Although Klippel-Trénaunay syndrome is a rare congenital (present at birth) disorder, it is the most common condition involving combined vascular malformations.

The syndrome is characterized by a localized or diffuse capillary malformation (port-wine stain) that overlies a venous malformation and/or lymphatic malformation with associated soft tissue and bone hypertrophy (excessive growth). The port-wine stain (caused by swelling of small blood vessels near the surface of the skin) is typically substantial, varicose veins are often quite numerous, and bone and soft tissue hypertrophy (overgrowth) is variable.

The affected limb is either larger or smaller than the unaffected limb. Hypertrophy occurs most commonly in the lower limbs, but may affect the arms, the face, the head or internal organs. Additionally, a wide range of other skeletal and skin abnormalities may be present (illustration 1). Patients may have an increased extremity circumference and an increase or decrease in the length of the limb.

Bony enlargement is usually not present at birth, but may appear within the first few months or years of life and may become particularly problematic during puberty. The affected area grows longer and thicker due to increased blood supply. Small vesicles (lymphatic blebs) appear within the capillary lesion and can bleed, become irritated and get infected. 

In young adults, the capillary malformation may thicken and become more prominent.