Helmet cells are a type of fragmented red blood cell (RBC) that are characterized by a large, smooth, variably concave defect interrupting an otherwise normal RBC contour. They are named for their resemblance to a helmet, with a straight border next to the semicircle and sharp, angular edges. Helmet cells are a subset of schistocytes, which are irregularly shaped RBCs that result from mechanical trauma in the bloodstream.
These fragments are often observed in conditions like microangiopathic hemolytic anemia (MAHA), where the red blood cells are subjected to shearing forces in the blood vessels. This shearing stress can be caused by various factors, including:
- Mechanical heart valves: These artificial valves can cause turbulence in blood flow, leading to RBC fragmentation.
- Disseminated intravascular coagulation (DIC): In this condition, small blood clots form throughout the body, damaging RBCs.
- Thrombotic thrombocytopenic purpura (TTP): This rare disorder involves a deficiency in an enzyme called ADAMTS13, leading to the formation of small blood clots that shear red blood cells.
- Preeclampsia: This condition, often occurring during pregnancy, can cause blood vessel damage and lead to RBC fragmentation.
Identifying helmet cells on a blood smear can be a significant indicator of these conditions.
