • What are the common diagnoses associated with flank pain?

Flank pain refers to pain occurring just below the 12th rib, encompassing the costovertebral angle and area lateral to that angle. Patients often describe flank pain as unilateral upper back pain. The initial differential diagnosis depends on the patient's age, gender, and comorbid illnesses. However, nephrolithiasis, pyelonephritis, and musculoskeletal strain account for most cases.

A careful history often suggests one of these possibilities or raises suspicion for a less common cause. For example, a history of chronic atrial fibrillation increases the likelihood of a renal vascular embolus. Splenic infarct as a cause of left flank pain is unusual but should be considered in patients with suspected endocarditis. If "red flags" arise in the history, life-threatening diagnoses such as rupturing abdominal aortic aneurysm (AAA) or retroperitoneal hemorrhage must also be considered.1

Unfortunately, few data exist on prevalence of the various causes of flank pain. Flank pain is often caused by sudden obstruction of a ureter by a renal calculus or renal colic. Renal colic tends to be sudden, severe, and debilitating. As the offending calculus descends through the collecting system, pain may also occur in the lower abdominal quadrants and genitalia, along with dysuria, frequency, urgency, and hematuria.2

Pyelonephritis commonly causes flank pain, particularly in women. Because women have shorter urethras than men, women have a greater incidence of lower urinary tract infections, which may ascend to one or both kidneys resulting in pyelonephritis. Pain is caused by inflammation of the kidney with stretching of the renal capsule; it may be less severe and more insidious than renal colic.3 A history of fever or dysuria suggests pyelonephritis, although dysuria may not occur in patients with indwelling urinary catheters.4,5 Occasionally, a kidney stone may obstruct the flow of urine, leading to the development of pyelonephritis.6 Such patients have both pyelonephritis and nephrolithiasis, the 2 most common causes of flank pain.

Musculoskeletal causes of flank pain are often clinically obvious. The patient usually describes a precipitating event, such as swinging a baseball bat or lifting a heavy object.


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