Pathophysiology of Kasabach-Merritt Syndrome
Kasabach-Merritt syndrome results from intratumoral platelet trapping within kaposiform hemangioendothelioma or tufted angioma, leading to consumptive coagulopathy with profound thrombocytopenia and hypofibrinogenemia. 1, 2
Core Pathophysiologic Mechanism
The fundamental process involves platelets and clotting factors being consumed and sequestered within the rapidly growing vascular tumor, creating a life-threatening consumptive coagulopathy. 3, 4 This is not a simple hemangioma—the American Academy of Pediatrics emphasizes that Kasabach-Merritt phenomenon occurs exclusively with kaposiform hemangioendothelioma (KHE) and tufted angioma (TA), never with infantile hemangiomas, which is critical for understanding both prognosis and treatment approach. 2
Tumor-Specific Characteristics
The vascular tumors in Kasabach-Merritt syndrome are aggressive and rapidly growing, distinguishing them from benign infantile hemangiomas. 1, 5 These tumors create abnormal vascular channels that:
- Trap platelets within the tumor vasculature, causing severe thrombocytopenia (often <10,000-20,000/μL) 3, 4
- Consume coagulation factors, leading to hypofibrinogenemia and disseminated intravascular coagulation-like picture 6, 3
- Generate microangiopathic hemolytic anemia from mechanical destruction of red blood cells passing through abnormal tumor vessels 3
Clinical Manifestations of the Coagulopathy
The consumptive coagulopathy manifests as:
- Hemorrhagic diathesis with spontaneous bleeding (epistaxis, conjunctival hemorrhage, periorbital ecchymosis, petechiae) 3, 4
- Life-threatening bleeding complications that can occur without warning 1, 5
- Multi-organ system failure in severe cases with diffuse hepatic involvement, including respiratory insufficiency from abdominal mass effect, abdominal compartment syndrome, and hypothyroidism from tumor production of type 3 iodothyronine deiodinase 1
Tumor Growth Dynamics and Thrombosis
The relationship between platelet counts and tumor behavior is bidirectional. While the tumor traps platelets causing thrombocytopenia, experimental evidence demonstrates that increasing platelet counts can paradoxically cause intravascular thrombosis of tumor vessels, leading to tumor growth inhibition. 7 This mechanism explains why some treatments that increase platelet production may have dual benefits—correcting the coagulopathy while simultaneously causing tumor regression through intratumoral thrombosis. 7
Critical Pathophysiologic Distinctions
Kasabach-Merritt syndrome should never be confused with thrombocytopenia from infantile hemangiomas, as the underlying tumors (KHE and TA versus infantile hemangioma) have completely different biological behaviors, natural histories, and treatment responses. 2 This distinction is essential because infantile hemangiomas typically involute spontaneously and do not cause consumptive coagulopathy, whereas KHE and TA with Kasabach-Merritt phenomenon require aggressive intervention. 1, 2
Hepatic Involvement Pathophysiology
When diffuse hepatic hemangioendothelioma occurs, the liver becomes almost completely occupied by hemangiomas, creating additional pathophysiologic complications beyond the coagulopathy itself, including compression of surrounding structures and metabolic derangements. 1