The Kasabach-Merritt phenomenon is the association of a rapidly enlarging vascular lesion anywhere on the body (commonly an extremity but also head and neck, thigh, sacrum, retroperitoneum) with consumptive coagulopathy (low fibrinogen, increased D-dimers) and thrombocytopenia because platelets are trapped into the tumor. Hemolytic microangiopathic anemia is also frequently present. It is exclusively associated with the vascular tumors kaposiform hemangioendothelioma (KHE) and tufted angioma (TA). These are benign vascular tumors that typically present in infancy, and are classified as having intermediate malignant potential as they are locally aggressive but are not known to metastasize. The Kasabach-Meritt phenomenon may affect up to 70% of all patients with KHE and up to 10% of patients with TA [1,2]. KHE and TA are vascular tumors made of irregular nodules with a complex vascular architecture. KHE presents as an enlarging, firm, purpuric cutaneous or soft tissue lesion with poorly defined margins and with spindle-shaped cells (active phase) at histology. TA presents as a series of violaceous macules and papules with a glomerular structure with crescentlike vascular cleft, before the active phase or during the regression phase at histology. Some accompanying lymphangiomatosis can be observed [1,2]. When the Kasabach-Meritt phenomenon starts, KHE or TA rapidly enlarges and becomes tense, purpuric, or ecchymotic, and painful. The swelling may be dramatic. On the contrary from infantile hemangiomas (common vascular malformation) the endothelial cells lining the lesion are negative for the glucose-transporter-1 (GLUT-1) isoform and for the Lewis Y (LeY) antigen. Careful surgical excision is performed when feasible but the lesion is often extensive, and a medical treatment is necessary: it includes high-dose steroids, vincristine, α- or β- interferon and platelet aggregation inhibitors such as ticlopidine and/or aspirin [3,4,5,6]. Propranolol is sometimes used with success [5]. Sirolimus with or without steroids is currently often used with excellent results. Platelet transfusion is best avoided because it generally results in enlarging the lesion and worsening the coagulopathy because of platelet trapping into the tumor [1,2,6,7,8,9]. 

The lesion usually appears during the first year of life. Mortality varies between 10 and 40%. Death usually occurs from life-threatening hemorrhage, cardiac failure, or invasion of the vascular lesion into local structures [2,6|.