Clozapine-induced pulmonary embolism in a patient with minimal pre-existing risk factors

Objectives: Although the risk of antipsychotic-induced venous thromboembolism (VTE) has been definitively established, guidelines recommending prophylactic anticoagulation do not yet exist. Several algorithms have been proposed that suggest possible prophylaxis with an anticoagulant medication on the basis of pre-existing VTE risk factors. We present a case of antipsychotic-induced VTE despite the patient's low-risk status so that practitioners may better understand which factors may or may not constitute a major risk in this population when making a determination about prophylactic anticoagulation.

Case summary: We present a patient case of a 56-year-old man with schizoaffective disorder who was treated with clozapine at an inpatient psychiatric unit. Although he would be classified as low risk for VTE on the basis of the proposed algorithms, he experienced a pulmonary embolism by day 17 of treatment and required transfer to a medical unit. This patient displayed sensitivity to other adverse effects associated with clozapine during his treatment course, including tachycardia, sialorrhea, enuresis, and bowel obstruction.

Practice implications: Many of the known risk factors for antipsychotic-induced VTE were not present in this patient, including immobility, hyperprolactinemia, and coagulation abnormalities. The recent initiation of clozapine and obesity seem to be the only identified risk factors, although malignancy and abnormal antiphospholipid antibody levels were not able to be ruled out. It is difficult to determine if this patient experienced a VTE owing to a relatively high degree of sensitivity to clozapine, as evidenced by the myriad of other adverse effects that he experienced. This case highlights the need to determine true antipsychotic-induced VTE risk factors, including evaluation of comorbid adverse effects that occur in addition to the VTE. This information will help to guide future decision-making regarding the risk versus benefit of providing prophylactic anticoagulation for patients during initiation of antipsychotic treatment.