Arteriovenous malformations (AVMs) are the leading cause of stroke in young people. Formed during embryonic development, an AVM is a tangle of dilated blood vessels that disrupts normal blood flow in the brain by pooling blood within its dense center or nidus. These vessels may enlarge over time.
Often "silent" for many years, AVMs typically begin to cause problems for patients between 10 and 30 years of age when they may experience severe headaches or seizures or suffer a stroke from a hemorrhage (bleeding) in the brain. While the first rupture may be fatal, more than 90 percent of patients survive with appropriate treatment.
The goal of treatment is to reduce this risk of fatal or debilitating hemorrhagic stroke. Twenty years ago, many patients were told their AVM was inoperable because the risks of treatment were considered too high. Today, advanced open skull surgical techniques, radiosurgery and endovascular embolization make the treatment of AVMs safer and highly effective.
Treatment begins with thorough, specialized imaging studies - usually computed tomography (CT), magnetic resonance imaging (MRI) and angiography - to determine precisely the size, location and blood-flow patterns of the AVM.
A multidisciplinary team of neurosurgeons, neuroradiologists, neurologists and neuropsychologists then discusses the case to develop an appropriate treatment plan. Current acceptable treatment usually involves some combination of embolization, open skull surgery and radiosurgery. It is important for the treating team to have all of these options in a state-of-the-art facility housing dedicated angiography suites, microsurgical-equipped operating rooms and radiosurgery.
Prior to radiosurgery treatment or surgery, patients with multiple AVMs or lesions larger than 3cm usually require embolization - the deliberate obstruction of a blood vessel. The goal of this procedure is to diminish the amount of blood flowing into the AVM by filling it with specially designed particles, micro-coils or glue. The "emboli" or clots formed by these agents plug the vessels of the AVM, making it more manageable during subsequent procedures by decreasing the amount of bleeding during surgery or by reducing the size of the nidus and thereby creating a smaller target for radiosurgery.
Embolization is performed by an interventional neuroradiologist, a physician specializing in intravascular techniques who works along with a team of skilled nurses and technicians.
During the embolization procedure, the patient usually remains awake and alert but is made comfortable with intravenous sedation and local anesthesia. The physician guides a very small microcatheter, a fine tube, into blood vessels leading to the AVM. To ensure precision and safety, the catheter's path is traced continuously by an X-ray monitoring system.
Before depositing the particles or glue, the neruoradiologist injects a small amount of Amytol into the blood vessel. This barbiturate momentarily anesthetizes the region of the brain fed by that vessel while a neuropsychological evaluation determines whether permanent occlusion (blockage) of the vessel can be safely performed. These tests of muscle strength, memory and cognitive function require the patient to be awake for the procedure. In unusual circumstances, embolization may be performed. These tests of muscle strength, memory and cognitive function require the patient to be awake for the procedure. In unusual circumstances, embolization may be performed with the patient under general anesthesia using EEG (brain wave) monitoring.
Following embolization, the patient is monitored overnight by specially-trained nurses in a neuro-intensive care unit and then usually remains in the hospital for observation for an additional two or three days. For large AVMs, multiple embolization procedures may be required to avoid changing blood flow patterns in the brain too rapidly or drastically.
Subsequent MRIs and angiograms reveal the portion of the AVM blocked by the procedure(s) and the multidisciplinary team then plans the next treatment phase.
It is important to note that blood vessels blocked by embolization may reopen even if completely blocked with any of the materials or glue utilized. In time, the body reacts to remove the "foreign" materials. This is why conventional surgery and radiosurgery are necessary even after very successful embolization of an AVM.
If the patient has already suffered a bleed, surgery is often recommended to quickly eliminate the risk of another hemorrhage. If the AVM is larger than 3cm, surgery is also usually indicated, often in conjunction with pre-operative embolization.
Using precise microsurgical tools, the neurosurgeon exposes the AVM and resects (removes) its nidus. At times, resecting all of the vessels feed the AVM may pose too great a threat of bleeding. In such cases, the residual AVM may be effectively treated after surgery with radiosurgery.
Gamma Knife radiosurgery either alone or in combination with interventional microradiology (embolization) techniques is very attractive in children with complex deep seated or brain stem AVMs. Given their long period of risk for recurrent hemorrhage, 25 to 50 percent chance of size increase and tendency toward seizures, observing these lesions and taking no treatment action is not very attractive.
Most studies suggest the risk of rebleed in children with AVMs is significantly greater than adults. Additionally, brain stem AVMs carry a high morbidity (complication rate) and mortality (death rate) in children. The goal is to reduce the size and flow of blood within the AVM when possible. Neuro-interventional techniques to occlude aneurysm within the middle of the AVM hopefully will reduce the risk of rebleeding, allowing enough time for radiosurgery to occlude the AVM.
Radiosurgery success is inversely related to the size and flow rate of AVMs. The usual time to resolution and occlusion in adults is two to three years for AVMs approximately 3cm in size. For reasons as yet unclear, children have a shorter time to obliteration of the AVM after radiosurgery treatment than adults. It is not unusual to see a child's AVM disappear in less than one year and even six months past radiosurgery. For lesions 3cm or less, the rate of complete occlusion approaches 80 percent with a less than 1 percent treatment mortality and less than 3 percent treatment morbidity. For larger AVMs less than 5cm, we have used embolization or multiple treatments of Gamma Knife radiosurgery spaced one to three years apart until complete obliteration has been confirmed by angiography.
There has never been a secondary tumor reported following radiosurgery for an AVM in a child, with the follow up in reported following radiosurgery for an AVM in a child, with the follow up in Europe being more than 20 years. Where appropriate with AVMs, radiosurgery is easy on the child, cost effective and allows a rapid return to all activities with minimal risk.
The effect of the focused radiation on the AVM is realized over many months. While the precise mechanism of radiosurgery's action on the AVM is still not clearly understood, it is thought to act on the endothelial cells lining its blood vessels. These cells tend to multiply after radiosurgery and then become thrombolytic or clot-producing. These clots diminish blood flow through the tangle of vessels, thereby reducing the risk of hemorrhage.
Successive MRIs and angiograms allow neurosurgeons and neuroradiologists to monitor blood flow through the AVM and observe its decrease in size. The size decreases over a period of one to three years. Occasionally, further radiosurgerical treatment is required for large AVMs after two or three years of observation. Research suggests the obliteration rate for small AVMs treated radiosurgically to range between 80 and 90 percent upon two-year follow-up.
Gamma Knife radiosurgery is playing a larger role than ever in children with specific neurosurgical disorders and tumors. It is likely its use will expand in the treatment of both low-grad and malignant intracranial tumors. Its use as an adjunct treatment to surgery and external beam radiation has become more defined in recent years. In some instances, it may replace surgery and/or standard fractionated (over time) radiation therapy. It will remain the gold standard for treating many AVMs.