Middle Cerebral Artery Aneurysms

Patient Selection

The middle cerebral artery (MCA) originates as the largest terminal branch of the internal carotid artery, the other major branch being the anterior cerebral artery. ¹^, ² The MCA origin is lateral to the optic chiasm, posterior to the medial and lateral olfactory striae, and inferior to the anterior perforated substance, and lies in the sphenoidal compartment of the sylvian fissure. The first (M1) or sphenoidal segment of the MCA courses laterally posterior and parallel to the sphenoid ridge. Ml bifurcates into superior and inferior trunks that are the M2 segments. Branches of Ml include lenticulostriate arteries, which perforate the anterior perforated substance, and there are usually cortical branches, most commonly the anterior temporal and polar temporal arteries.

The anatomy of the MCA and the aneurysm should be reviewed. The direction of projection, size, and morphology (saccular, fusiform, dissecting, infectious, traumatic, proximity to and potential adherence of MCA branches, presence of atherosclerosis, calcifications, and thrombosis) of the aneurysm should be scrutinized on three-dimensional catheter or computed tomography (CT) angiography. For the most common aneurysm of the MCA, which is located at the MCA bifurcation, it is important to determine the length of the Ml segment preoperatively, which, on the anteroposterior angiogram view, shows the depth of the aneurysm. The lenticulostriate branches nearly always originate along the superior and posterior parts of the MCA; thus, the arachnoid should be opened and the Ml manipulated on the frontal and inferior sides.

Most MCA aneurysms are diagnosed following subarachnoid hemorrhage (SAH) and/or intracerebral hemorrhage. ¹^, ² They also are the most common incidentally found aneurysm. Rarely, giant MCA aneurysms present with symptoms related to mass effect, ischemia, or seizures ( ▶ Fig. 9.1, ▶ Fig. 9.2, ▶ Fig. 9.3).

Fig. 9.1 A giant thrombosed middle cerebral artery aneurysm that arises from the left inferior M2 branch can be seen on (a) cranial computed tomography, (b) magnetic resonance angiography, and (c) T2-weighted magnetic resonance imaging scans. The patient was a 32-year-old man who presented with a seizure. Lateral (d) and anterior posterior (e) catheter angiography shows the aneurysm arising from the inferior M2 trunk. (f) The aneurysm was treated by trapping and bypass by suturing the superficial temporal artery to the M2 branch distal to the aneurysm.

Fig. 9.2 A giant fusiform middle cerebral artery (MCA) aneurysm. (a) The aneurysm was found incidentally on a skull radiograph, and the initial lateral carotid angiogram shows the calcified part of the aneurysm with filling of MCA branches beyond the aneurysm. (b) A magnetic resonance study at that time shows the aneurysm in the left sylvian fissure. The 49-year-old woman was followed; 10 years later, she presented with headaches and dysphasia, and a computed tomography (CT) scan (c, d) showed marked enlargement of the aneurysm with intra-aneurysmal thrombosis and edema in the brain surrounding it. (e) Axial T1-weighted gadolinium-enhanced and (f) axial T2 images show the giant, thrombosed aneurysm with the suggestion of a serpentine channel through the aneurysm. (g) A left internal carotid catheter angiography lateral view shows the channel through the aneurysm with three distal MCA branches emerging from the distal end of the aneurysm. Sequential oblique views early (h) and late (i) arterial phase show that the three branches are on the deep side of the aneurysm sac and course up to the surface over the top of the aneurysm. The aneurysm was treated by bypassing two superficial temporal artery branches into two of the MCA branches and then filling the proximal end of the aneurysm inflow with coils. (j) Postoperative angiogram 6 months later shows occlusion of the aneurysm with coils on the lateral view of the internal carotid injection and (k) revascularization distally of the MCA. The CT scan (l, m) shows resolution of much of the aneurysm mass. The patient recovered fully and remained well at last follow-up 4 years later.

Fig. 9.3 A 68-year-old woman with type 2 diabetes and hypertension presented with multiple syncopal episodes and was found to have a giant fusiform left middle cerebral artery aneurysm on plain (a) and enhanced (b) axial CT scans and axial T2-weighted (c) and time-of-flight magnetic resonance imaging angiography (d). Catheter angiography (e, f) with a 3D reconstruction (g) demonstrates the arterial inflow from the M1 segment of the middle cerebral artery and a branch arising from M1 proximal to the aneurysm. A dilated portion of the middle cerebral artery emanates from the saccular part of the aneurysm giving rise to two branches (double arrowheads) as well as another branch directly from the sac (arrowhead). The patient underwent trapping of the aneurysm and bypass by anastomosis of one superficial temporal artery branch into the middle cerebral artery branch arising directly from the sac (arrowhead) and a second into the larger distal branch (double arrowheads). The distal clip tapping the aneurysm was placed proximal to the bifurcation of the two distal branches to allow filling of both through the one anastomosis. Postoperative CT angiography shows the two bypasses (double arrows, h). Postoperative T2-weighted magnetic resonance imaging shows collapse of the aneurysm and preservation of the brain parenchyma (i, j). The patient recovered fully.

9.2 Indications and Contraindications for Surgery

The main indications for surgery are for ruptured or symptomatic MCA aneurysms. In patients with SAH, we aim to secure the aneurysm as early as possible after SAH to minimize the risk of further hemorrhage and also before the onset of angiographic vasospasm and delayed cerebral ischemia. Emergent treatment may be indicated in cases where there is a sizable intracerebral hematoma that needs to be evacuated to improve the neurological condition ( ▶ Fig. 9.4). In general, the authors treat all patients with ruptured aneurysms except those who have bilaterally fixed pupils or worse, and those in whom recovery is deemed highly unlikely. The decision to adopt conservative treatment assumes that poor neurological status is not due to factors such as intracerebral hematoma or hydrocephalus. If evacuation of an intracerebral hematoma is necessary, the aneurysm has to be secured at the same surgery.

Fig. 9.4 (a) Cranial computed tomography showing subarachnoid hemorrhage and (b) a hematoma in the temporal lobe on the left. The aneurysm can be seen in the sylvian fissure (arrow). (c) Angiography shows the aneurysm, which (d) was clipped uneventfully.

The urgency of the situation will dictate whether a catheter angiogram can be performed prior to surgery. In a patient with an intracerebral hematoma that needs to be removed acutely, a CT angiography can give adequate information on the location and size of the aneurysm so that surgery can be performed. We rely heavily on CT angiography and reserve catheter angiography for complex or giant aneurysms and those where endovascular treatment is contemplated, such as unruptured aneurysms or those with favorable morphology and unfavorable surgical characteristics (advanced patient age, poor medical risk, neck calcification, severe atherosclerosis, etc.). The next indication is to secure unruptured aneurysms in the case where the risk of aneurysm rupture over time is considered to exceed surgical risk.

9.3 Alternative Considerations to Surgery

The alternatives to surgery are endovascular treatment or no treatment. ³ No treatment for the aneurysm may be appropriate in poor-grade patients after SAH or for unruptured aneurysms. Management of unruptured aneurysms is controversial, and the surgeon must be convinced that the risk of morbidity and mortality from surgery is lower than that due to the natural history of the aneurysm if left alone. ⁴^, ⁵

Once a decision has been made to secure an aneurysm, the next consideration is whether clipping or coiling is more appropriate. MCA aneurysms tend to have a wide neck with branches arising at the base, which has generally made them more suitable for clipping. Endovascular technology is advancing, although efficacy studies of new devices have rarely been done. Management decisions have to be made on a case-by-case basis.

9.4 Risks

Specific risks associated with craniotomy for MCA aneurysm clipping include intraoperative aneurysm rupture, incomplete aneurysm occlusion, seizures, occlusion of significant branches or perforators with clip application leading to varying degrees of neurological deficit, and injury to the frontalis branch of the facial nerve. Occlusion of MCA branches can lead to motor weakness, expressive or receptive dysphasia, higher-functioning disturbance, visual field defects, parietal signs, and even death due to brain swelling or severe neurological deficit.

9.5 Preoperative Preparation

9.5.1 Medications

Nimodipine is commenced after diagnosis in patients with SAH. ⁶ The usual oral dose is 60 mg every 4 hours. Maintenance of normovolemia is imperative in the perioperative phase. There is no uniform agreement on choice of fluid replacement, but our practice is to use 0.9% NaCl with close monitoring of electrolytes and sodium and potassium supplementation as necessary. Anticonvulsants such as phenytoin or levetiracetam may be administered to patients who have had a seizure, who are at high risk of having one (intracerebral hematoma), or who would be potentially harmed by one (poor-grade patients, those with already increased intracranial pressure).

9.5.2 Other Factors

Smooth induction of anesthesia without altering the blood pressure is important so as to reduce the risk of aneurysm rupture or rerupture. The head pins should only be applied when it can be assured that the patient is adequately anesthetized. We do not use lumbar drainage but place an external ventricular drain (EVD) preoperatively in patients with neurological symptoms and signs due to hydrocephalus or intraoperatively via Paine’s point when additional brain relaxation is needed. ⁷ An arterial line and urinary catheter are used in all patients. Central venous lines are optional. Always be prepared for massive uncontrolled bleeding. Intraoperative hypothermia was not beneficial in a large, randomized study, so despite experimental data to the contrary, its use has to be considered questionable at this time.

For complex or giant aneurysms where prolonged temporary clipping or bypass procedures may be necessary, we use electroencephalography monitoring so we can induce burst suppression if necessary ( ▶ Fig. 9.1, ▶ Fig. 9.2, ▶ Fig. 9.3).

9.6 Operative Procedure

9.6.1 Positioning

The patient is positioned supine with a rolled sheet under the ipsilateral shoulder to allow neck rotation (Video 9.1). The bed is flexed and put in a 20-degree reverse Trendelenburg position. A three- or four-pin radiolucent head holder is applied. The normal angle from the anterior clinoid process to the pterion is just over 45 degrees. For MCA aneurysms, turning the patient’s head 45 degrees to the contralateral side means the operative pathway will be almost vertically downward along the sphenoid ridge. Intraoperative neuronavigation can be used to guide positioning and, if required, EVD insertion and locating where an intracerebral hemorrhage presents closest to the brain surface. This reduces the need for retraction of the temporal lobe, and often only the frontal lobe needs to be retracted. The head is also extended to allow the frontal lobe to fall away naturally once dissection has begun. Finally, the rotated, extended head is elevated upward to facilitate venous return. In this position, the malar eminence is the highest point. The head is then fixed. Mannitol, 1 g/kg, and furosemide (20–40 mg) are given intravenously in most cases with SAH except in cases such as an elderly person with minimal SAH where brain relaxation will not be needed.

9.6.2 Skin Incision and Craniotomy

The authors use a standard pterional craniotomy. We only shave a line of hair along the incision and a line along where an EVD might exit posteriorly. Once the patient has been positioned and draped (standard craniotomy drape), an incision is begun 1 cm anterior to the tragus at the level of the zygoma extending initially vertically and slightly posteriorly and then gently curving over the superior temporal line to end at the hairline just across the midline. This preserves the superficial temporal artery so that it can be used for intraoperative angiography. The temporalis muscle is reflected in a single layer with the scalp flap, which makes it easier to preserve the frontalis branch of the facial nerve. The temporalis fascia can be cut with a knife so it does not shrink down if cut by cautery. The muscle can be divided with cautery. Dissection of the muscle off the bone should preserve the periosteal layer attached to the muscle. A cuff of fascia and muscle along the superior temporal line can be left behind to reapproximate the muscle to the region of the superior temporal line with sutures at the end of the procedure.

The musculocutaneous flap is covered with saline-soaked gauze and retracted with fish hooks that can be attached to the carbon fiber basal frame. A free bone flap is fashioned with up to three burr holes joined up with a craniotome. The first burr hole is at the keyhole above the frontozygomatic suture and inferior to the superior temporal line. Additional burr holes can be made just above the posterior end of the zygomatic arch at the lower end of the skin incision and above the superior temporal line at the posterior margin of the scalp incision. If after removing the bone flap the dura is tense, additional brain relaxation can be achieved by mild hyperventilation. Also, we insert an EVD into the frontal horn of the lateral ventricle once the dura is open. ⁷ This is done by aiming perpendicularly to the brain from the top of a triangle 2.5 cm back along the sylvian fissure and 2.5 cm superiorly.

Further bone removal is then directed at removing the lateral one-third of the sphenoid wing using either a high-speed drill or rongeurs. For an anteriorly directed MCA aneurysm, be aware that the aneurysm could be adherent to the dura of the sphenoid ridge. A clue to this is if there is subdural blood on the CT scan, suggesting the aneurysm has eroded through the arachnoid. Early retraction of the brain away from the dura can precipitate early intraoperative rupture in these cases. The dura is opened in a U– or C-shaped fashion and retracted toward the musculocutaneous flap. Extradural bleeding can be controlled with standard tack-up stitches.

9.6.3 Operative Approach

The operation proceeds under the operating microscope (Video 9.1). There are three main approaches to MCA aneurysms. The transcortical approach is through the superior temporal gyrus. The two transsylvian approaches are opening of the sylvian fissure from medial to lateral or from lateral to medial. The author almost exclusively uses a lateral to medial transsylvian approach. The main disadvantage of this approach is that the aneurysm may be encountered before proximal control is obtained.

The superior temporal gyrus approach is usually for cases where there is a large intracerebral hematoma. In these cases, we make a cortical incision in the anterior part of this gyrus, enter the clot cavity, and suction out some of the clot. The aneurysm is anterior so we suction out enough clot away from the aneurysm to achieve brain relaxation and then return to open the sylvian fissure. The rest of the clot is removed after the aneurysm is clipped, either through the cortical incision or via the pathway where the aneurysm ruptured through into the brain. The medial to lateral transsylvian approach involves elevation of the frontal lobe, identification of the optic nerve, entry into the chiasmatic and carotid cisterns, and dissection of the sylvian fissure from proximal to distal. The advantage is early proximal control, but it requires greater retraction of both frontal and temporal lobes. This is probably preferable to the lateral to medial approach other than for unruptured aneurysms or maybe until one gains confidence in microdissection.

For the lateral to medial approach, the arachnoid over the fissure is opened sharply with an arachnoid knife starting 2 to 3 cm posterior to the sphenoid ridge ( ▶ Fig. 9.5 and ▶ Fig. 9.6). The sylvian veins tend to run along the surface of the temporal lobe, so dissection should begin on the frontal side of these veins. As one proceeds deeper, retraction of the edges of the fissure with a patty and suction tip allows sharp dissection of bridging fibers. Self-retaining tapered retractors can be used. Cortical veins that bridge the fissure can be coagulated and divided. Once an M3 or M2 branch is identified, it can be followed proximally to the MCA bifurcation. Subarachnoid hematoma can be suctioned away. The arachnoid can also be cut with microscissors. Once the MCA bifurcation is identified, the next step depends on the location of the aneurysm. If the aneurysm is at the MCA bifurcation, then it will have been seen and the next step is to get proximal control by exposing the Ml. This is done by leaving the aneurysm dome undisturbed as much as possible and dissecting near the base of the aneurysm to expose the proximal artery. If the aneurysm is along the M1 segment, further dissection is necessary and is best along the anterior and inferior sides of the Ml.

Fig. 9.5 (a) A cranial computed tomography scan of a ruptured middle cerebral artery aneurysm showing subarachnoid hemorrhage (SAH) and a hematoma in the temporal lobe. The patient was taken to surgery immediately and the aneurysm exposed and clipped, as shown by intraoperative angiography unsubtracted (b) and subtracted (c) views performed by retrograde injection of the superficial temporal artery. (d) Initial exposure of the sylvian fissure using a lateral to medial approach shows the SAH upon opening of the arachnoid over the lateral part of the fissure. (e) The clot is suctioned away and a distal M2 branch is identified. (f) This is followed proximally to the aneurysm (asterisk), with the course of the middle cerebral artery shown by the curved dark lines. (g) The aneurysm is clipped with a clip that curves along the neck of the aneurysm. The collapsed sac of the aneurysm is seen distal to the clip (asterisk).