Fulminant Thromboinflammatory Syndrome Following an Influenza-like Illness in an Adolescent: Clinical Insights from a Case Report.

Infection-associated thromboinflammation may result from dysregulated interactions between innate immune activation, endothelial activation, and coagulation. Although influenza-like illnesses are generally self-limited in young individuals, severe respiratory infections have been associated with thrombo-inflammatory complications involving coagulation dysregulation and endothelial activation. We describe an 18-year-old previously healthy woman who developed extensive left iliofemoral deep vein thrombosis, bilateral pulmonary embolism, and progressive systemic venous and intracardiac thrombosis following an influenza-like illness. Despite therapeutic anticoagulation, the clinical course was complicated by acute compartment syndrome, progressive tissue ischemia, circulatory failure requiring vasopressor support, multiple organ dysfunction, and ultimately below-knee amputation. Laboratory investigations revealed thrombocytopenia, severe hypofibrinogenemia, markedly elevated D-dimer levels, reduced Protein C activity, and increased von Willebrand factor antigen (>500%), consistent with overt disseminated intravascular coagulation and endothelial activation. A positive anti-PF4/heparin immunoassay raised concern for heparin-induced thrombocytopenia; however, the subsequent clinical and laboratory evolution supported overt DIC as the predominant consumptive coagulopathy, while HIT could neither be confirmed nor definitively excluded. This case highlights a fulminant thrombo-inflammatory syndrome temporally preceded by an influenza-like illness of unconfirmed etiology and the diagnostic challenges posed by overlapping features of thrombotic-predominant overt disseminated intravascular coagulation and anti-PF4/heparin antibody positivity.