27 Distal Middle Cerebral Artery Aneurysms

10.1055/b-0038-162156

27 Distal Middle Cerebral Artery Aneurysms

Wuyang Yang and Judy Huang

Abstract

Distal middle cerebral artery (MCA) aneurysms, located distally to the M2 segment of the MCA, are particularly rare and constitute less than 5% of all MCA aneurysms. Uncertainty may arise in the management of distal MCA aneurysms, especially regarding the decision of whether or not to treat and selection of optimal treatment modality. Multiple factors may confound these decisions, including ruptured presentation, presence of intracerebral hemorrhage (ICH), aneurysm size, location, morphology, overall clinical status of the patient, presence of aneurysm calcification, and intraluminal thrombosis. The development of treatment strategy should take these factors into account and individualize the treatment modality or combination of modalities, which include open microsurgical techniques such as clip reconstruction or bypass, endovascular treatment, and also conservative management. This chapter discusses the decision-making algorithm that informs the management of distal MCA aneurysms, with specific emphasis on how anatomical and clinical factors may ultimately impact treatment selection.

Introduction

Distal middle cerebral artery (MCA) aneurysms are defined as aneurysms occurring after the MCA bifurcation or trifurcation, namely, those originating from the M2 to M4 (or M5) segments of the MCA. They are rare lesions comprising only 1.1 to 5% of all MCA aneurysms, with most located at the M2 and M3 segments. Unlike aneurysms of the proximal MCA, the etiology of distal MCA aneurysms is often infectious, and distal MCA aneurysms are reportedly less likely to rupture. The distinct anatomical features of distal MCA aneurysms must be considered in treatment selection using either microsurgical or endovascular techniques.

Major controversies in decision making addressed in this chapter include:

Whether treatment is indicated.
Microsurgical versus endovascular treatment for M2 segment distal MCA aneurysms.
Selection of treatment modality for M3–M4 segment distal MCA aneurysms.
Selection of treatment modality for giant, fusiform, dissecting, and mycotic aneurysms.

Whether to Treat

The natural history of distal MCA aneurysms is less well understood due to their heterogeneity and rarity, but accepted risk factors for aneurysmal rupture are believed to be applicable to distal MCA aneurysms. These include aneurysm size, irregular morphology, history of subarachnoid hemorrhage (SAH), hypertension, positive family history, and symptomatic presentation. Tailored treatment decisions must take these factors into account when deciding whether the patient should be treated. Large series of distal MCA have estimated around 70% of these lesions are smaller than 7 mm, and over 75% were unruptured at presentation ( 2 in algorithm ), with most saccular in shape. These aneurysms theoretically harbor low risk of subsequent hemorrhage, and may be managed conservatively if risk of treatment is believed to exceed the patient and aneurysm risk profiles ( 11, 12 in algorithm ). For aneurysms with high risk of hemorrhage characterized by large or giant size, prior history of SAH, or irregular shape, treatment with microsurgical or endovascular techniques may achieve near 100% obliteration rate with good functional outcomes in 70 to 90% of all treated patients ( 9, 10 in algorithm ). For ruptured aneurysms ( 2 in algorithm ), endovascular or microsurgical treatment is often warranted to prevent further hemorrhages ( 4, 5, 8 in algorithm ), and may be combined with a hemicraniectomy for decompression if presenting in poor grade condition or concomitant intracerebral hemorrhage (ICH) ( 6, 7 in algorithm ).

Anatomical Considerations

The M1 segment originates from the terminal bifurcation of the supraclinoid internal carotid artery. The M1 bifurcates at the limen insula, and most MCA aneurysms occur at this bifurcation, which divides into superior and inferior MCA trunks (▶ Figs. 27.1 and ▶ 27.2 ). Over 85% of MCA bifurcations occur proximal to the genu, and most bifurcations were immediately adjacent to the termination of M1. Aneurysms occurring beyond the genu are classified as distal MCA aneurysms, comprising those occurring on the insular segment (M2), opercular segment (M3), and cortical segment (M4). The distal M4 branches are also referred to as the terminal segment (or M5). Starting from the M2 segment, the MCA courses through the operculoinsular compartments of the sylvian fissure and eventually reaches the cortical surface, which marks the origin of the M4 segment. The largest branching in M2 occurs at the anterior part of the insula, with anterior branches supplying the anterior frontal and temporal areas, and the posterior branch travels a longer distance to supply the posterior parietal region. Approximately 80 to 90% of distal MCA aneurysms were found in the M2–M3 segment, with most located in the M2 segment, followed in frequency by the M2–M3 junction and M3 locations.

**Algorithm 27.1** Decision-making algorithm for distal middle cerebral artery aneurysms. ICH, intracerebral hemorrhage.

**Fig. 27.1** Artist′s illustration depicting the anatomy of the four segments of the middle cerebral artery. (Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

An anatomical advantage of distal MCA aneurysms for microsurgical treatment, particularly at the M2 and M3 segments, is provided by the relatively superficial location and proximity to the sylvian fissure (▶ Figs. 27.1 – 27.3 ). In contrast, anatomical disadvantages for treatment include those located at multiple branch points beyond the MCA bifurcation, thereby increasing difficulty for intraoperative localization of the aneurysm, as the correlation of an easily identified distal MCA branch on computed tomography angiography (CTA) or angiography to its intraoperative location in the operative field may be challenging. A higher prevalence of fusiform aneurysms without a clear neck might be associated with distal MCA aneurysms, and lack of collateral circulation necessitates optimal clip reconstruction in order to prevent stenosis of the small parent artery. Additionally, the decreasing luminal diameter of the branches also renders difficult endovascular access.

**Fig 27.2 (a)** Anteroposterior (AP) view of a left carotid injection digital subtraction angiogram (DSA). Two aneurysms are demonstrated, with one at the middle cerebral artery (MCA) bifurcation and another at the M2 segment. The distal MCA segments are labeled accordingly (M2–M4 segments). **(b)** Oblique view of the same patient. ACA, anterior cerebral artery; ICA, internal carotid artery.

Classification

Classification of distal MCA aneurysms can be either by location or by etiology of the aneurysm. The relatively more proximal distal MCA aneurysms are comprised of those at M2 and M2–M3 junction, where the majority of aneurysms occur. The most distal MCA aneurysms occur at M3–M4 (or M5) segments. In regard to etiology, distal MCA aneurysm can be categorized as noninfectious and infectious (mycotic) aneurysms.

Workup

Clinical Evaluation

The presentation of distal MCA aneurysms may be distinct from aneurysms arising at or proximal to the MCA bifurcation. Ruptured presentation with SAH or ICH is seen in only 20% of distal MCA aneurysms; these patients may present with symptoms such as headache, loss of consciousness, and acute onset of weakness or aphasia. Approximately 50% of ruptured patients have concomitant ICH, and hydrocephalus may occur in 20% of all patients. Around 70% of the patients may harbor multiple intracranial aneurysms.