Kaposiform haemangioendothelioma

Clinical features

• Violaceous, indurated, ill-defined plaque, mass or "bruise-like" infiltrate, often with overlying ecchymotic discolouration.
• Distribution — extremities (especially upper limb, trunk, head/neck, retroperitoneum); often deep-seated with overlying superficial component.
• Onset typically in infancy (~50% congenital) or early childhood; less commonly in adults.
• M:F roughly equal.
• Rapid initial growth, then slower expansion; may infiltrate fascia, muscle and underlying organs.
• Complications:
  • Kasabach-Merritt phenomenon (KMP) — life-threatening consumptive coagulopathy in ~70% — see next section.
  • Local infiltration, pain, functional impairment of the affected limb.
  • Lymphoedema after resolution of the active phase.
• Differential — infantile haemangioma (much more common, GLUT1+, no KMP); congenital haemangioma; tufted angioma (overlapping spectrum); cutaneous angiosarcoma (rare in children).

Kasabach-Merritt phenomenon (KMP)

• Life-threatening consumptive coagulopathy occurring in ~70% of KHE.
• Pathogenesis — intratumoral platelet trapping and consumption, with secondary fibrinogen consumption and microangiopathic haemolysis.
• Laboratory features:
  • Severe thrombocytopenia (often <10 × 10⁹/L).
  • Hypofibrinogenaemia.
  • Elevated D-dimer.
  • Microangiopathic haemolytic anaemia (schistocytes, low haptoglobin).
• Clinical — bleeding from biopsy / venepuncture sites, intratumoral haemorrhage with sudden tumour expansion, GI / CNS / pulmonary haemorrhage.
• Mortality 10–30% in untreated cases.
• Important — KMP is associated with KHE and tufted angioma, NOT with infantile haemangioma.

Histology & molecular

• Infiltrative ill-defined nodules of slit-like vascular channels and spindled endothelial cells with focal "kaposiform" architecture (resembling Kaposi sarcoma) and microthrombi within vascular spaces.
• "Glomeruloid" tufts of capillaries — reminiscent of tufted angioma.
• Endothelial markers — CD31+, CD34+, ERG+, FLI-1+; D2-40+ (lymphatic).
• GLUT1 NEGATIVE — distinguishes KHE / tufted angioma from infantile haemangioma (GLUT1+).
• HHV-8 negative (excludes Kaposi sarcoma).
• Recurrent GNA14 somatic mutations described.

Management

• Multidisciplinary — paediatric haematology / oncology, vascular anomalies team, dermatology, plastic surgery.
• Refer urgently to a specialist vascular anomalies / paediatric oncology centre.
• First-line: sirolimus (mTOR inhibitor) — has revolutionised KHE treatment; oral once daily; highly effective for both tumour shrinkage and KMP resolution; well-tolerated; trough monitoring.
• Alternative first-line: vincristine ± corticosteroids (historic standard) — IV vincristine weekly for 6+ months; high-dose oral / IV corticosteroids during initial KMP.
• Other systemic options — interferon-α (seizures and neurological side effects in infants — caution); propranolol (less effective in KHE than in infantile haemangioma); aspirin / ticlopidine.
• Platelet transfusion is reserved for active bleeding only — repeated transfusion fuels intratumoral consumption.
• Cryoprecipitate / fresh frozen plasma for hypofibrinogenaemia and active bleeding.
• Surgical excision rarely feasible due to infiltrative growth; reserved for residual disease after medical control or focal symptomatic lesions.
• Embolisation in selected cases.
• Long-term surveillance for residual disease, lymphoedema and functional impairment.

Prognosis

Local mortality 10–30% in the era before sirolimus; substantially improved with modern multidisciplinary care. Distant metastasis is exceptionally rare. Long-term sequelae — residual mass, lymphoedema, functional impairment, persistent KMP-related coagulopathy in some.