A 32-year-old woman is brought to the emergency department by her husband due to sudden onset of confusion and strange behavior. She has also developed small, purple spots on her lower legs and has had a low-grade fever for the past 24 hours. Her medical history is unremarkable. Vital signs show her temperature is 38.1°C (100.6°F), blood pressure is 130/85 mmHg, and heart rate is 105/min. On physical examination, she is disoriented to time and place. Petechiae are noted on both shins.
Laboratory studies show the following:
Hemoglobin: 8.5 g/dL
Platelet count: 22,000/μL
Serum Creatinine: 1.6 mg/dL (Baseline: 0.8 mg/dL)
Total bilirubin: 3.2 mg/dL
Lactate dehydrogenase (LDH): 980 U/L
Prothrombin time (PT) and Partial thromboplastin time (PTT): Normal
Peripheral blood smear: Significant numbers of schistocytes
Which of the following is the most likely underlying mechanism of this patient's condition?
The correct answer is:
B) Deficiency of a von Willebrand factor-cleaving protease
This patient is presenting with the classic pentad of thrombotic thrombocytopenic purpura (TTP):
Microangiopathic hemolytic anemia (MAHA): Elevated LDH, indirect bilirubin, and schistocytes.
Thrombocytopenia due to consumption in microthrombi.
Neurologic symptoms such as confusion, headache, or focal deficits.
Fever.
Renal insufficiency.
The pathophysiology involves a deficiency (usually acquired via an autoantibody) of ADAMTS13, a metalloprotease responsible for cleaving ultra-large von Willebrand factor (vWF) multimers. Without the molecular scissors, the massive vWF multimers persist and cause spontaneous platelet aggregation and microthrombi formation in small vessels. As red blood cells pass through these partially occluded vessels, they are sheared, resulting in the schistocytes (helmet cells) seen on the peripheral smear.
Answer choice A: Autoantibodies against platelet glycoprotein IIb/IIIa, is incorrect. This describes immune thrombocytopenic purpura (ITP). While ITP causes isolated thrombocytopenia, it does not cause microangiopathic hemolysis (no schistocytes) or the multi-organ involvement seen in TTP.
Answer choice C: Direct destruction of platelets by bacterial Shiga toxin, is incorrect. This describes hemolytic uremic syndrome (HUS), which is closely related to TTP. However, HUS is more common in children, typically follows a diarrheal illness (EHEC O157:H7), and is characterized by more prominent kidney injury rather than neurologic symptoms or fever.
Answer choice D: Mutation in the gene encoding the ADAMTS13 enzyme, is incorrect. While hereditary TTP (Upshaw-Schülman syndrome) exists, it is exceptionally rare. The vast majority of adult-onset TTP is due to an acquired autoantibody against the protease, not a genetic mutation.
Answer choice E: Widespread activation of the coagulation cascade, is incorrect. This describes disseminated intravascular coagulation (DIC). While DIC also presents with schistocytes and thrombocytopenia, it is characterized by abnormal coagulation studies (elevated PT/PTT and low fibrinogen). In TTP and HUS, the coagulation cascade is not activated, so PT and PTT remain normal.
Key Learning Point
Thrombotic thrombocytopenic purpura (TTP) is a medical emergency. It is caused by an acquired deficiency of ADAMTS13, leading to ultra-large vWF multimers and microthrombi. The diagnosis should be suspected in any patient with thrombocytopenia and schistocytes. Treatment must be initiated urgently with plasmapheresis to remove the autoantibodies and replace the deficient enzyme.
