26 Middle Cerebral Artery Aneurysms
Abstract
Middle cerebral artery (MCA) aneurysms account for 14 to 20% of all intracranial aneurysms. The MCA is the largest and most variable of all intracranial arteries. Compared to other aneurysms, nonruptured MCA aneurysms are more likely to become large before detection. The most common location of MCA aneurysms is at the bifurcation in up to 90% of cases. Most common clinical presentation is subarachnoid hemorrhage and/or intraparenchymal (IPH) hemorrhage. Computed tomography (CT) and CT angiography are the initial imaging evaluation. Digital subtraction angiography is the gold standard for aneurysm visualization and characterization. When selecting the modality of treatment of MCA aneurysms, factors such as patient′s age, clinical condition, aneurysm size and morphology, inclusion of a branch vessel in the neck, and presence of IPH should be taken into consideration. Microsurgical clipping is historically the treatment of choice and well described; however, given the right conditions certain endovascular techniques can have clinical equipoise. Their classic bifurcation location makes recurrence likely if simple coil technique is used. Stent-assisted coiling, novel neck reconstruction devices, and intrasaccular flow diversion are good alternatives if microsurgical clipping is not feasible or because of patient preference. Overall, the outcome of MCA aneurysms is good to excellent. Large series have shown favorable outcome in greater than 95 and greater than 75% of cases of un-ruptured and ruptured aneurysms, respectively. Appropriate clinical and radiological follow-up is mandatory in all intracranial aneurysms.
Introduction
Middle cerebral artery (MCA) aneurysms account for 14 to 20% of all intracranial aneurysms. They are most commonly located at the bifurcation of the first segment (M1). Compared to other intracranial aneurysms, MCA aneurysms are more likely to become large before detection, to cause cerebral mass effect symptoms, and to present with intraparenchymal hematoma. They often have broad necks and diverging branches that originate from the base of the dome. As such, they are more challenging to treat with endovascular techniques.
Major controversies in decision making addressed in this chapter include:
Whether or not treatment is indicated.
Open versus endovascular treatment for ruptured and unruptured MCA aneurysms.
Management of MCA aneurysms that present with intracerebral hematoma (ICH).
When should an advanced surgical technique (aneurysmorrhaphy, bypass) be considered?
Whether to Treat
As with other intracerebral aneurysms, the risk of rupture increases with size and subarachnoid hemorrhage (SAH) from a previous aneurysm in the same patient. For MCA aneurysms less than 7 mm in diameter in patients with no previous SAH, the rupture rate is 0.1 to 1.0% per year. The International Study of Unruptured Intracranial Aneurysms 1 (ISUIA-1) and the follow-up study ISUIA-2 were designed to assess the natural history of unruptured intracranial aneurysms and treatment-related morbidity and mortality and to compare outcomes of surgical and endovascular repair. The 5-year cumulative rupture rates for MCA aneurysms were as follows: 0% for aneurysms less than 7 mm with no previous SAH from other aneurysms and 1.5% with previous SAH from other aneurysms; 2.6% for aneurysms sized from 7 to 12 mm; 14.5% for aneurysms sized from 13 to 24 mm; and 40% for aneurysms larger than 25 mm. Predictors of poor surgical outcome relevant to MCA aneurysm treatment included aneurysm size larger than 12 mm and patient age older than 50 years. A total of 1,179 (29%) MCA aneurysms were included in these trials, of which 475 (40%) did not receive treatment, 650 (55.1%) underwent clipping, and 54 (0.45%) underwent coiling. For ruptured aneurysms, treatment is clearly indicated unless the patient is moribund or very elderly (for patients older than 80 years, outcomes are very poor with or without treatment). A prospective Japanese study found the rate of rupture per MCA aneurysm per year (95% confidence interval [CI]) to be 0.23 (0.09–0.54) for 3- to 4-mm aneurysms, 0.31 (0.10–0.96) for 5- to 6-mm aneurysms, 1.56 (0.74–3.26) for 7- to 9-mm aneurysms, 4.11 (2.22–7.66) for 10- to 24-mm aneurysms, and 16.87 (2.38–119.77) for greater than 25 mm aneurysms. A population-based Finnish study found an annual incidence of 1.1%. MCA aneurysms are considered medium-risk aneurysms. The vast majority of ruptured MCA aneurysms should be treated, and for the most part first-line treatment should be microsurgical clipping. For unruptured aneurysms, size and patient′s remaining life expectancy are the primary determinants of whether treatment should be undertaken or not ( 1 in algorithm ).
Anatomical Considerations
The MCA is the largest and most variable of all intracranial arteries from an anatomical perspective. It is divided into four segments (the MCA candelabra): M1 (sphenoidal), M2 (insular), M3 (opercular), and M4 (cortical). The length of the MCA is variable. Short M1 segments have surgical implications because aneurysms on such vessels are located deep within the sylvian fissure. Branches of the MCA are important for surgical orientation, treatment implications, and rescue techniques. The M1 segment has medial and lateral lenticulostriate arteries that must be respected relative to any treatment approach undertaken. Anomalies of the MCA candelabra are not uncommon and include duplications of the MCA (in 3% of cases) and origin at the anterior cerebral artery (3%).
Pathophysiology/Classification
Intracranial aneurysms can be classified by size, etiology, location, and morphology. According to size, they are classified as small (<5 mm), medium (5–10 mm), large (11–25 mm), and giant (>25 mm). Large and giant aneurysms are seen more frequently on the MCA, and they represent up to 10% of MCA aneurysms. On the basis of etiology, MCA aneurysms can be classified as saccular or berry, fusiform, dissecting, infectious, and traumatic. Most MCA aneurysms are saccular. Fusiform MCA aneurysms are rare; however, they can grow large, and incorporate multiple branch vessels. Dissecting MCA aneurysms are rare and associated with infection and connective tissue disorders. The dissection is located between the internal elastic lamina and medial layers. Angiographically, there is asymmetric narrowing or a ripple appearance due to the separated intima. Affected patients can present with infarction. If parent vessel patency has been preserved, endovascular stenting or trapping with a surgical bypass procedure may be considered. Infectious aneurysms are located more distally (see Distal MCA Aneurysms, Chapter 27). Traumatic MCA aneurysms are also distal, associated with a skull fracture, and can present with delayed rupture.
According to location, the most common site is at the bifurcation (90%), followed by the M1 segment (in 10% of cases). Bifurcation aneurysms are uniformly referred for surgical consideration. Proximal MCA aneurysms are divided into superior wall and inferior wall types. Superior-wall aneurysms are located at the origin of the lenticulostriate arteries with a frontal lobe projection. These aneurysms are small, making them challenging for endovascular treatment. Inferior-wall aneurysms have a temporal lobe projection. Distal MCA aneurysms are described in Chapter 27.
Workup
Clinical Evaluation
SAH with or without ICH is the most common clinical presentation of an MCA aneurysm. Large aneurysms can present with mass effect, seizures, and infarction. However, nowadays many MCA aneurysms are found incidentally during evaluation of unrelated symptoms.
Imaging
Typically, a diagnostic cerebral angiogram (DSA) is performed that includes a three-dimensional reconstruction of the aneurysm. For large aneurysms, computed tomographic angiography (CTA) is also recommended to evaluate for calcifications and possible partial thrombosis of the aneurysm.
Treatment
Choice of Treatment and the Influence of Intracerebral Hematoma
Traditionally, all MCA aneurysms recommended for treatment have been treated with microsurgical clipping. While this remains the general rule, several authors have begun to report their experience with endovascular treatment for both ruptured and unruptured aneurysms ( 1 , 8 in algorithm ). The difficulty or ease of MCA aneurysm treatment varies, depending on aneurysm size and configuration, inclusion of the branch vessels in the neck, presence of SAH, and experience of the surgeon or interventionist (▶ Figs. 26.1 and ▶ 26.2 ). MCA aneurysms presenting with a large temporal or sylvian ICH require urgent craniectomy and clot evacuation with exploration and treatment of the aneurysm simultaneously is the rule. Often the aneurysm can be clipped without removing the clot first, which then can be easily and safely removed ( 2 in algorithm ). Many MCA aneurysms are unsuitable for endovascular options due to several reasons including the higher chance of main branch occlusion and the fact that their classic bifurcation location makes recurrence more likely. MCA aneurysms are also commonly readily accessible surgically and therefore they are preferentially clipped at most cerebrovascular centers (▶ Fig. 26.1 ). There are some reports of coiling of the aneurysm, followed by clot evacuation. This is considered a nonstandard treatment by experienced cerebrovascular surgeons. If a craniotomy is undertaken for an ICH, the presence of shift out of proportion to the hematoma should prompt consideration of a hemicraniectomy. There is controversial practice of coiling the aneurysm and then taking the patient for a craniotomy for clot evacuation with no attempt at clipping ( 3 in algorithm ). For those ruptured aneurysms without hematoma, timing of surgery should be as early as reasonably possible. Since much of the risk of rebleeding is in the first 24 hours through 2 weeks, the earliest opportune time should be selected ( 4 in algorithm ).
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