Conservative Management (“Observation”) of Intracranial AVMs

Introduction

Current management of intracranial arteriovenous malformations (iAVMs) includes several treatment modalities to choose from. However, the initial decision for both patient and physician is whether the lesion should be treated or not. The overall risk of treatment must be balanced against the risks of continued observation (natural history) of the lesion. If treatment seems the best option, the modality of treatment is chosen based on careful assessment of the risks and potential benefits of each treatment in the patient’s particular case. Surgical excision (resection), endovascular embolization, radiosurgery, or combined-modality therapy should each be considered. These treatment options are covered in other chapters of this text. Our goal for the current chapter is to consider the natural history of iAVMs and how to determine whether a conservative course of observation might be the lowest-risk option for an individual patient.

Initial Evaluation and Classification

The initial phase of evaluation of an iAVM requires gathering information. This typically includes obtaining a clinical history and imaging to delineate the details of the AVM in order to be able to make an informed decision about the most appropriate next step. Patient demographic variables also weigh heavily in the decision. The workup usually includes multiple diagnostic tests, especially CT and MR angiograms and standard MRI studies. Additionally, functional MRI can be helpful in discerning the relationship between the AVM and critical structures of the brain, including areas that control motor function and speech. A catheter-based angiogram is often critical to help evaluate the AVM vasculature and visualize the main vessels of supply to the lesion as well as the actual “nidus” (mass of tangled vessels) and the venous drainage. Once the information from this workup is complete, a discussion should ensue regarding the risks of continued observation versus the risks of intervention.

In order to understand the data that inform decisions about the risk for individual patients, it is necessary to have some familiarity with the classification of iAVMs. This topic is covered in detail elsewhere in this book (see, in particular, Chapters 8 and 19 ); here, we provide only a brief overview of the most commonly used classification system, the Spetzler-Martin grading system, to facilitate the discussion of management options.

The Spetzler-Martin grading system was specifically designed to predict the outcome of surgical treatment. It has also been extensively used to classify iAVMs in general and discuss other treatment modalities and natural history, although the utility of this more general application is questionable. The Spetzler-Martin system is a composite five-point classification system based on three main criteria: maximum diameter of the nidus: (1) (< 3 cm: 1 point, 3–6 cm: 2 points, > 6 cm: 3 points), (2) venous drainage (superficial: 0 points, deep: 1 point), and (3) eloquence of the area of the brain (noneloquent: 0 points, eloquent: 1 point). Eloquence refers to particular areas of the brain that control essential functions, including speech, language, and vision, where damage can lead to focal neurologic deficits. Accordingly, the risks of intervention may be deemed higher when these areas are involved. Examples of eloquent areas include but are not limited to motor and sensory cortices, auditory and visual cortices, speech areas, basal ganglia, thalamus, hypothalamus, and brainstem. A higher Spetzler-Martin grade denotes increased complexity of the lesion and a higher risk of a poor outcome with surgical excision.