Venous malformations occur when veins do not properly form. As a result of this anomalous development, venous malformations may include vessels that are irregular or enlarged, and may lack the valves that prevent backflow in normal venous structures. Sometimes these anomalies appear as small, bluish lesions in the skin. In other patients, they may also involve larger abnormal vessels, which can pose a risk for blood clots. Venous malformations may involve any body area, and may be limited to the skin or arise in deeper tissues. They may also be found in conjunction with lymphatic malformations in some patients (mixed vascular malformations).
Although the exact cause of most venous malformations is unknown, physician-scientists have found that improper assembly of the smooth muscle cells normally forming the outer layer of veins can result in venous malformations in some cases. In other instances, venous malformations may be associated with other vascular anomalies, such as glomuvenous malformations or blue rubber bleb nevus syndrome.