Venous malformations (VMs) are the most common sub-type of vascular malformation and consist of malformed veins that do not work properly. Over time, venous malformations typically have slow and gradual enlargement. However, rapid enlargement may occur in certain situations such as surgery, trauma, infection or hormonal changes associated with puberty, pregnancy or menopause. VMs may occur anywhere in the body including the skin, soft tissues, muscles and internal organs.

Although the exact cause of these lesions is unknown, they are thought to be caused by errors in the formation and development of veins during fetal development. A deficiency of smooth muscle cells in the vein walls is known to be an important factor. VMs are not caused by any known drug, medication or environmental factor that the baby may have been exposed to during the pregnancy. Even though venous malformations are present at birth, they may not be identified until adolescence or even adulthood.

Some individuals with venous malformations have been found to have a genetic change in the TIE-2 gene. Most commonly, this mutation is not inherited and is found only in the affected tissue(s). However, this mutation may be inherited and has been found in families who have multiple family members with venous malformations.