Middle Cerebral Artery Aneurysm

Diagnosis and Management

پوستی مسعود دکتراعصاب و مغز جراحی متخصص

قزوین رازی بیمارستان

ANATOMY

• The MCA is the largest and most complex of the cerebral arteries. Some of its branches are exposed in most operations in the supratentorial area, whether the approach is to the cerebral convexity, parasagittal region, or along the cranial base

• The MCA arises as the larger of the two terminal branches of the internal carotid artery.

• Its origin is at the medial end of the sylvian fissure, lateral to the optic chiasm, below the anterior perforated substance, and posterior to the division of the olfactory tract into the medial and lateral olfactory striae. From its origin, it courses laterally below the anterior perforated substance and parallel, but roughly 1 cm posterior, to the sphenoid ridge.

The MCA is divided into four segments: Ml (sphenoidal),M2 (insular), M3 (opercular), and M4 (cortical) . The M1 begins at the origin of the MCA and extends laterally within the depths of the sylvian fissure. It courses laterally, roughly parallel to and approximately 1 cm (range, 4.3–19.5 mm) posterior to the sphenoid ridge in the sphenoidalcompartment of the sylvian fissure. This segment terminates at the site of a 90-degree turn, the genu, located at the junction of the sphenoidal and perculoinsular compartments of the sylvian fissure.

The perforating branches of the MCA enter the anterior perforated substance and are called the lenticulostriate arteries . There is an average of 10 (range, 1–21) lenticulostriate arteries per hemisphere (36). Lenticulostriate branches arise from the prebifurcation part of the M1 in every case and from the postbifurcation part of the M1 segment in half of the hemispheres.The branches of the middle cerebral artery are important for surgical orientation, treatment paradigms and implications, andsalvage techniques. The M1 has multiple lenticulostriate arteries that are divided into two groups. The medial lenticulostriatearteries enter the anterior perforating substance superiorly and supply the lentiform nucleus, the caudate, and the internal capsule. The lateral lenticulostriate arteries are more variable in their location,traverse the basal ganglia, and supply the caudate nucleus

Middle Cerebral Artery Aneurysms

• The middle cerebral artery is one of the most common sites of saccular aneurysms. They most commonly arise at the level of the first major bifurcation or trifurcation of the artery. The angulation with which the bifurcating trunks arise from the main trunk forms the turn or curve. These aneurysms usually point laterally in the direction of the long axis of the prebifurcation segment of the main trunk.

Middle cerebral aneurysms. A, scalp incision andcraniotomy for approaching aneurysmsarising on the middle cerebralartery. B, operative view providedby a right frontotemporalcraniotomy. The right sylvian fissurehas been split to provide thisview of the optic nerves and thecarotid and middle and anteriorcerebral arteries. Brain spatulas areon the temporal and frontal lobes.C, middle cerebral aneurysms areusually located at the bifurcationnear the genu of the artery. Thearrows show the direction ofhemodynamic force at the aneurysmsite. The medial, intermediate,and lateral lenticulostriatearteries arise from the middle cerebralartery. D, aneurysm arisingon an early bifurcation. E, aneurysmarising at a large lenticulostriatebranch. F, aneurysm arisingat an early branch. A., arteries,artery; A.C.A., anterior cerebralartery; C.A., internal cerebralartery; Fr., frontal; Int., intermediate;Lat., lateral; Len.Str., lenticulostriate;M.C.A., middle cerebralartery; Med., medial; O.N., opticnerve; Temp., temporal.

CLASSIFICATION OF MIDDLE CEREBRAL ARTERY ANEURYSMS

• MCA aneurysms can be classified in many ways, including by morphology, location, etiology, and size.

Classification by Morphology

• Saccular aneurysms are the most commonly encountered, followed by fusiform aneurysms.

• Extremely dysmorphic or distal aneurysms are usually infectious and are classically identified on distal M4 branches.

Classification by Location• Bifurcation and trifurcation aneurysms represent up to 90% of all MCA

aneurysms and are the lesions most uniformly referred for surgical consideration.

• Aneurysms of the Ml segment are second in frequency to bifurcation aneurysms and are composed of lenticulostriate or anterior temporal artery saccular aneurysms.In patients with multiple intracranial aneurysms, the frequency of proximal MCA aneurysms tends to increase, and nearly three fourths of patients with multiple intracranial aneurysms harbor an MCA aneurysm.

• Rinne and colleagues examined 561patients with MCA aneurysms and found that 39% of patients with MCA aneurysms had multiple intracranial aneurysms, significantly higher than the 20% classically quoted for other parts of the intracranial circulation.additionally, these investigators found that the multiplicity of aneurysms increased the risk for poor outcome.

Classification by Etiology• Saccular • With regard to etiology, the precise pathogenesis of the common saccular (or berry) aneurysm is incompletely understood.

These aneurysms classically form at sites of arterial curves or branching. Hemodynamic forces are likely to be an important contributing factor in the forced segmentation of the arterial elastic membrane, which may be an important factor in the aneurysm formation cascade.

• Fusiform Aneurysms• Fusiform lesions are most frequently seen in the posterior circulation and have far different management than saccular

aneurysms . Although rare, there is no doubt that they represent a different disease process than saccular aneurysms, and in the MCA distribution, they can grow quite large, incorporating multiple branch vessels. MCA fusiform aneurysms can be found in all segments of the artery although classically they are found at the bifurcation.

• Infectious Aneurysms• Infectious or mycotic aneurysms are most commonly found along the distal branches of the cerebral arteries.They are

typically secondary to infectious emboli, With subsequent aneurysm formation. Usually, bacterial endocarditis (65%) is implicated in infectious aneurysms, but other idiopathic bacterial or fungal source have been implicated. Other embolic sources include neoplastic disease, such as choriocarcinoma and atrial myxoma.

• Dissecting Aneurysms• Dissecting MCA aneurysms are rare and may be associated with infection, connective tissue diseases such as Marfan's

syndrome, cystic medial degeneration, and fibromuscular dypslasia.• congenital weakness of the vessel wall with rupture of the elastic interna.

• Traumatic Aneurysms• Traumatic aneurysms are uncommon and are most often associated with the ACA and its branches because of its proximity

to both the skull base and the faLx.Traumatic MCA aneurysms are unusual, are most classically associated with a skull fracture, and have a high rupture rate. These lesions are most frequently distal on M3 and M4 segments and often present Witlh delayed rupture (average, 4.7 days) after tlh inciting trauma.Classically, these are managed with surgical trapping and excision with or without bypass.

Classification by Size

• Small(<5mm), medium (5 to 10 mm), large (11 to 25 mm), and giant (>25 mm).Most M1-lenticulostriate aneurysms are quite small (which often precludes endovascular treatment).

• Cerebral aneurysms that reach large (>15 mm) or giant (>25 mm) size are more frequently seen in the MCA distribution than in other arterial distributions and can represent up to 9% of MCA aneurysms.

PRESENTATION AND EVALUATION OFMCA ANEURYSMS

• SAH or ICH is the most common presentation of MCA aneurysms. Because of their propensity to become quite large before detection, they may occasionally become symptomatic without SAH. Giant aneurysms are reported to cause seizures more often than smaller ones, and this may be due to mass effect, ischemic changes, or repeated subclinical hemorrhages.

• Evaluation and treatment of patients With aneurysms varies depending on whether an aneurysm has ruptured or not. Nonruptured aneurysms are increasingly being discovered incidentally because of the frequency of use of screening studies (CT, CTA,MRA) in the evaluation of any number of unrelated symptoms.

• In symptomatic cases, CT usually reveals the presence of SAH however, a lack of blood necessitates a LP if the history is suggestive of an aneurysm rupture.In patients with SAH on CT or LP, we typically perform a cerebral angiogram, with three-dimensional reconstruction of any pathology if possible.

Basic surgical principles

• 1.Proximal control: The parent artery should be exposed proximal to the aneurysm. This allows control of flow to the aneurysm if it ruptures during dissection

• Exposure of the ICA above the cavernous sinus will give proximal control for aneurysms arising at the level of the Post.Com or Ant.choroidal A.

• The supraclinoid carotid or the preaneurysmal trunks of the MCA or ACA should also be exposed initially to obtain proximal control of MCA and ACA aneurysms.

• 2.Exposure rule : If possible, the side of the parent vessel away from or opposite to the site on which the aneurysm arises should be exposed before dissecting the neck of the aneurysm. The dissection can then be carried around the wall of the parent vessel to the origin of the aneurysm.

• 3.Neck before fundus: The aneurysmal neck should be dissected before the fundus. The neck is the area that can tolerate the greatest manipulation, has the least tendency to rupture, and is to be clipped.But unfortunately the origin of parent A. trunk and perforating vessels make this difficult .So The dissection should not be started at the dome, because this is the area most likely to rupture before or during surgery.

• 4. Separating and preserving of all perforating arterial branches: Separating the perforators, if tightly packed against or adherent to the aneurysm may be facilitated by lowering the blood pressure or by temporary clipping of the parent artery.

• 5.Bleeding: If rupture occurs during microdissection, bleeding should be controlled by applying a small cotton pledget to the bleeding point and concomitantly reducing mean arterial pressure. If this technique does not stop the hemorrhage, temporary occlusion with a clip or occluding balloon can be applied to the proximal blood supply, but only for a brief time.

• 6. The bone flap should be placed as low as possible to minimize the need for retraction of the brain in reaching the area• 7.After clipping:After the clip is applied, the area should always be inspected, sometimes with intraoperative angiography, to

make certain the clip does not kink or obstruct a major vessel and that no perforating branches are included in it• 8.Broad neck base: If an aneurysm has a broad-based neck that will not easily accept the clip, the neck may be reduced by

bipolar coagulation. Nearby perforating arteries are protected with a cottonoid sponge during coagulation.