Vascular malformations of the brain result from the failure of the normal process of embryonic vascular development. Many terms have been used to describe cerebral vascular lesions. The generally accepted nomenclature classifies vascular malformations into four types, based on their histopathological characteristics. These types are telangiectasias, cavernous malformations, venous malformations, and true arteriovenous malformations. The lesions are assigned to a given category based on the vessels involved (arteries, veins, capillaries) and the presence or absence of intervening brain tissue.

Telangiectasias are small lesions, usually less than 1 cm in diameter, composed of capillary-type vessels separated from each other by normal-appearing brain parenchyma. These vessels are usually identified at autopsy and rarely present with haemorrhage. They are not seen on angiography because of the small size of the vessels involved. Cavernous malformations are well defined lesions that can reach a size of 10 cm, or more. These lesions are composed of sinusoidal-type vessels in apposition to each other without any recognizable intervening neural parenchyma; they may bleed repeatedly. Affected patients can suffer seizures or step-wise neurological dysfunction, with some recovery between haemorrhages. Cavernous malformations are thought to be lesions with slow blood flow and fairly low pressure in the vessel lumens. They do not appear on angiography, yet are visible on MRI and CT scans. Calcification is not uncommon. Venous malformations are composed of anomalous veins with no direct arterial input. The veins in the malformation are separated by completely normal parenchyma. These malformations are usually detected incidentally and do not tend to bleed. Venous malformations drain normal brain tissue, and their surgical removal should be avoided because of the high risk of venous infarction. True arteriovenous malformations are often the most important lesions from the clinical point of view. They are composed of arteries with direct shunting to arterialized veins, which appear as high flow lesions on angiography (Fig. 1) 2303. Brain parenchyma can be seen between the vessels of the lesion, but the neural tissue is usually abnormal and gliotic. Associated cerebral aneuryms are present in approximately 10 per cent of patients with arteriovenous malformations.

Arteriovenous malformations vary in their size, location, and degree of blood shunting, and can manifest themselves with a variety of symptoms. They frequently become symptomatic during childhood or early adulthood. Approximately 50 per cent of arteriovenous malformations become apparent initially with intracranial haemorrhage; this can be intraparenchymal, intraventricular, subarachnoid, or may occur in a combination of these locations. Seizures are the second most frequent clinical presentation, occurring in one-third to one-half of patients with arteriovenous malformations. The seizures may be grand mal or focal, depending on the site and location of the malformation. Headaches are probably the third most common clinical presentation; however, the relationship between headaches and arteriovenous malformations is difficult to define precisely. With the increased availability of CT and MRI studies, many patients with headaches undergo a radiological study; a small proportion of these patients have vascular lesions. Other less common clinical presentations of arteriovenous malformations include hydrocephalus due to ventricular system obstruction or elevated venous pressure, cranial bruits, and high output cardiac failure in infants with very high flow and large malformations.

The need for treatment and the type of therapy required for arteriovenous malformations must be assessed on the patient's age, his medical condition and the likelihood of further morbidity from the lesion, particularly in the form of recurrent haemorrhage. The risk of haemorrhage from an unruptured arteriovenous malformation is about 4 per cent per year. Unlike aneurysms, the risk of rebleeding during the first few weeks after haemorrhage is low.

The diagnosis of suspected arteriovenous malformation is usually confirmed by CT scan, MRI studies and angiography. Figures 1 and 2 2303,2304 show an angiogram and MRI of a young patient who presented with seizures. The MRI demonstrates the characteristic lesion with irregular boundaries and flow void consistent with blood flow through anomalous arteries and veins.

Unless an intraparenchymal or subdural blood clot warrants acute surgical intervention, removal of an arteriovenous malformation can be delayed until the patient has recovered from the acute effects of haemorrhage. Passage of time allows stabilization from any neurological deficit resulting from a parenchymal haemorrhage: this may take between 3 and 4 weeks. Most arteriovenous malformations of the cerebral convexity are roughly conical in shape, with their apex pointing towards the ventricle. Lesions deep within the cerebral hemispheres or brain-stem may not be related to the ventricular system.

Microsurgical techniques have contributed to the reduction in morbidity and mortality associated with surgery. There are usually several main feeding arteries to the nidus of the arteriovenous malformation: these vessels should be coagulated and transected first during the operation. Some part of the arteriovenous malformation is usually present on the brain surface (Fig. 3) 2305: removal of a small amount of brain tissue around the lesion defines a plane around the periphery of the structure. Coagulation and transection of feeding arteries as they are encountered is then undertaken. The venous drainage of an arteriovenous malformation is left intact until the final stages of excision, when the draining veins are ligated and the lesion removed (Fig. 4) 2306.

When lesions adjacent to eloquent brain tissue are to be removed the location of the dissection plane between the malformation and adjacent gyri is crucial to avoid injury to the motor cortex, the speech cortex, or other eloquent tissue.

Techniques of preoperative embolization have advanced greatly. An intravascular catheter can be inserted into the feeding artery of an arteriovenous malformation through a femoral approach, allowing transvascular embolization with a variety of agents. Such embolization can appreciably reduce blood flow to the lesion, reducing the risk of neurological deterioration due to surgical removal of the lesion. This technique also provides a means of occluding blood supply to the depths of an arteriovenous malformation, which are difficult to reach surgically. Arteriovenous malformations can also be treated using stereotactically directed beams of heavy ions, or standard radiotherapy from cobalt sources to produce changes in the vascular wall. This results in occlusion of the lumens of vessels in the malformation over a period of 2 to 3 years.

Surgically accessible arteriovenous malformations can be managed with low morbidity and mortality. Larger inoperable lesions may be amenable to transvascular embolization or radiosurgery. Arteriovenous malformations are often best evaluated by a team of physicians, including a neurosurgeon, an interventional neuroradiologist, a radiation therapist, and a neurologist.

Ojemann RG, Heros RC, Crowell RM, eds. Surgical management of cerebral vascular disease. 2nd edn. Baltimore: Williams & Wilkins, 1988.

Wilson CP, Stein BM, eds. Intracranial Arteriovenous Malformations. Baltimore: Williams & Wilkins, 1984.

Yasargil MG. Microneurosurgery Vol. IIIA. AVM of the brain, History, Embryology, Pathological Considerations, Hemodynamics, Diagnostic Studies, Microsurgurical Anatomy. New York: Thieme Medical Publishers, Inc., 1987.

Yasargil MG. Microneurosurgery Vol. IIIB. AVM of the brain, Clinical Considerations, General and Special Operative Techniques, Surgical Results, Nonoperational Cases, Cavernous and Venous angiomas, Neuroanesthesia. New York: Thieme Medical Publishers, Inc., 1987.