Venous malformations comprise either superficial or deep veins that are abnormally formed and dilated. They are the most common type of asymptomatic vascular lesion. Although they usually are present at birth, they may not be seen until years later into adolescence or even adulthood.

The natural history of a venous malformation is slow, steady enlargement. However, events such as surgery, trauma, infection, or hormonal changes associated with puberty, pregnancy or menopause may cause rapid expansion. These lesions may be present in the skin, mucous membrane or in any organ system (illustrations 1-2), including the brain, bowel, liver, or spleen.

Although the exact cause of these lesions has not yet been determined, a deficiency of smooth muscle cells in the vein walls is known to be a critical factor. There is no evidence that drugs or medications taken during pregnancy, or environmental exposures that may have occurred during that time can cause venous malformations. There have been some families identified with multiple venous malformations that have alterations in the genes responsible for the communication of endothelial cells and smooth muscle.