Capillary lymphatic venous malformations (CLVMs) are rare blood vessel anomalies characterized by an abnormal network of capillaries, lymphatic vessels and veins. Patients with CLVMs have a localized or diffuse capillary malformation (port-wine birthmark) that overlies venous and lymphatic malformations.

CLVMs have variable appearances and complications depending on the location(s), involved body structures and the proportion of which vascular components (capillaries, veins and lymphatic vessels) make up the malformations. Patients with these combined malformations may also have varying degrees of soft tissue overgrowth and abnormal bone growth. Overgrowth most commonly affects the extremities but may occur in other areas including the internal organs. This abnormal soft tissue and bone growth may result in different lengths of the limbs. Additionally, a wide range of other skeletal and skin abnormalities may be present.

Capillary lymphatic venous malformations form prior to birth. Although the exact cause of CLVMs is unknown, they are thought to be caused by errors in the formation and development of the blood vessels during fetal development. CLVMs are not caused by any known drug, medication or environmental factor that the baby may have been exposed to during the pregnancy. Most CLVMs are identified within the first years of life.

The classification of capillary lymphatic venous malformations also includes two named syndromes: Klippel-Trénaunay syndrome (KTS) and CLOVES syndrome.

KTS is characterized by the presence of a capillary venous malformation with or without a lymphatic malformation and soft tissue / bone overgrowth of an affected limb.

In contrast, the overgrowth in CLOVES syndrome is lipomatous or fatty. In addition to the CLVM, individuals with CLOVES syndrome may also have a high flow component, such as an arteriovenous malformation (AVM). The acronym CLOVES stands for: congenital lipomatous overgrowth, vascular malformations, epidermal nevi, scoliosis / skeletal and spinal [abnormalities].

Some individuals with CLVM and overgrowth have been found to have a mutation of the PIK3CA gene in the overgrown tissue. This mutation is not inherited and is only found in the affected tissue(s).