Arteriovenous Malformation (AVM)
What is an AVM?
Arterio-Venous Malformations, or AVMs, are defects of the circulatory system (faulty connections between arteries and veins) that are generally believed to arise during embryonic or fetal development, or soon after birth. Normally, oxygenated blood is pumped by the heart through branching tubes called arteries to the brain, where it enters a fine network of tiny vessels called capillaries. It is in these capillary beds where the blood nourishes the tissues. The “used” (deoxygenated) blood then passes back to the heart through branching thin walled tubes called veins.
An AVM can be thought of as a “short circuit” because the capillaries are bypassed by the malformation. Instead of the capillaries, the blood does not go to the tissues but is pumped through much larger sized blood vessels that connect the artery directly to the vein, and back to the heart without ever giving nutrients and oxygen to the tissues. These connections that replace the capillary are called shunts and a collection of shunts is called a nidus. The consequence of this is that the very high pressure in the artery is no longer dampened by the capillaries, and the veins now experience the same high pressure as the arteries. Arteries have thick walls to handle the very high pressures and the veins have thin walls as they are only meant to withstand low pressures. The high arterial pressure can rupture the veins resulting in bleeding in the brain called a hemorrhagic stroke. Small AVMs may be under higher pressure and may present more often as hemorrhage. Larger AVMs may present as seizures more often because their size makes them more likely to involve the cortex.
Symptoms, Incidence and Risk Factors
Unlike brain aneurysms, there are no well-established risk factors for AVM formation, growth and rupture. It is not known how AVMs develop in the fetus and typically do not cause any problems in the fetus. Unfortunately, most people discover that they have an AVM when they bleed and the patient experiences an extremely severe headache or the patient suffers a seizure. The greatest potential danger posed by AVMs is hemorrhage. Studies suggest that AVMs have an annual rupture risk of 2 to 4 percent per year. Other factors that may affect the risk of hemorrhage include the type of venous drainage and the presence of aneurysms in the AVM. Episodes of bleeding may remain undetected at the time they occur because they are not severe enough to cause significant neurological damage. But massive, even fatal, bleeding episodes do occur.
Some AVMs present with a stroke-like event due to “stealing” blood flow from neighboring brain territory. Less commonly, an unruptured AVM can present with persistent headaches, with or without nausea and vomiting (i.e., raised intracranial pressure), seizures or neurological symptoms such as weakness, numbness in the body, vision changes or speech problems.
It is estimated that the lifetime risk of bleeding from AVMs is in excess of 50%. This is a significant risk particularly that each bleed has a 1 in 10 chance of causing death, and if you survive the bleed there is a more than 50% chance of causing disability (stroke). Larger AVMs occasionally may cause progressive neurological problems by putting pressure on the brain or by altering the blood flow. For these reasons aggressive treatment of AVMs is undertaken to eliminate these risks.
AVMs occur about equally in both sexes and in different races, although a slight predominance of AVM’s among males has been noted in virtually all studies. The mode of inheritance is yet to be explained and thus far there are no recommendations for screening for it. About 0.14% of the U.S. population has AVMs. Given a population now of about 250,000,000 in the United States, that suggests that about 350,000 people in the country may have an AVM.
Up to 9% of the patients diagnosed with an AVM have multiple AVM’s, most of which have an associated vascular syndrome called hereditary hemorrhagic telangiectasia, also known as Rendu-Osler-Weber syndrome. This is an autosomal dominant disorder of vascular structure, affecting the brain as well as the nose, skin, lungs, and gastrointestinal tract.
Risk of Bleeding from an AVM:
- 10 years —33.5%
- 20 years —55.8%
- 30 years —70.6%
- 40 years —80.3%
- 0 years —86.8%
Diagnosis
As mentioned above, many people with AVMs will never experience any adverse symptoms. With age, however, the risks of a rupture increase. The average age of an AVM diagnosis is 31 years old. The consequences of a rupture can be quite severe, and in many cases preventative treatment is needed.
Patients whom are having symptoms of a ruptured AVM are evaluated typically with a CT/CTA scan. This is still the best imaging technique to check for an acute hemorrhage. Once bleeding is confirmed on a CT scan, definitive diagnosis of an AVM is made by angiography (diagnostic cerebral angiogram). Here, a contrast dye is injected through a catheter device inserted usually in a thigh (femoral) artery. From here, the dye eventually enters one or more of the main brain arteries, where it is imaged with X-rays. An AVM often appears early during the injection as an abnormal number of expanded and tortuous vessels. There may be one or more aneurysms associated with the AVM, and these can appear as sacs or balloons (variable size and number) coming off the parent arteries or within the tight coil of the AVM nidus itself.
MRI and MRA are also very helpful particularly with localizing the AVM. The advantages of these methods are that they are less invasive than cerebral angiography, yet they still provide excellent information regarding where exactly the AVM is located (i.e., which part of the brain, which important brain functions may be involved, what critical structures lie nearby, and how best to approach the AVM when considering a treatment option). However, MRI/A may not detect the smallest of aneurysms associated with AVMs as well as cerebral angiography. Once the AVM is detected, the individual should be carefully evaluated for any findings that may indicate an increased risk of hemorrhage. Once the location and appearance of the AVM is decided, definitive plans for treatment are made.
Treatment Options
Treatment options for AVMs include endovascular techniques (embolization), open surgery, radiation therapy, or a combination of these methods. Each treatment has its advantages and disadvantages. Embolization is most often used as a preparatory step prior to surgery or radiosurgery. Each patient with an AVM is treated on an individualized basis, taking into consideration the age of the patient, presence of other significant medical conditions, the site (especially the “eloquence” of brain involved by the AVM; see below) and size of the AVM, whether there is a history of previous AVM hemorrhage in that patient, and the type and risks of treatment option most suitable for that AVM and person.
Embolization is an angiogram based procedure and a method of blocking the blood vessels of the AVM. Under X-ray guidance, a small tube called a catheter is guided from the femoral artery in the leg up into the area to be treated. Once the area is reached, a liquid embolic material (Onyx or NBCA glue) is slowly injected into the arteries feeding the AVM and the nidus to slow the flow of blood in the feeder artery. This same therapy can also be used to treat aneurysms associated with the AVM, especially aneurysms located on the feeder arteries. Endovascular therapy is very helpful in high-flow AVMs and AVMs whose feeder arteries may be difficult to reach surgically, but this method alone rarely cures an AVM. It is best thought of as a helpful adjunct for subsequent radiation therapy and/or surgery. Embolization may facilitate the final surgery by reducing the number of blood vessels feeding the malformation, as well as reducing the blood flowing into the malformation. Having an embolization procedure prior to radiation reduces the size of the AVM, making the area that needs to be radiated smaller. The smaller the area of brain radiated leads to a smaller chance that the radiation will damage the brain tissue around the AVM.
Risks of the Embolization Procedure: As with any other type of neurosurgery, endovascular therapy carries its own set of risks. Overall, the risk of death or significant neurological disablilty associated with this option is about 4-5% in total. Disability may be from parent vessel rupture, vessel occlusion causing a stroke, or tearing of the vessel (dissection).
What to Expect During and After an Embolization Procedure: During the embolization the patient is placed under general anesthesia. Neurological and neurophysiological testing is performed before and after a medication is injected into the artery to be occluded. This can help to determine if the vessel that feeds the AVM also feeds normal and important portions of the brain. After this, a permanent agent is injected into the AVM and the catheter removed. This is repeated for each vessel that feeds the AVM.
After the embolization, the patient usually spends the night in the Neurological Intensive Care Unit (NICU) where he/she is monitored closely. Patients are usually hospitalized for two nights for each embolization and may require several embolizations at 2-6 week intervals. There may be some mild headaches after the embolization related to the blood vessels of the AVM clotting, or some nausea related to some of the medicines that are given. Another common side effect is pain in the groin in the area where the catheter had been introduced during the procedure. Most if not all these side effect resolve within 24 to 48 hours. At home recovery is rapid and most patients report feeling “normal” as early as one or two days after the procedure.
Surgical Removal of an AVM: The advantages of surgery are the immediate removal of the hemorrhage and reduction in the rehemorrhage risk of an AVM. This is achieved through a craniotomy, which is the opening of the skull, followed by meticulous identification, isolation and disconnection of the arterial branches feeding the AVM, followed by the veins draining the AVM. The goal of surgery is for the AVM to be carefully removed in one piece.
Particular attention to tight blood pressure regulation is vital after surgery to avoid secondary hemorrhage. The surgical risks for removal of an AVM are 1-10% chance of significant neurological complications.
Stereotactic Radiosurgery (SRS): Radiosurgery involves delivery of a focused beam of radiation to the nidus of the AVM. It may involve one or several treatments. It is painless and generally well tolerated by patients. In very rare cases, it can cause secondary tumors, and in some cases, impairment of brain function (especially important in kids whose brains are more rapidly developing), and delayed swelling (brain edema) or more uncommonly, cystic radiation necrosis (an area of the brain containing a collection of dead cells). SRS is certainly a good option for treating AVMs when those AVMs are located in very deep regions of the brain (e.g., brainstem, thalamus).
For AVMs that are not treatable by surgery, then radiation is the method of choice. The disadvantage is that radiation is a gradual cure of the AVM, generally over the course of two to three years, and during this waiting period the patient is still at risk for hemorrhage. It takes time for the radiation to damage the AVM nidus and cause it to shrink and scar down. In addition, radiation only works best with small AVMs and does not work as well with large AVMs. Any AVM smaller than 3 centimeters in greatest diameter is considered small.