Master Hemolytic Uremic Syndrome (HUS) with Picmonic for Medicine

HUS is commonly preceded by an infectious gastroenteritis. Shiga toxin from E. coli O157:H7 enters the bloodstream through the inflamed intestinal mucosa, binds to Gb3 receptors on endothelial cells, and triggers endothelial damage and platelet activation. This is the mechanism behind typical, diarrhea-associated HUS, which is the most common form in children.

Shiga toxin directly injures endothelial cells, particularly in renal glomeruli. The endothelial damage exposes the subendothelial matrix, which triggers platelet aggregation and microthrombus formation, a hallmark of HUS. This is the pathophysiologic basis of microangiopathic hemolytic anemia and thrombocytopenia.

Thrombi in small vessels shear passing red blood cells, producing schistocytes (helmet cells) on peripheral smear. This destruction of red blood cells leads to hemolytic anemia, elevated LDH, indirect hyperbilirubinemia, and low haptoglobin, which are important USMLE high-yield lab findings.

Platelet consumption within microvascular thrombi causes low platelet counts. Clinically, this may manifest as easy bruising, petechiae, or mild bleeding, although severe bleeding is uncommon.

Microthrombi deposition in glomeruli causes renal ischemia, tubular injury, and acute kidney injury. Lab findings include elevated BUN and creatinine, hematuria, proteinuria, and electrolyte abnormalities such as hyperkalemia and metabolic acidosis. This is the reason HUS is a leading cause of acute kidney injury in children.

Schistocytes appear as helmet-shaped RBC fragments on peripheral smear. This is a high-yield USMLE sign of MAHA in thrombotic microangiopathies like HUS and TTP.

Lactate dehydrogenase (LDH) is elevated due to red blood cell destruction in MAHA. High LDH is a supportive lab marker for HUS and reflects cellular injury.

Dialysis is required in severe cases with BUN >80 mg/dL, symptomatic uremia, or refractory electrolyte disturbances, especially hyperkalemia or fluid overload. Pediatric patients may recover renal function after supportive care, but some may progress to chronic kidney disease.

Treatment is primarily supportive, including intravenous hydration to maintain renal perfusion, electrolyte monitoring and correction, and RBC or platelet transfusions if indicated. Antibiotics are generally avoided in typical HUS because they may increase Shiga toxin release. In atypical HUS, complement inhibition with eculizumab is used.