Chapter 23

### INTRODUCTION

All aspects of the management of intracranial arterial disease have undergone major transformations in the past two decades. Alternatives are now available for conditions previously treatable only with a single modality. Moreover, some conditions that would have been considered untreatable in the past now have safe and effective therapies. Although the credit for these advances is attributable to many factors, several fundamental achievements are recognized. First, the advances in noninvasive diagnostic neuroimaging, namely, computed tomographic (CT) and magnetic resonance (MR) imaging and their respective variants have resulted in the increased detection of asymptomatic vascular lesions and improved diagnostic sensitivity. As will be discussed later, this early detection presents a clinical dilemma in determining the appropriate management on the basis of the natural history of specific lesions. Second, the refinement of microneurosurgical techniques has resulted in improved surgical outcomes. This experience has also allowed us to recognize the limitations of cerebrovascular surgery. Third, the advances in neurological critical care have resulted in improved morbidity rates and shorter hospitalizations. Finally, the emergence of neuroendovascular therapy into a multidisciplinary field involving neurosurgeons, neurologists, and radiologists has changed practice patterns dramatically. The ongoing explosion in endovascular advances has allowed for catheter-based therapeutics in virtually every aspect of cerebrovascular disease.

In this chapter, the current concepts of the major categories of intracranial vascular disease are reviewed. Given the inherent limitations of covering such a broad subject in one chapter, some discussions are very cursory and superficial. The topics most likely to be encountered in clinical practice, namely, aneurysms, ischemic stroke, and arteriovenous malformations (AVMs), are treated more rigorously.

### STRUCTURE OF INTRACRANIAL ARTERIES

#### Histology

To gain insight into the numerous pathological conditions of the human intracranial arteries and to develop sound therapies for these disorders, a review of the gross and microscopic structure of normal arteries is necessary. The walls of the cerebral arteries are composed of three regions. From the lumen to the outer surface, these layers are the tunica intima, tunica media, and tunica adventitia. The intima is composed of an endothelial cell layer and the internal elastic lamina. The media is relatively thin and is composed of 4 to 20 smooth muscle layers that account for roughly half the thickness of the artery wall.1 Collagen fibers and fibroblasts mostly comprise the adventitia. Innervation of the cerebral arteries is by myelinated and unmyelinated nerve fibers coursing in the adventitia or between the media and adventitia.2

There are notable differences between the structure of the intracranial and systemic arteries. The internal elastic lamina is extremely convoluted and well developed in the cerebral arteries.3 The internal elastic lamina has long been thought to account for most of the mechanical strength of the artery.4 Cerebral arteries have a smaller wall-to-lumen ratio than their systemic counterparts.5 In contrast to peripheral arteries, intracranial arteries have few elastic fibers within ...

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