helmet cell

**Definition:** Helmet cells are schistocytes, which are RBC fragments produced when red cells are sheared. They have a flat or straight border with a bite-like or concave edge, often with one or two pointed projections ("horns"). Related forms include triangular cells and keratocytes (horn cells).

**Pathophysiology:** RBCs are mechanically damaged as they pass fibrin strands in small vessels or turbulent flow across abnormal surfaces. The result is intravascular hemolysis with fragmented cells.

**Major causes:** - **Microangiopathic hemolytic anemia (MAHA)**, caused by fibrin or platelet microthrombi: - TTP: ADAMTS13 deficiency or autoantibody, leading to ultra-large vWF multimers. Classic pentad: fever, anemia, thrombocytopenia, renal dysfunction, neurologic changes. - HUS: Shiga toxin from EHEC O157:H7 or Shigella, causing the triad of anemia, thrombocytopenia, and AKI. - DIC: low fibrinogen, elevated PT/PTT, and elevated D-dimer. - HELLP syndrome and malignant hypertension. - **Mechanical heart valve** or other prosthetic hemolysis, including a paravalvular leak. - **Other:** severe burns, march hemoglobinuria, and vasculitis.

**Lab findings:** - Anemia with elevated reticulocyte count. - Elevated LDH and indirect bilirubin. - Low haptoglobin and hemoglobinuria or hemosiderinuria. - **Negative Coombs test**, which separates MAHA from autoimmune hemolytic anemia. - Thrombocytopenia in TTP, HUS, and DIC. - Peripheral smear shows schistocytes, including helmet cells.

**Key distinctions:** - TTP and HUS have normal PT/PTT. DIC has prolonged PT/PTT. - Spherocytes suggest autoimmune hemolysis or hereditary spherocytosis, not mechanical injury. - Bite cells (G6PD deficiency) result from phagocytic removal of Heinz bodies by splenic macrophages, which is a different mechanism from helmet cells. - Schistocytes may also appear in severe iron deficiency or thalassemia, but in much smaller numbers.

**Management:** Treat the underlying cause. - TTP: urgent plasma exchange, steroids, and caplacizumab or rituximab. Avoid platelet transfusion unless there is life-threatening bleeding. - HUS: supportive care. Avoid antibiotics in EHEC infection. - DIC: treat the trigger and replace blood products as needed. - Prosthetic valve hemolysis: consider surgical repair if severe.