Arteriovenous fistulae and malformations

D: An arteriovenous fistula or malformation is an abnormal communication between an artery and vein that bypasses the capillary bed.

A: Congenital: Fistulae can be divided into haemangiomas, e.g strawberry naevi, and malformations (AVMs). The latter is divided into low flow or high flow (e.g. hepatic or pulmonary AVM).

Acquired: Trauma, tumours (e.g. glomus tumour, hepatoma, hypernephroma and sarcomas), infection, inflammation (e.g. aortocaval fistula) or iatrogenic (e.g. Brescia-Cimino fistula for haemodialysis or portocaval shunt in portal hypertension).

A/R:| AVMs are associated with many different syndromes, e.g. Klippel-Trenaunay, Kasabach-Merritt, Sturge-Weber, von-Hippel-Lindau and Rendu-Osler-Weber syndrome (the latter is caused by a defect in the protein, endoglin, which carries a 30% risk of developing a pulmonary AVM).

E: AVMs such as cutaneous haemangiomas are common, the others less so.

H: Presentation is variable, depending on the site and size of the AVM and symptoms may be due to local, peripheral or systemic effects (see below). Congenital AVMs involving the skin are often visible from birth (haemangiomas appear ~4 weeks after birth). Malformations usually grow with age, often during puberty or pregnancy in women, and those within internal organs may only be detected once complications develop; e.g. brain AVMs may cause haemorrhage, epilepsy or a neurological deficit. Other presentations include varicose veins, limb swelling or pain.

E: Cutaneous haemangiomas are usually scarlet in colour, firm and cannot be emptied of blood on compression. Internal AVMs may be revealed by an overlying bruit, possibly with reduced distal pulses and an " pulse pressure. There may be signs of cardiac failure in large AVMs. There may be focal neurological signs in cerebral AVMs, especially after a complication such as haemorrhage.

P: Congenital AVMs are thought to arise due to anomalous embryonic vascular development and can involve arteries, veins, capillaries or lymphatics. Haemangiomas result from endothelial hyperplasia and contain abundant mast cells, whereas endothelial cells in malformations demonstrate normal turnover.

I: Imaging of the AVM can be carried out by Duplex scanning, CT or MRI scanning or invasively, by angiography.

Other: Quantification of AV shunting can be carried out using 99mTc-human albumin microspheres. These are introduced into an artery and are too large to pass through capillaries. Those passing through AVMs are trapped in the lungs and quantified using gamma camera.

M: Conservative: Cutaneous haemangiomas usually undergo spontaneous regression at the end of the first year of life.

Interventional radiology: In the case of internal AVMs, using metal coils or tissue adhesive or particles to embolise the AVM.

Surgery: Often difficult, but excision (after pre-op embolisation) may be possible in the case of small and accessible AVMs. Stereotactic radiosurgery using a gamma knife has been used on small brain AVMs, but takes 2 years for full effect.

C: Depends on size and location.

Local: Cosmetic disfigurement, ulceration, bleeding, local pressure, hydrocephalus.

Distal: Ischaemia of peripheral tissues,

Systemic: High-ouput cardiac failure in the case of large AVMs.

P: Depends on site and aetiology. 90% of haemangiomas regress by 5-10 years. 1-4% annual risk of haemorrhage in cerebral AVMs.

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