25 Skull Base Approaches in Cerebrovascular Surgery

Skull base approaches are commonly used in the treatment of cerebrovascular pathologies, such as intracranial aneurysms, arteriovenous malformations (AVMs), dural arteriovenous fistulas (dAVFs), and brainstem cavernous malformations (BCMs). Aneurysms that are most amenable for a skull base approach are aneurysms of the anterior circulation, (vertebro-)basilar artery aneurysms, and posterior circulation aneurysms.1,2 In many cases, anterior communicating artery aneurysms or complex middle cerebral artery aneurysms can be treated best with a skull base approach by minimizing brain retraction and optimizing exposure and maneuverability.^3–6

Due to the central location in the skull, BCMs are mostly treated by skull base approaches to ensure adequate exposure of the brainstem.

Anatomy

Review the cerebrovascular anatomy in Chapter 2, Fig. 2.20, page 65, as well as the classification systems of the segments of the internal carotid artery (ICA) and vertebral artery (VA) (Fig. 2.21, page 67).

Investigations and Imaging

• Digital subtraction angiography (DSA) and computed tomography angiography (CTA) are used to assess the angioarchitecture and projection of the aneurysm.

• Plain computed tomography (CT) with bone setting is used to delineate the anatomy of the skull base.

• Magnetic resonance imaging (MRI) is mandatory for BCMs to determine the optimal approach.

• Additionally, diffusion tensor imaging (DTI) can be helpful in determining the safest way to access the brainstem.⁷

Table 25.1 Skull Base Approaches to Cerebrovascular Pathology8–10

Abbreviations: AComA, anterior communicating artery; AICA, anterior inferior cerebellar artery; BA, basilar artery; BCM, brainstem cavernous malformations; ICA, internal carotid artery; PCA, posterior cerebral artery; PICA, posterior inferior cerebellar artery; SCA, superior cerebellar artery; VB, vertebrobasilar.

Treatment

Table 25.1 summarizes the potential skull base approaches for intracranial aneurysms and brainstem cavernous malformations.

Revascularization Techniques in Skull Base Surgery

Revascularization of intracranial arteries is sometimes necessary when the internal carotid artery (ICA) must be sacrificed. Reasons for occluding the ICA are the presence of an intracranial saccular, dissecting or blister aneurysms that cannot be secured by straightforward clipping or endovascular treatment, and skull base tumors that invade the arterial wall.^10–12

• Cerebral revascularization surgery of the ICA is rarely indicated.

• A very selective approach with assessment of cerebrovascular reserve is advised to achieve the best outcome.

• Revascularization techniques include direct reconstruction, interposition graft or extracranial-intracranial (EC-IC) bypass.

• The most commonly used grafts are the saphenous vein and radial artery.

Indications

Most aneurysms of the ICA are treated by endovascular methods, such as coiling, balloon occlusion, or placement of a flow-diverting stent. If this treatment fails or is not feasible, or if the aneurysm cannot be clipped, intracranial revascularization surgery is indicated.

• The decision to occlude or resect the ICA for skull base tumors is controversial. In the era before the advent of stereotactic radiosurgery (SRS), it was performed in about 3% of cases.13 Nowadays, remnants of skull base tumors can be treated by SRS, making complete tumor resection in most cases unnecessary. Therefore, revascularization surgery in skull base tumors is only indicated in very selective cases.¹⁴

Etiology/Pathophysiology

Sacrificing the ICA in skull base tumors is considered after evaluation of the following four aspects of the tumor: pathology, extent of infiltration, location,¹² and cerebrovascular reserve. The pathological diagnosis and the clinical behavior of the tumor are important factors to consider. Malignant tumors of the skull base where long-term control can only be achieved after complete surgical eradication, or “benign” skull base tumors with recurrent growth despite several treatment attempts may be indications to sacrifice the ICA.^12,15 The invasion of the vessel wall by the tumor, in contrast to encasement of the vessel, makes complete tumor resection impossible. In such cases, when complete surgical eradication is indicated and no additional therapy is possible, the ICA needs to be resected as well to achieve a complete tumor resection.

• A more rostral location of the tumor makes resection of the ICA more dangerous, due to the increased difficulty in approaching the artery and the presence of important branches and perforators.¹²

Clinical Presentation

Patients present with neurologic deficits related to the skull base/brain involvement, or intracranial (mostly subarachnoid) hemorrhage due to a ruptured aneurysm. Rarely, a patient can develop severe epistaxis due to rupture of a pseudoaneurysm of the ICA as a result of tumor invasion or radiation necrosis.¹⁶

Investigations and Imaging

The first step is to image the intracranial vessels and identify their relation with the surrounding structures to assess whether ICA sacrifice is necessary. Perform MRI/magnetic resonance angiography (MRA) and DSA. DSA is helpful in identifying the optimal site for arterial occlusion and bypass, examining the collateral circulation, and determining the size of the intended recipient and donor vessels. Additionally, a balloon test occlusion (BTO) can be helpful in assessing the stroke risk, although the 4 to 7% risk of (transient) complications and the 15% false-negative rate should be taken into account (see also page 301).^17,18 Assessment of cerebral vascular reserve during temporary ICA occlusion, which can be investigated by different studies (Table 25.2), might be a good adjunctive to determine whether the patient can tolerate ICA sacrifice. When a bypass procedure is planned, a preoperative DSA of the extracranial arteries, a preoperative Doppler investigation of the saphenous vein, and the (modified) Allen’s test to check the patency of the ulnar artery can be useful.

Table 25.2 Imaging Methods that Can Investigate Cerebral Blood Flow and Cerebral Vascular Reserve19

Treatment

There are different types of revascularization procedures, such as direct reconstruction by direct suture or patch (type 1), interposition grafting (type 2), EC-IC bypass (type 3), and direct intracranial revascularization (type 4).²⁰ The most common procedure in skull base aneurysms and tumors is the EC-IC bypass. There are different types of EC-IC bypass (Table 25.3); each has benefits and drawbacks.

• The procedure of the EC-IC bypass using a graft, which is most commonly used for bypasses to the ICA, is discussed in detail below.

Preoperative

• When choosing the type of graft, consider the following factors: A saphenous vein is larger in diameter and can therefore provide a higher blood flow to the brain in comparison with the radial artery graft. On the other hand, the saphenous vein has lower long-term patency rates, a higher risk of kinking, and may cause a caliber mismatch with small intracranial arteries (< 2 mm). The drawbacks of the radial artery are its shorter length and its tendency to become spastic.¹¹

• Patients are given acetylsalicylic acid preoperatively to reduce the risk of graft thrombosis.

Table 25.3 Types of Revascularization Procedures

*See text.

• General12:

Normocapnea should be maintained: a partial pressure of carbon dioxide (PCO₂) of 35 to 40 mmHg.

Mannitol is administered if necessary for brain swelling.

Patients are allowed to become mildly hypothermic (34–36°C)

Electroencephalograph (EEG) monitoring is optional.

Deep sedation/burst suppression and elevation of the blood pressure by 20% from baseline is induced just before temporary occlusion of the intracranial artery.

• Surgical^10,11,15:

The necessary surgical sites are prepared: head, neck, lower arm or leg.

The head is positioned in a radiolucent head holder.

Graft harvesting is done following general surgical principles. A plastic surgeon (for radial artery harvest) or cardiothoracic surgeon (for saphenous vein harvest) can assist.

The graft is marked proximally and distally, as well as on the superficial wall to identify whether the graft rotated during tunneling.

The graft is flushed and distended with heparinized saline (1,000 heparin units in 500 mL saline), and periadventitial tissue is sufficiently denuded at both ends for a length of 1 to 2 cm.

The donor vessel—common carotid artery (CCA), internal carotid artery (ICA), or external carotid artery (ECA)—is exposed in the neck following general surgical principles.

A craniotomy is performed. It is tailored to expose the recipient artery as well as the pathology that needs treatment.

The recipient artery is exposed and occluded just distal to the pathology with a permanent clip, and the area of the arteriotomy is trapped by placing a second, temporary, clip.

The graft is cut in a “fish-mouth” shape to optimize the diameter of the anastomosis.

An arteriotomy of the recipient vessel is performed and flushed with heparinized saline.

A end-to-side bypass is performed (with running or interrupted sutures) using 8-0 monofilament plastic sutures for an ICA anastomosis and 9-0 monofilament plastic sutures for middle cerebral artery (MCA) anastomosis

After checking the patency of the bypass, a temporary clip is placed on the graft, close to the anastomosis, and the graft is flushed with heparinized saline.

The temporary clip on the recipient artery is removed.

With use of a chest tube, and with attention paid so as not to kink the vessel, the graft is tunneled in a preauricular-subzygomatic or retroauricular direction toward the cervical incision, unless the IMAX is used for the proximal anastomosis.

The donor vessel is temporary clamped and an 8- to 10-mm arteriotomy is performed.

Related posts:

Hi-Tech Tools in Skull Base Surgery Pediatric Skull Base Skull Base Imaging Skull Base Infections Skull Base Reconstruction Techniques Transcranial Approaches to the Skull Base

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