Medical Challenge Question 12 Answer
Answer is B. This patient who is a runner is suffering from exercise-induced hemolysis.
Exercise-induced hemolysis also called march hemoglobinuria or runner's hemolysis is caused by erythrocyte damage from repetitive mechanical trauma. Trauma happens with activities like running or marching, resulting in intravascular hemolysis with loss of iron in the urine. In this patient the urinalysis is positive for blood, but there are no erythrocytes. This indicates iron loss in the urine.
Low haptoglobin combined with elevated LDH and CK are indicative of red cell destruction, aka hemolysis. Hemolysis causes increased levels of free hemoglobin in the plasma, which is filtered by the kidneys, resulting in hemoglobinuria. With prolonged intravascular hemolysis, patients may become secondarily iron deficient through iron loss in the urine. Patients with this entity may show iron deficiency anemia with evidence of urine iron loss, indicated by the presence of hemosiderin in sloughed tubular cells by Prussian blue staining. Exercise-induced hemolysis is considered a benign condition and is treated by removal or reduction of the traumatic cause of erythrocyte injury.
Strenuous exercise is a well-described cause of either gross or microscopic hematuria. Although the mechanism for hematuria associated with exercise has not been fully elucidated, it is considered a benign condition and should resolve with termination of exercise. This patient has evidence of hemoglobinuria but not of hematuria given the lack of erythrocytes in her urine, making this a less likely diagnosis.
An inflammatory myopathy, such as polymyositis, may cause muscle damage and the finding of positive blood on the urine dipstick without significant hematuria. However, inflammatory myopathies are usually associated with muscle weakness and tenderness and an elevated creatine kinase level, which are absent in this patient.
Rhabdomyolysis is caused by injury and necrosis of muscle with release of intracellular muscle contents into the circulation; myoglobin may be detected in the urine as hemoglobin in the absence of erythrocytes. However, rhabdomyolysis is usually associated with muscle pain and significant elevations in circulating muscle enzyme levels, including creatine kinase. This patient has no muscle tenderness and her creatine kinase level is normal, making this diagnosis unlikely.
References:
Dang CV. Runner's anemia. JAMA. 2001 Aug 8;286(6):714-6. PMID: 11495622