Kasabach-Merritt syndrome (KMS), also known as Kasabach-Merritt phenomenon, is characterized by an enlarging vascular lesion, profound thrombocytopenia, consumptive coagulopathy, and often a microangiopathic hemolytic anemia. KMS is associated with vascular tumors, especially kaposiform hemangioendothelioma (KHE) and tufted angioma (TA). Whether KMS complicates true hemangiomas of infancy is controversial, although there have been cases associated with diffuse neonatal hemangiomatosis and visceral hemangiomas.

Affected infants typically present with a solitary cutaneous lesion either at birth or within the first year of life, although multiple cutaneous or visceral vascular tumors are also encountered in affected infants. KMS is thought to result from platelet trapping within the hemangioma, with subsequent consumption of clotting factors, leading to intralesional bleeding and a rapid enlargement of the vascular lesion. In the midst of the consumptive coagulopathy, easy bruising and even frank bleeding occur.

The rapidly enlarging vascular lesion can compress vital organs and may lead to high-output heart failure. KMS has been associated with a mortality of between 30%-40%, mostly secondary to hemorrhage.