Surgical Principles: Techniques, Goals, and Outcomes

Pearls

Definitive treatment is only achieved with complete obliteration or excision of the iAVM.
In cases of ruptured iAVMs, surgery is usually delayed several weeks if the patient is neurologically stable.
The surgical goal must be complete iAVM resection, taking all risk factors into consideration preoperatively.
All feeding vessels to the iAVM mass (nidus) are taken “before” the draining vein.
Surgery is best performed in high-volume centers where patient selection is of utmost priority.

Introduction

Intracranial arteriovenous malformations (iAVMs)—also known as brain or cerebral AVMs—are tangles of abnormal arteries and veins that reside within or on the surface of the brain. The specific cause of iAVMs is unknown. Long believed to be congenital lesions that develop during embryogenesis, iAVMs may rarely develop in some individuals after birth. Although they likely develop early in life and can cause symptoms at any time after development, iAVMs most often come to attention during the third, fourth, and fifth decades of life. While the specific subject to be detailed in this chapter is the surgical management of iAVMs, and other chapters address AVM pathology and imaging, as well as clinical decision-making regarding the treatment of iAVMs, some brief background is worth reviewing to serve as a backbone for the ensuing discussion.

The blood vessels of the human circulatory system comprise five main types, three on the arterial side and two on the venous. The arterial system, which consists of arteries, arterioles, and capillaries in descending order of diameter, serves to deliver oxygen and nutrient-rich blood from the heart to the tissues of the body. The venous system collects “used blood” from the tissues to deliver it to the heart, which sends it back to the lungs to receive fresh oxygen and subsequently pumps it back to the tissues via the arterial system. In normal circulation, oxygenated blood passes from arteries to arterioles to capillaries. Capillaries, which are the diameter of a single red blood cell, allow the passage of oxygen, fluid, and nutrients through their walls and into the tissues. The used blood then passes from venules to progressively larger veins, which ultimately flow into the vena cava, the main trunk of the venous tree that returns blood to the heart.

On the arterial side, as the circulation transitions from larger to smaller branches, the pressure within the system is diffused, such that by the time blood reaches the venous side, the veins are under low pressure. The structure of arteries and veins is adapted to handle their particular hemodynamic pressures. Arteries have a thick muscular layer that can withstand high pressure; this muscular layer also changes the diameter of the arteries to accommodate variation in blood pressure. Veins, conversely, have little muscle in their thin walls and, as a result, are incapable of withstanding high fluid pressure.

An iAVM is a pathological short circuit between arteries and veins, with no intervening capillaries, venules, or normal brain. The abnormal blood vessels create a nidus, or a 3D blob of ensnarled “branches” receiving blood under high pressure and high flow from feeding arteries. The blood “swirls” within the nidus and enters the veins draining blood from the iAVM under high pressure. As a result, the veins that drain an iAVM are subjected to pressures above their normal physiological limit. In addition, the arteries that feed the iAVM and the abnormal channels within the iAVM are subjected to supranormal hydrodynamic forces, which can weaken their walls and lead to focal outpouchings known as aneurysms. The walls of these aneurysms are prone to give way, as are the abnormal vessels within the iAVM and the draining veins. Intracranial AVMs may come to attention in a variety of ways. Sometimes they cause headaches that may be indistinguishable from migraine headaches; they may also irritate the surrounding brain and cause seizures or epilepsy. If large enough, they may “steal” blood from the surrounding normal brain, because the pathway of least resistance is through the iAVM as opposed to the normal surrounding circulation; this steal effect then causes dysfunction of the normal brain. The primary cause of morbidity and mortality from iAVMs, however, is brain hemorrhage resulting from the above-mentioned hemodynamic factors.

Preoperative Planning

Risks and benefits of surgery/informed consent

At its most basic, the decision to surgically treat an iAVM weighs the natural history risk of living with the AVM untreated vs the surgical risk of a complication. If the natural history risk is substantially higher than the surgical risk, in general, removal of the iAVM is a reasonable course of action. In contrast, if the surgical risk is equal to, or greater than, the natural history risk, then surgical excision should not be recommended. Incumbent upon the surgeon proposing surgery is to explain the risks and benefits on each side of the equation, to inform the patient and their family, answer questions as honestly and completely as possible, and allow the patient to make an educated decision that is best for them. This process, which is known as informed consent, is critical to ensure that the goals of surgery, the expected range of outcomes, and potential complications are understood before surgery is pursued.

The risk of surgery is unique to each patient and varies. Factors that combine to predict overall risk are related to the patient generally and iAVM specifically. Patient factors that are important to consider include, but are not limited to, chronological and medical age and systemic comorbidities and illnesses, as these best predict fitness to tolerate the stress of surgery and may contribute to complications of surgery and undergoing general anesthesia. AVM factors that contribute to surgical risk include location, size, number and location of arterial feeders and draining veins, and the speed at which blood passes through the iAVM, among others. Surgeon-related factors also influence risk, including the experience of the surgeon and their team. High-volume centers where this type of surgery is performed regularly often have better outcomes than facilities where it is performed less frequently.

Preoperative imaging

To determine which iAVMs should be operated on, the surgeon must carefully review the preoperative imaging, which should be multimodal. Diagnostic angiography provides invaluable data about the size of the AVM, the exact anatomy of the arteries that feed into it and the veins that drain it, the speed at which blood transits through the lesion, the pattern of venous outflow, and the presence of associated aneurysms—all of which are important factors in determining how challenging removing the AVM will be. MRI is also important in many cases to better determine the relationship of the AVM to the surrounding normal brain. In the case of an AVM that is near or within eloquent structures, such as the movement or speech centers of the brain, functional MRI (fMRI) is often helpful to determine the risk of surgery preoperatively; fMRI is a specialized study during which the patient is given movement and language tasks to map areas of the brain involved in those functions and determine their spatial relationship to the iAVM.

Preoperative embolization

An embolus is a mass that travels through the bloodstream and lodges in the arterial system at a point at which the arterial diameter is less than the size of the mass, resulting in a blockage of blood flow. Emboli can be naturally occurring—e.g., a blood clot that forms somewhere in the bloodstream—or introduced as part of a medical procedure. The process of injecting an embolic agent for the medical purpose of therapeutically blocking a blood vessel or series of blood vessels is called embolization. In some circumstances, a cerebrovascular surgeon may request embolization of an iAVM prior to surgery. In general, embolization of an iAVM is done by colleagues in interventional neuroradiology, doctors trained in minimally invasive treatment of blood vessel abnormalities. The interventional neuroradiologist navigates a tiny, flexible catheter, or tube, to a target artery or arteries that feed the iAVM and delivers an embolic agent, usually a liquid similar to glue, to either block a feeding artery or a set of arteries supplying blood to the iAVM or make a cast of the iAVM nidus. The overall goal is to make surgical removal of the iAVM safer by limiting blood loss or preventing collateral brain injury. The goal of embolization is usually directed toward one of two objectives: (1) blocking the feeding arteries at the base of the iAVM that are obscured from view on the underside of the lesion and are typically approached last, making them treacherous; or (2) reducing the overall blood flow through the iAVM nidus to simplify surgery. No matter the objective, as a rule, for preoperative embolization to be justified, the overall risk reduction for surgical removal provided by the embolization must be greater than the risk of the embolization itself. Embolization may also be used to eliminate associated aneurysms that would not be readily identified and treatable at surgery.

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