Vascular Considerations in Neurotologic Surgery

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Chapter 66 Vascular Considerations in Neurotologic Surgery

Robert F. Spetzler, Shaun C. Desai, Vivek R. Deshmukh, Shervin R. Dashti

The foundation for management of vascular lesions of the petrous bone and cerebellopontine angle (CPA) relies on an understanding of the relevant skull base anatomy and vascular anatomy of this region. Vascular lesions are quite varied and include carotid dissections; carotid and vertebrobasilar aneurysms; arteriovenous fistulas; cavernous malformations; and hypervascular tumors including glomus tumors, hemangiomas, and hemangioblastomas, among others. This chapter presents a concise and clinically relevant review of the anatomy and the diagnostic and therapeutic strategies for these complex lesions.

VASCULAR ANATOMY OF THE PETROUS BONE AND CEREBELLOPONTINE ANGLE

Internal Carotid Artery

The petrous segment of the internal carotid artery (ICA) begins as the vessel enters the periosteal-lined carotid canal, whereas the cavernous segment originates as it exits the apex of the petrous portion of the temporal bone and traverses the posterior part of the cavernous sinus. The petrous portion of the ICA, which lies immediately posterior to the bony eustachian tube but anterior to the jugular foramen, can be divided into vertical and horizontal segments that connect at the genu. The ICA is lined by cervical sympathetic ganglia throughout its course in the petrous canal, and aneurysms or lesions of the vessel that compress the nerves can cause a classic Horner’s syndrome.¹ The ICA is also lined by a venous plexus in the distal part of the canal. The greater superficial petrosal nerve, which originates from the geniculate ganglion of the facial nerve, usually courses above and parallel to the horizontal portion of the petrous ICA. Special caution should be used when exposing the middle fossa floor and the petrous ICA because excessive traction of this nerve can cause postoperative facial nerve palsy.² Similarly, the trigeminal ganglion lies over the medial portion of the petrous ICA, but it is usually separated from the vessel by dura or a variable layer of bone.

The petrous ICA has four main branches of importance to the neurotologic surgeon: (1) caroticotympanic artery, (2) stapedial artery, (3) artery of the pterygoid canal (vidian), and (4) periosteal artery.^1,³ The caroticotympanic artery is often said to be a remnant of the embryonic hyoid artery, and arises from the proximal vertical part of the petrous ICA laterally and supplies the tympanic cavity. It anastomoses most commonly with the inferior tympanic artery. The stapedial artery is the source of the middle meningeal artery during development, but it rarely persists into adulthood. If present, a persistent stapedial artery passes through the floor of the middle ear and runs superiorly in a bony canal, in the obturator foramen of the stapes, and finally through the fallopian canal into the middle cranial fossa.³ Finally, the artery of the pterygoid canal (vidian) and the periosteal artery are often thought of as small collateral routes most likely branching off from the horizontal segment of the petrous portion or from the internal maxillary artery, and may have relevance in interventional embolization procedures as described later on.¹

The intracavernous segment of the ICA begins at the level of the petrolingual ligament near the foramen lacerum, and most consistently gives off two branches known as the dorsal (meningohypophyseal trunk) and lateral (artery of the inferior cavernous sinus) main stem arteries.⁴ The intracavernous ICA is commonly divided into five segments for anatomic orientation, including a posterior vertical, posterior bend, horizontal, anterior bend, and anterior vertical segments.¹

The dorsal main stem artery generally branches from the central one third of the convex outer margin of the posterior bend of the ICA. There is great variability in the origin of the branches of the dorsal main stem, but most investigators agree that three vessels—the tentorial artery (artery of Bernasconi-Cassinari), dorsal meningeal artery, and inferior hypophyseal artery—exist in some form.³ The tentorial arteries travel along the tentorium and may contribute blood supply for tentorial or proximal falcine meningiomas and tentorial dural arteriovenous fistulas, and blood supply to portions of CN III and IV.^3,⁵ The inferior hypophyseal artery supplies the periphery of the anterior pituitary gland and forms a “circulus arteriosus” with the dorsal meningeal arteries around the root of the dorsum sella. The distal branches of the dorsal meningeal arteries may enter the internal auditory meatus as described in a few patients, and may represent a remnant of the fetal trigeminal artery.³

The second main branch, the artery of the inferior cavernous sinus, generally arises from the central one third of the horizontal segment of the cavernous ICA. Although there is some variability, most studies agree that this vessel supplies various nervous components of the cavernous sinus.

Although the dorsal and lateral main stem vessels are the most common branches given off by the intracavernous segment of the ICA, other arteries have been described. McConnell’s capsular artery, when present in about 8% of the population, originates from the horizontal segment of the intracavernous ICA, and supplies any combination of the inferior and peripheral aspect of the anterior lobe of the pituitary gland, the diaphragma sella, and the floor of the sella turcica.⁶ A persistent fetal trigeminal artery, although rare with an incidence of 0.06% to 0.6%, usually arises from the posterolateral or posteromedial aspect of the intracavernous ICA.³ It is most notable in the literature for producing unique clinical scenarios, including trigeminal neuralgia, oculomotor palsies, hyperprolactinemia, and an increased incidence of intracranial aneurysms.³ Finally, the ophthalmic artery and a superior hypophyseal artery have been shown to originate from the cavernous ICA in 1% to 7.5% and 16% of individuals, respectively.^3,⁷

External Carotid Artery

Proximal to its terminal bifurcation, the external carotid artery gives rise to an anterior and posterior group of vessels, the latter of which consists of three arteries that relate to the petrous portion of the temporal bone. The ascending pharyngeal artery usually originates at or near the main bifurcation of the ICA and external carotid artery, and supplies the meninges around the jugular foramen as it passes through the foramen lacerum. As it travels upward with the carotid arteries, it gives off the inferior tympanic artery, which travels through the tympanic cavity with Jacobson’s nerve (inferior tympanic) via the tympanic canaliculus.² The occipital artery arises from the posterior surface of the external carotid artery and traverses upward between the posterior belly of the digastric muscle and the internal jugular vein, and then medial to the mastoid process. After passing the longissimus capitis muscle, the vessel courses deep to the splenius capitis muscle, finally terminating at the fascia between the attachment of the sternocleidomastoid and the trapezius muscles at the superior nuchal line.² Its branches supply several muscular and meningeal branches and often anastomose with branches of the external carotid and vertebral arteries.² Its most notable branch is the mastoid artery, which supplies the posterior portion of the mastoid bone and runs through the mastoid foramen. Finally, the posterior auricular artery arises above the posterior belly of the digastric and travels between the parotid gland and the styloid process. The artery divides into posterior auricular and occipital branches, and develops a stylomastoid branch, which supplies the structures of the stylomastoid foramen and the facial nerve.²

Although not technically a direct branch of the external carotid artery, but rather a branch of the internal maxillary artery, the middle meningeal artery has clinical relevance in the various neurotologic approaches. As this vessel courses through the foramen spinosum of the sphenoid bone between the two roots of the auriculotemporal nerve, it enters the middle cranial fossa, dividing into two or three main branches. When performing extradural approaches to the floor of the middle fossa, the foramen spinosum is located anteromedial to the geniculate ganglion and anterolateral to the carotid canal. These branches—the anterior branch (frontalis), posterior branch (parietal), and middle branch—supply most of the supratentorial dura and calvaria. On entering the cranium, it also gives off the superficial petrosal artery, which enters the hiatus of the facial canal and supplies the facial nerve. Manipulation of this branch during transpetrosal approaches can result in facial nerve injury.² Finally, the middle meningeal artery also gives rise to the superior tympanic artery just superior to the foramen spinosum, which runs with the superficial petrosal nerve through the superior tympanic canaliculus and supplies the canal of the tensor tympani muscle.

Vertebrobasilar Arteries

The vertebrobasilar system has three main branches that pass through the CPA, and any damage to these delicate branches can lead to various brainstem stroke syndromes. The major arterial vessels to consider include the posterior inferior cerebellar artery (PICA), the superior cerebellar artery (SCA), and, most relevant to the CPA, the anterior inferior cerebellar artery (AICA).

The AICA typically originates from the lower part of the basilar artery and divides into rostral and caudal trunks as it traverses the central part of the CPA. This main bifurcation often occurs proximal to the facial and vestibulocochlear nerves, and the two trunks formed are usually nerve-related.⁸ These branches can be divided into a premeatal segment, meatal segment, and subarcuate loop. In one study, a laterally convex curve or “loop” from the meatal segment was found to be medial of the internal acoustic meatus lying in the CPA in 33%, at the entrance of the meatus in 27%, and entering the internal auditory canal (IAC) in 40% of patients.⁸ A second laterally convex curve, known as a subarcuate loop, can also be seen in the subarcuate fossa and often gives the appearance of an M configuration.⁸ Although rare, in a few cases the lateral convex loop of the AICA can be embedded in the dura or bone covering the subarcuate fossa, or both.⁹ In such cases, freeing the artery from the dura and, if necessary, drilling the bone around the margin may aid in freeing this segment in anticipation for exposure of the posterior wall of the internal acoustic meatus.⁹

In the area of the CPA, the various branches of the AICA give rise to several nerve-related arterial branches from which originate the (1) labyrinthine (IAC), (2) recurrent perforating, (3) subarcuate, and, less commonly, (4) cerebellosubarcuate arteries. The labyrinthine arteries follow the vestibulocochlear nerve into the IAC, supplying the nerves, dura, and bone of the canal, and eventually terminate by giving rise to the anterior vestibular and common cochlear arteries, which supply the inner ear. Although the labyrinthine arteries most commonly arise from the premeatal segment of the AICA, some studies have reported anomalous cases where it originated directly from the basilar artery or from the PICA, or recurrent perforating, subarcuate, and cerebellosubarcuate arteries; however, this difference may be partially explained by the exact anatomic definition of the arteries in the different studies.⁸ The recurrent perforating arteries often course near the meatus and along the facial and vestibulocochlear nerves before supplying the brainstem. Finally, the subarcuate artery usually travels medial to the meatus, traversing the subarcuate fossa and petromastoid canal, sending branches to the petrous apex, the bone of the semicircular canals, and the posterosuperior portion of the vestibule.

The PICA, by definition, originates from the vertebral arteries and usually bifurcates into a medial and lateral trunk at the level of the telovelotonsillar fissure; however, its branches are extremely variable. Because it is located inferiorly, it passes very close to the roots of the lower cranial nerves. The PICA can arise from outside the dura, and at any point from the intradural course of the vertebral artery.

The SCA originates most commonly from the basilar artery below, but adjacent to the origin of the posterior cerebral artery. The SCA generally bifurcates into rostral and caudal trunks, and branches from either make up the cortical arteries, which supply the upper two thirds of the petrosal surface, including both lips of the petrosal fissure.⁸ The first branch of the cortical arteries, when present in about half of the population, is referred to as a marginal branch, and its surface area supplied is inversely related to the supply of the AICA. Several anastomoses exist between the marginal artery and the AICA.

Dural Venous Sinuses

One must have an intimate knowledge of venous anatomy in neurotologic surgery to decrease the risk of complications from venous congestion. The petrosal vein (Dandy’s vein) parallels the trigeminal nerve just beneath the tentorium and drains into the superior petrosal sinus. The superior petrosal sinus, which lies at the attachment of the tentorium cerebelli to the superior margin of the petrous ridge, connects the cavernous sinus to the lateral extent of the sigmoid sinus. In addition to draining the petrosal veins that drain the cerebellum and brainstem, it also receives vessels from the inferior surface of the temporal lobe and the cavernous sinus. The transverse sinus begins as a confluence of sinuses at the level of the internal occipital protuberance between the tentorium cerebelli and occipitalis bone and runs laterally to join the sigmoid sinus. The anastomotic vein of Labbé, which drains the temporoparietal region, bridges the inferior surface of the temporal lobe to the transverse sinus. This vein is of special importance in petrosal approaches to the skull base because its injury can lead to speech disturbance and contralateral hemiparesis or hemiplegia.² This vein is particularly at risk during subtemporal exposures. Excessive retraction of the temporal lobe can result in avulsion of the vein from its insertion into the relatively nonmobile transverse-sigmoid junction.

The sigmoid sinus not only drains the superior petrosal sinus, but also receives tributaries from the brainstem, cerebellum, and occipitalis, and vertebral emissary veins. The angle formed between the superior petrosal and sigmoid sinuses and the middle fossa dura represents the sinodural angle, which is an important anatomic landmark. The sinus curves medially and forward, crossing the occipital bone, and finally through the jugular foramen where it enters the superior bulb of the internal jugular vein. Finally, the inferior petrosal sinus connects the cavernous sinus to the medial wall of the bulb of the internal jugular vein as it traverses the petro-occipital fissure and drains the clival area.

At its entrance to the jugular foramen, the jugular vein is separated from the inferior petrosal sinus by CN IX, X, and XI. This foramen is located at the lower end of the petro-occipital fissure, and is divided into a larger lateral opening that drains the sigmoid sinus and into a small medial petrosal part that receives the inferior petrosal sinus.² The jugular bulb is inferior to the posterior floor of the middle ear cavity and generally lies inferior to the ampulla of the posterior semicircular canal; however, its location within the tympanic cavity is highly variable. Often, a high-riding jugular bulb can extend superiorly as high as the lateral semicircular canal and can interfere with visualization in Trautmann’s triangle in transpetrosal approaches as discussed in the next section.² Finally, the internal jugular vein runs inferiorly from the bulb and is located posterolaterally to the ICA.

VASCULAR CONSIDERATIONS IN NEUROTOLOGIC APPROACHES

Transpetrosal Approaches

The transpetrosal approach, also known as the transtemporal approach, consists of three craniotomy techniques that open the posterior fossa dura anterior to the sigmoid sinus, through the posterior aspect of the petrous pyramid (Fig. 66-1). This set of approaches requires removal of 1 to 2 cm of retrosigmoid bone to allow for posterior displacement of the sigmoid sinus. A detailed explanation of the surgical technicalities and indications and exposure to the various approaches is provided elsewhere.

FIGURE 66-1 Surgical corridor afforded by the various transpetrosal approaches.

(Courtesy of Barrow Neurological Institute.)

Retrolabyrinthine

In this approach, bone is removed up to the bony capsule of the semicircular canals, which provides a limited exposure to the midportion of the CPA. Although this approach was first described as a root entry zone in tic douloureux, it has more recently been used for much less common problems, including vascular decompression of CN VIII for tinnitus and chronic vertigo, and for approaching midbasilar artery aneurysms. More commonly, this approach is used in combination with others, including a subtemporal exposure for transtentorial lesions abutting the brainstem.

After the initial incision is made behind the postauricular sulcus, and the mastoid air system is thoroughly exenterated, the posterior limit for drilling is the sigmoid sinus. Using a large diamond burr, the sigmoid sinus must be skeletonized carefully. Some authors have suggested that a thin bone plate be left intact over the anterior aspect of the sinus to protect the vessel from the rotating shaft of the drill as it goes deeper into the temporal bone.² If this bone plate begins to restrict sigmoid retraction, however, one can always fragment it to facilitate posterior mobilization. Other surgeons believe there is no need for preservation of this bony island, and they remove it and guard the vessel with a retractor. Either way, further exposure must be attained by retracting the sinus superiorly from the jugular bulb. In selected patients, the sigmoid sinus can be ligated to improve exposure; however, this can occur only when angiography shows a patent communication between the two transverse sinuses. Mastoid emissary veins are usually encountered and can be controlled with bipolar cautery; similarly, any lacerations to the sinus can readily be controlled with extraluminal tamponade with hemostatic gauze. Intraluminal packing of the sigmoid sinus is not recommended because there is always a risk for embolization of the packing material.

Translabyrinthine

In the translabyrinthine approach, the semicircular canals and vestibule are completely removed, which allows for excellent access to the IAC and exposure of the CPA. The main indication for this approach is for acoustic neuromas; it also is indicated for meningiomas, epidermoids, and other tumors of the CPA when hearing has been compromised. This approach has also been described for aneurysms of the midportion of the basilar artery.

In a translabyrinthine approach, the same precautions must be taken for the sigmoid sinus as discussed earlier for the retrolabyrinthine approach. Sigmoid mobilization must be achieved, however, to have adequate access between the sinus and the external auditory canal. Although there is some anatomic variation to the sigmoid sinus, full exposure can usually be achieved after decompression from the transverse-sigmoid junction superiorly to the jugular bulb inferiorly. A high anterior sigmoid course or a large sinus may inhibit access to the inferior CPA; however, this should not be a problem with acoustic neuroma removal because this neoplasm becomes more accessible in the operative field as it is debulked. Finally, when drilling the mastoid bone toward the semicircular canals, the superior petrosal sinus should be exposed at the sinodural angle.

During the labyrinthectomy portion of the procedure, as the lateral and superior semicircular canals are drilled away, bleeding may occur. This is most likely a result of the subarcuate artery because it courses in the bone near the center of the superior semicircular canal. The boundaries of Trautmann’s triangle, the patch of posterior fossa dura in front of the sigmoid sinus, are also important. One limb of the V-shaped incision extends below to the superior petrosal sinus, and the other limb extends above the jugular bulb. The jugular bulb sometimes can lie atypically high behind the posterior semicircular canal and impede the creation of the inferior bony trough, interfering with access to the IAC. This variation can increase the chance of unintentional bleeding and create a potential source for air emboli. If this occurs, better exposure of the superior aspect of the IAC can be achieved by elevating the temporal dura and extending the superior trough to the level of the tentorium. Decompression and inferior displacement of the jugular bulb can always be done, however, if the former option fails to reveal exposure.

As the dural flap is reflected posteriorly to expose the meatal structures and the CPA, the subarcuate artery or AICA may be encountered. As noted earlier, the subarcuate artery usually stems from the AICA and passes through the dura on the upper posterior wall of the meatus. This vascular anatomy tends to be variable, however, and in some patients the subarcuate artery, along with its origin from the AICA, may be incorporated into the dura on the posterior face of the temporal bone.⁹

Transcochlear

The transcochlear approach is mainly an anteromedial extension of the translabyrinthine approach; the entire inner ear is removed, and the facial nerve is posteriorly rerouted. The surgical extension allows for access to the midclival region, prepontine cistern, and anterior aspect of the CPA. This approach has lost favor in recent years because of substantial morbidity (deafness, facial nerve palsy), although is still indicated in cases of preexisting facial palsy and for life-threatening vascular lesions, such as midbasilar artery aneurysms.

The beginning steps of the transcochlear approach represent the same vascular considerations as in the translabyrinthine approach. Similarly, the boundaries in this approach have several important vascular relationships. Medially, the drilling extends to the edge of the clivus, exposing the inferior petrosal sinus from the jugular bulb below to the superior petrosal sinus above. Anteriorly, the ascending portion of the petrous carotid is exposed. After extending dissection medially into the clivus and opening the dura, the initial segment of the AICA and its origin from the basilar artery can be achieved.

Middle Fossa Approaches

The middle fossa approaches are a group of procedures that give complete exposure to the IAC and a limited view of the CPA. Several variations exist to this set of approaches, which vary in the exposure of the IAC and CPA. Only Kawase’s approach is discussed here, however.

Kawase’s Approach

The middle fossa–transpetrous apex approach, or Kawase’s approach, has mainly been used for lesions that traverse the middle fossa and a small part of the posterior fossa. These include dumbbell-shaped tumors such as petroclival meningiomas and trigeminal schwannomas, and lesions in the petrous apex. This approach can also be used to access aneurysms of the mid-basilar artery and lower basilar artery.

Kawase’s approach involves a small posterior fossa craniotomy, after a middle fossa opening, by removing the medial portion of the petrous pyramid and the lateral aspect of the clivus. The ultimate goal is exposure of the anterior CPA and the ventral surface of the pons. As the dura is carefully elevated from the floor of the middle fossa, special attention should be paid to exposing the middle meningeal artery. If necessary, this artery can be sacrificed and divided at the foramen spinosum to avoid excessive bleeding. The boundaries of Kawase’s approach are the IAC posteriorly, petrous carotid artery anteriorly, inferior petrosal sinus inferiorly, and abducens nerve medially.

The boundaries of the rhomboid-shaped bony window in the petrous pyramid have several important vascular relationships. The most important is the inferior border of the petrous carotid artery. Often, this vessel can limit posterior fossa exposure. Some authors have advocated that displacement of the intrapetrous carotid artery from its bony canal can be achieved; however, doing so is controversial and probably not justified because of its risk for significant morbidity. More commonly, if more exposure is needed, the horizontal segment of the petrous carotid artery can be skeletonized, leaving a surrounding thin eggshell of bone intact allowing for slightly more mobilization and visualization. A 7 mm length segment of the petrous carotid artery may be exposed without a covering of bone in the floor of the middle fossa deep to the greater petrosal nerve.

Exposure of the Petrous Carotid

The petrous ICA may require exposure in particular for revascularization procedures. The petrous ICA is typically located in Glasscock’s triangle, and can be exposed by drilling bone posteromedial to foramen ovale and anterior to the arcuate eminence. The greater superficial petrosal nerve runs superficial to and parallel to the petrous ICA, and can be sacrificed if necessary with resultant disturbance in lacrimation. As stated earlier, excessive traction on the greater superficial petrosal nerve can cause facial nerve palsy.

VASCULAR LESIONS OF THE PETROUS BONE AND CEREBELLOPONTINE ANGLE

Cerebral Aneurysms

Petrous Internal Carotid Artery Aneurysms

Presentation and Pathophysiology

Aneurysms of the petrous ICA are rare, although their true incidence is unknown. They are often discovered incidentally on computed tomography (CT) scans for unrelated purposes. They are usually asymptomatic; however, patients can present to an otolaryngologist with a wide variety of signs and symptoms, including cranial nerve palsies, Horner syndrome, epistaxis, vertigo, pulsatile tinnitus, and otorrhagia.¹ The triad of otorrhagia, epistaxis, and neurologic deficit is almost pathognomonic for a petrous ICA aneurysm.¹⁰ On otoscopic examination, patients with aural symptoms can present with a vascular retrotympanic mass, which is often mistaken for a glomus tympanicum tumor. Biopsy of such a lesion can lead to severe hemorrhage and can be catastrophic. Spontaneous rupture can cause dramatic hemorrhage into the eustachian tube or middle ear or both, and manifest as massive epistaxis or otorrhagia or both with the risk of death from exsanguination.

These lesions can be classified further as true aneurysms or pseudoaneurysms, which can alter treatment. True aneurysms have walls that are continuous with the unaffected portion of the parent vessel and can develop from a congenitally weakened wall, whereas pseudoaneurysms solely involve the adventitial layer of the vessel and develop when a thrombus and fibrous tissue capsule form in response to a traumatic injury. The pathophysiology of petrous ICA aneurysms is unclear, but traumatic, mycotic (by direct seeding from the middle ear and eustachian tube), and congenital causes have been implicated.¹¹ Trauma is a significant cause of petrous ICA pseudoaneurysms in particular because of the predisposing anatomic arrangement, with a stationary ICA segment connected to a distal mobile cervical segment, predisposing the vessel to stretch forces from forceful trauma. Iatrogenic trauma, especially from temporal bone surgery and myringotomy, has been a well-documented source of petrous ICA aneurysms.¹

Management and Treatment

After the diagnosis is confirmed by cerebral angiography, the question of management arises. Management of petrous ICA aneurysms remains undefined, with no established treatment paradigm in place. Most authors agree, however, that patients with ruptured petrous ICA aneurysms should undergo urgent treatment to stop active bleeding or to prevent future hemorrhage. Similarly, patients with unruptured petrous ICA aneurysms who experience major chronic symptoms from cranial nerve involvement should undergo intervention. A third group of patients, with incidental findings on CT and who are asymptomatic or experience minimal and minor symptoms, raise the question of whether intervention is necessary. Most surgeons agree that the treatment of asymptomatic patients should be approached on a case-by-case basis. It is reasonable to recommend conservative management, including close observation with serial angiography (magnetic resonance or CT angiography may suffice). Change in lesion size or morphology should prompt further consideration for treatment. Management of petrous ICA dissections should primarily be based on the degree of resultant stenosis of the true lumen. For non–flow-limiting dissections, antiplatelet therapy usually suffices. For dissections with hemodynamically or clinically significant stenoses, stent placement for artery repair should be considered.

Various interventional options exist for patients with petrous ICA aneurysms, including endovascular and surgical therapies. Endovascular techniques consisting of coil embolization or stent coiling should be considered the first line of therapy in symptomatic patients or in patients with ruptured aneurysms. For aneurysms that are not amenable to reconstructive procedures, parent vessel sacrifice (deconstructive procedures) should be considered. A balloon occlusion test with hypotensive challenge is usually performed to evaluate for adequate collateral flow and to assess for any neurologic symptoms or signs of insufficient blood supply. This test produces a high false-negative rate, however, and 2% to 22% of patients with negative findings have been found to have early or delayed ipsilateral cerebral ischemia after permanent occlusion.¹ Also, a false-positive result may be obtained if the procedure is complicated by thromboemboli.

If the transient occlusion produces no new neurologic deficits, and if adequate collateral flow exists, occlusion of the ICA can be considered. Detachable balloons were frequently used in the past. More recently, coil occlusion has become the predominant modality for parent vessel sacrifice. Although coils have the advantage of providing a rapid and permanent arterial occlusion, they also have an increased risk of clot formation and distal embolization compared with immediate vessel occlusion with balloon embolization.¹² Some reports have shown formation or growth of cerebral aneurysms after balloon or coil embolization.¹

Numerous case reports have documented the successful use of stents to treat ruptured petrous ICA aneurysms. Cheng and colleagues ¹² and Auyeung and associates¹³ described the successful use of covered stents in three hemodynamically unstable patients who had massive epistaxis as a result of rupture of petrous ICA pseudoaneurysms that were radiation induced. A covered stent may be placed in the petrous ICA because of its lack of major arterial branches, and can be considered for pseudoaneurysms because of their lack of surrounding support. Cheng and colleagues¹² concluded that porous stents can alter blood flow pattern, which readily results in stasis and thrombosis of the aneurysm; they cited numerous reports of the successful use of secondary coiling in case of stent failure. Drawbacks to stents include stent-induced intimal hyperplasia, which can cause a hemodynamically significant stenosis and thromboembolic events.¹⁴ Long-term studies are still ongoing to determine its overall efficacy and patency rates; however, initial studies show a promising role for this technology.

If a patient develops neurologic sequelae after a balloon occlusion test, revascularization should be considered through an extracranial-intracranial bypass. Similarly, a patient with no neurologic deficits on temporary occlusion who exhibits marked asymmetric decrease in hemispheric blood flow (<30 mL/100 g/min) should be a candidate for a revascularization procedure because of the high false-negative rate of the screening test.¹⁵ Some groups advocate universal revascularization in patients who undergo carotid artery sacrifice. The rationale for universal revascularization includes the high incidence of false-negative results from balloon test occlusion, the risk of delayed ischemic complications, and the risk for delayed intracranial aneurysm formation because of changes in intracranial flow and increased shear stress at points of bifurcation.

Surgical options should also be considered if endovascular techniques fail, and include cerebral revascularization procedures with high-flow or low-flow bypass. Umenzu and coworkers ¹⁶ described a 21-year-old man who had angiographic evidence of a petrous ICA. He failed balloon occlusion twice and eventually was successfully treated via a superficial temporal artery–to–middle cerebral artery bypass.¹⁶ Five patients with petrous ICA aneurysms were treated successfully by saphenous vein interposition graft through an extradural cervical carotid artery–to–petrous ICA bypass at another institution.¹⁷ Surgical techniques described include a modified infratemporal fossa approach and a combined subtemporal and preauricular infratemporal fossa approach.

Liu and colleagues ¹ describe a less invasive approach in which a submandibular cervical carotid artery–to–supraclinoid ICA interpositional saphenous vein graft precluded the need for direct exposure of the aneurysm or extensive drilling of the temporal bone. From our experience of eight patients with traumatic ICA dissections, we have had great success with saphenous vein ICA bypasses (cervical ICA–to–petrous ICA) with only one patient experiencing early postoperative graft occlusion that was amenable to immediate thrombectomy.¹⁸ Relative contraindications to direct surgical repair or bypass include poorly controlled active hemorrhage and lesion extending distal to the carotid canal, which would impede exposure.

High Cervical Internal Carotid Artery Aneurysms

Aneurysms of the high cervical ICA are rare and usually occur as a result of trauma, dissection, and atherosclerosis, and iatrogenically from carotid artery surgery.¹⁹ Most often, these vascular lesions reach clinical attention after causing symptomatic distal embolization, thrombosis, mass effect, or hemorrhage. Management of an aneurysm in this location is similar to management of aneurysms in the petrous ICA; a balloon occlusion test should be performed to assess for collateral circulation. If the test shows insufficient supply, a revascularization procedure is necessary. Several techniques have been proposed, including a sacrifice of the ICA followed by a standard bypass graft to the middle cerebral artery, exclusion of the aneurysmal ICA by an extra-anatomic cervicopetrous bypass, and a cervicosupraclinoid bypass.¹⁹ If the aneurysm is small, and if the ICA tortuosity permits, treatment with a stent (bare or covered) can be considered. Small cervical ICA aneurysms typically can be managed conservatively with the patient placed on aspirin therapy to prevent thromboembolic complications.

Vertebrobasilar Aneurysms

Numerous approaches have been described for vertebrobasilar aneurysms, and most often have depended on the precise location of the lesion in the vessel. Aneurysms of the upper third of the basilar trunk are considered more accessible via an orbitozygomatic or subtemporal approach, whereas aneurysms arising from the vertebral artery usually can be accessed via a retrocondylar far-lateral approach.^20,²¹ Surgical access to aneurysms of the lower basilar trunk and vertebrobasilar junction have been controversial, however, and various approaches have been attempted, including the subtemporal with drilling of Kawase’s triangle, suboccipital-retromastoid, far-lateral transcondylar, and transoral-transclival.²⁰ Exposure is often limited because these vascular lesions are deep within the surgical field and surrounded by vital structures, including brainstem and cranial nerves.

More recently, lateral approaches that traverse the petrous bone have gained favor as the approach of choice for such aneurysms. In a series of nine patients with aneurysms of the lower basilar trunk and vertebrobasilar junction, direct clipping via a transpetrosal approach was successfully performed in eight.²⁰ The operative technique consisted of a combined supratentorial/infratentorial craniotomy via burr holes bordering the transverse sinus that were connected using a high-speed drill. After craniotomy, a posterior petrosectomy is performed with careful attention to preserve the labyrinthine structures (particularly the posterior semicircular canal). The temporal dura is excised parallel to the transverse sinus, followed by removal of the presigmoid dura in Trautmann’s triangle to the level of the superior petrosal sinus. This sinus is ligated, the tentorium is transected, and the structures are retracted to expose the basilar artery to the level of the vertebrobasilar junction and to the vertebral artery. The origin of the aneurysm is dissected and clipped. A major advantage over this approach and other skull base approaches compared with standard craniotomies is minimal retraction of brain tissue.

Dural Arteriovenous Malformations

Dural arteriovenous malformations are extremely rare, constituting 10% to 15% of all intracranial arteriovenous malformations.⁵ This vascular anomaly is believed to derive from venous thrombosis of a dural sinus with consequent collateral revascularization that can lead to abnormal fistulous connection between arteries and veins. Dural arteriovenous malformations can be high-risk or low-risk lesions; this differentiation is based primarily on the presence of venous drainage into the cortical veins, which confers high risk for future hemorrhage. Most patients with high-risk cases present with intracranial hemorrhage or progressive neurologic deficit attributed to venous hypertension. Low-risk fistulas are frequently discovered incidentally or manifest with pulse-synchronous bruit.

Low-risk fistulas can generally be observed or treated with palliative embolization if the patient is intolerant of the bruit. High-risk fistulas should be treated because of the risk for future hemorrhage. Dural arteriovenous malformations are usually amenable to endovascular treatment because the arterial and venous sites of the fistula can usually be accessed. When surgery is necessary, endovascular exploration should still be attempted because it helps guide the surgical approach by simplifying the anatomy and allowing the surgeon to pinpoint the exact location of the fistula. Preoperative arterial embolization may also reduce intraoperative blood loss.

Inadequate access to the arterial or draining venous system or multiple sites of fistulization may necessitate surgical management. Endovascular and surgical techniques should complement each other in the management of this group of vascular malformations. For patients with high-risk dural arteriovenous fistulas, treatment with single-mode or multimodal therapy results in a high likelihood of achieving excellent long-term outcomes: 95% have attained Glasgow Outcome Scale scores of 4 or 5 in certain studies.⁵ The following sections discuss the surgical approaches to dural arteriovenous fistulas traversing the two most common locations for this lesion to develop: the transverse-sigmoid junction and the tentorium.

Transverse and Sigmoid Fistulas

The junction of the transverse and sigmoid sinuses is the most common location for dural arteriovenous fistulas. Lesions in this area usually manifest with headache and pulse synchronous bruit. Lesions without cortical venous drainage may be managed conservatively with palliative embolization considered for intractable bruit. Embolization techniques have increasingly become the treatment of choice for dural arteriovenous fistulas located at this sinus junction; however, surgical management may be necessary if transvenous access cannot be attained or in cases of multiples sites of fistulization along the sinus. High-risk lesions are defined by cortical venous drainage.

Two types of fistulas have been observed: fistulas with a single leptomeningeal vein along the tentorium in the supratentorial compartment and fistulas in which the entire sinus is involved. Patients with a single fistula focus should have the transverse sinus exposed at the origin of the retrograde draining vein in the region of the tentorium. A lumbar drain should be placed to relax the brain and help with exposure. The arterialized draining vein is identified and coagulated by cauterization or clip-ligation. In the second type of tentorial fistula, the surgeon should expose a significant portion of the sinus for direct vascular access. The sinus can be cannulated intraoperatively and obliterated with coils. Care must be taken, however, to avoid interrupting drainage from the normal venous circulation, particularly from the vein of Labbé. Alternatively, the transverse sinus can be skeletonized, and dural borders above, below, and medial to the sinus can be coagulated. Postoperative angiography should be performed in all cases to ensure adequate obliteration of the fistulas.

Tentorial Fistulas

Preoperative angiograms must be reviewed closely to identify the exact focus of the anomalous fistulous connection. The surgical approach depends on the location of the fistulous connection in the tentorium. If the fistula is situated in the anterior third of the tentorium, an orbitozygomatic approach is most likely the best option; a presigmoid transpetrosal approach is also a reasonable alternative.

After achieving exposure by opening the sylvian fissure or retracting the temporal lobe, the tentorium is followed posteriorly until venous drainage is identified most likely through the basal vein of Rosenthal. The vein is coagulated or clip-ligated at the point of dural attachment. Lesions of the middle third of the tentorium, with venous drainage from the petrosal vein or Dandy’s vein, generally require a retrosigmoid-suboccipital approach. Lumbar drainage is recommended to allow for cerebellar relaxation. After achieving cerebellar retraction, the wing of the tentorium is followed medially to the point of the fistula. If the network of normal draining veins complicates identification of the leptomeningeal drainage, microvascular Doppler ultrasonography showing retrograde venous flow or intraoperative angiography can be performed. Venous drainage is clip-ligated or coagulated.

Finally, a midline supracerebellar-infratentorial approach is recommended for dural arteriovenous malformations traversing the posterior aspect of the tentorium. Cauterization of occipital artery feeders and the meningeal supply during the approach significantly reduces blood flow to the fistula and facilitates surgical revision. Preoperative arterial embolization of external carotid artery branches decreases blood loss and can help mitigate eventual obliteration of the cortical draining veins. When the dura is opened, and the fistula is identified, the vein is cauterized or clip-ligated. Postoperative angiography is recommended to confirm obliteration of the lesion.

Cavernous Malformations and Cavernous Hemangiomas

Cavernous malformations and hemangiomas of the petrous bone are rare and often manifest with deficits of CN VII and VIII. Other, more life-threatening presentations are possible, however, including epidural hemorrhage as described by Kessler and associates in 1956.²² There are two subtypes of cavernous malformations, and they are histologically identical. Extra-axial cavernous hemangiomas typically affect the petrous bone, and intra-axial cavernous malformations within the posterior fossa are typically located within the brainstem or cerebellum. Cavernous hemangiomas are most commonly located at the fundus of the IAC or in the region of the geniculate ganglion, and can often spread to the CPA.²³ These lesions generally produce symptoms by compressing the surrounding cranial nerves causing resultant palsies. They have the capability of perineural invasion, however, which would necessitate resection of the nerve with a cable graft placement.²³

These vascular malformations can often resemble an acoustic neuroma in presentation clinically and radiographically, but they can be distinguished from a meningioma based on negative angiography.²⁴ Diagnosis is best made with thin-section, gadolinium-enhanced magnetic resonance imaging (MRI). They are often sharply defined and enhanced compared with cholesteatomas and schwannomas.²³ The presence of calcification within the lesion strongly suggests a hemangioma.

Sundaresan and colleagues ²⁴ described three patients with vascular lesions in the petrous bone, two of which were cavernous hemangiomas, and concluded that these lesions can most likely be distinguished from neuromas by their presentation. The authors observed that small IAC or CPA hemangiomas tend to have early-onset hearing loss associated with marked facial weakness or fasciculation or both. In contrast, neuromas in the same locations, which also manifest with hearing loss, seldom involve such a severe motor palsy of CN VII.²⁴

Madden and Sirimanna ²⁵ described a case of a cavernous hemangioma located in the IAC, with the same presentation. Because the patient had permanent sensorineural hearing loss, the group decided on a translabyrinthine approach with a facial nerve graft. The patient did not regain function after 1 year, however.²⁵ Rocco and coworkers²⁶ described a similar presentation of hearing loss and severe facial weakness (House-Brackmann grade IV) except that final pathology revealed a cavernous malformation in the IAC with hematoma formation into the CPA.²⁶

Posterior fossa cavernous malformations are rare and account for only a few cavernous malformations affecting the craniospinal axis. Most of these lesions are approached via standard posterior fossa craniotomies. Cavernous malformations with a ventral and deep location may require a transpetrosal approach, however, to minimize brain retraction during exposure. An anterior transpetrosal approach (Kawase’s approach) was used successfully for a symptomatic cavernous hemangioma that was located in the ventrolateral surface of the pons as described by Saito and colleagues.²⁷ MacDonald and associates²⁸ used an anterior, subtemporal, medial transpetrosal approach for a pontine cavernous hemangioma, which caused intraparenchymal hemorrhage resulting in a moderate right hemiparesis. Most of these lesions can be accessed by a retrosigmoid craniotomy with a generous unroofing of the sigmoid sinus and a few millimeters of the presigmoid bone to allow for retraction of the sinus (see Fig. 66-2).

FIGURE 66-2 A, Axial T2-weighted MR image of the brain shows large pontine cavernous malformation in a patient with progressive left hemiparesis. B, Stealth neuronavigation trajectory views during right retrosigmoid craniotomy. C, Postoperative axial T2-weighted MR image of the brain shows complete resection of cavernous malformation. The patient’s strength improved significantly on his left side, and he had House-Brackmann grade II right facial weakness and preserved hearing.

(Courtesy of Barrow Neurological Institute.)

VASCULAR TUMORS OF THE PETROUS BONE AND CEREBELLOPONTINE ANGLE

Role of Preoperative Embolization

Surgical excision of hypervascular central nervous system tumors in the temporal bone and CPA can be dangerous, with the potential for staggering blood loss. The need to mitigate intraoperative tumoral hemorrhage has fostered the development and refinement of various neuroendovascular techniques for preoperative embolization of tumors. This concept of interrupting blood supply to a tumor in anticipation for surgical excision has gained popularity for hypervascular tumors such as meningiomas, hemangiopericytomas, hemangioblastomas, paragangliomas, and juvenile nasopharyngeal angiofibromas.²⁹ In the following sections, the principles of embolization, choice of embolic agents, and potential complications associated specifically with glomus tumors, hemangiomas, and hemangioblastomas of the petrous bone and CPA are discussed.

Glomus Tumors (Paragangliomas)

Paragangliomas consist of a slow-growing group of benign neoplasms known as glomus tumors that arise from remnants of neural crest paraganglion cells. These tumors are of special interest to neurotologic and head and neck surgeons as they occur in the temporal bone (glomus tympanicum and glomus jugulare), the carotid bifurcation (glomus caroticum), and the upper parapharyngeal space (glomus vagale). Four percent of paragangliomas have documented catecholamine secretion, which can cause a pheochromocytoma-type syndrome.²⁹ If catecholamine secretion is suspected and documented, the patient must be pretreated with α and β blockade before embolization and surgical resection. Similarly, preoperative testing should include angiography to define intracranial and extracranial arterial supply to the tumor and involvement of any dural venous sinus. In such cases, the patency of both transverse-sigmoid systems must be evaluated to determine if the involved sinus can be sacrificed without causing intracranial venous congestion.

Temporal glomus tumors often obtain their blood supply from the petrous branches of the ICA, the most common of which is the vidian artery, and from the cavernous-carotid branches, the most common of which is the clival branch of the meningohypophyseal trunk. The most frequent neoplasm arising in the middle ear—glomus tympanicum—seldom requires preoperative embolization, and can usually be resected with conventional tympanoplasty techniques (Fig. 66-3).

FIGURE 66-3 A and B, Axial (A) and coronal (B) postcontrast MR images of the brain show right-sided glomus jugulare tumor encroaching on the cerebellopontine angle and underlying right cerebellar edema. C and D, Right internal (C) and external (D) carotid artery angiographic injections show prominent vascular blush at right skull base. The tumor was embolized with NBCA glue through ascending pharyngeal, posterior auricular, and occipital branches of right external carotid artery. E and F, Postembolization angiographic injections of right internal (E) and external (F) carotid arteries show significant flow reduction through glomus jugulare tumor. G, Postcontrast coronal MR image of the brain shows successful resection of tumor.

(Courtesy of Barrow Neurological Institute.)

Alternatively, preoperative embolization may help with removal of glomus jugulare lesions, particularly tumors with extension intracranially. Preoperative embolization of these tumors helps to reduce intraoperative blood loss (which facilitates meticulous neurovascular dissection at the skull base), increases the extent of tumor resection, and decreases the overall length of postembolization surgery. These tumors tend to have a multicompartmentalized arterial supply. For embolization to be beneficial, each compartment should be catheterized selectively and embolized. The inferomedial compartment is most often supplied by the ascending pharyngeal artery. The posterolateral compartment is supplied by the stylomastoid branch of the occipital artery or posterior auricular artery or both. The anterior compartment is supplied by the internal maxillary artery and caroticotympanic arteries of the petrous ICA. Finally, the superior compartment is supplied by various branches of the middle meningeal artery. Balloon test occlusion with preoperative sacrifice is an option for tumors with significant supply from the ICA.

Although complications are rare, they do occur. Inadvertent embolization of the vasa vasorum can lead to a palsy of the lower cranial nerves (CN IX-XII). Marangos and Schumacher ³⁰ and Herdman and colleagues³¹ documented two cases of facial palsy after embolization of a glomus jugulare tumor. One patient had recovered completely 1 year after treatment, and the other had improved significantly.^30,³¹ Pandya and coworkers³² described a herniation syndrome that followed direct puncture and absorbable gelatin sponge (Gelfoam) embolization of a glomus jugulare tumor. They theorized that significant tumor infarction exacerbated posterior fossa edema.³²

Carotid body tumors usually occur sporadically, whereas bilateral tumors occur in 5% to 10% of cases, with 2% to 6% of lesions showing malignant characteristics such as distant metastatic spread.²⁹ Surgical excision of these tumors is particularly challenging because of their hypervascular nature and propensity to involve cranial nerves (Fig. 66-4). Relevant literature on the use of preoperative embolization techniques for carotid body tumors is scarce, and mostly relegated to a few small single-institution reports.

FIGURE 66-4 A and B, Postcontrast axial (A) and coronal (B) short tau inversion recovery MR images show large left-sided carotid body tumor. C and D, Angiogram with left common carotid artery injection shows splaying of internal and external carotid arteries by tumor. There is also a large tumor blush. The tumor was embolized with Onyx-15 through multiple left external carotid artery branches. E and F, Repeat left common carotid artery injection shows near-complete occlusion of discernible tumor vessels. One day after embolization, the patient underwent dissection of the left neck with gross total resection of carotid body tumor.

(Courtesy of Barrow Neurological Institute.)

LaMuraglia and associates ³³ preoperatively embolized 11 patients with carotid body tumors supplied by the ascending pharyngeal artery, ascending cervical branches of the thyrocervical trunk, and vertebral arteries. They found that the technique significantly decreased intraoperative bleeding compared with nonembolized lesions. One patient had transient aphasia that resolved in 24 hours. The authors concluded that preoperative embolization should be performed for tumors larger than 3 cm, and should be followed by surgical resection within a few days. They also advocated for temporary balloon dilation occlusion with hypotensive challenge if the carotid artery is extensively involved because embolization might be difficult owing to the tumor’s location at the carotid bifurcation.³³ Feeders to carotid body tumors tend to have short pedicles, which makes stable microcatheter positioning for embolization challenging, and increases the risk for nontarget embolization.

Hemangioblastomas

Hemangioblastomas constitute 1.1% to 2% of craniospinal tumors, and most frequently occur within the cerebellar hemispheres and less commonly at the CPA, vermis, or brainstem.²⁹ These benign, highly vascularized tumors most commonly arise sporadically. Twenty percent are associated, however, with the autosomal dominant genetic disorder, von Hippel–Lindau disease. Because these neoplasms are hypervascular, they are exceedingly difficult to resect, particularly in the intricate areas from where they tend to develop. Severe intraoperative hemorrhage is a significant contributor to the morbidity and mortality rates associated with the resection of hemangioblastomas.

Based on angiography, blood supply is typically via the PICA and, less commonly, via the AICA or branches of the SCA. Dural branches of the vertebral artery, such as the posterior meningeal artery, may supply superficial lesions. The criteria for preoperative embolization of hemangioblastomas include large tumors (often >3 cm), with well-defined arterial feeders that are inaccessible surgically. Embolization using polyvinyl alcohol (PVA) particles or N-butyl cyanoacrylate (NBCA) glue should be performed with a microcatheter tip that is placed beyond the normal branches.³⁴ The risk of embolization is particularly high with hemangioblastomas because the feeding arteries are often pial vessels.

Experience with preoperative embolization of hemangioblastomas in the petrous bone or CPA has been limited mostly to small studies performed at single institutions. Conway and associates ³⁵ had mixed success with the use of preoperative embolization in 4 of 40 patients with hemangioblastomas. Although the embolization alone was sufficient to arrest symptom progression in one patient with a sacral hemangioblastoma, another patient had a lateral medullary infarction after attempted embolization of a medullary hemangioblastoma. The authors concluded that embolization should be reserved for tumors with large, surgically inaccessible arterial feeders.³⁵ Tampieri and colleagues³⁶ treated two patients successfully with large hemangioblastomas, one spinal and one involving the posterior fossa, with only minimal blood loss on surgical resection. Eskridge and coworkers³⁷ reported one complication in nine patients with craniospinal hemangioblastomas: A patient developed malignant posterior fossa edema associated with hydrocephalus after treatment with polyvinyl alcohol. The surgeons concluded that embolization facilitated tumor manipulation and surgical resection.³⁷ Finally, from this author’s experience of 35 patients who underwent preoperative embolization with NBCA liquid adhesive, one of the five patients treated for a hemangioblastoma experienced a complication.³⁴ In this case, distal branches of the PICA were inadvertently occluded during glue embolization of a posterior meningeal artery. The patient had initial mild dysmetria; however, the deficit had resolved by the 12 month follow-up examination.

Hemangiomas

The role of preoperative embolization of hemangiomas is unclear because of their low overall incidence.

Role of Cerebral Revascularization and Bonnet Bypass

Untreated head and neck cancer that involves the skull base often is associated with a poor prognosis, especially if it involves the ICA. The risk of ICA rupture because of malignant cervical tumor invasion can be 18%, which highlights the need for ICA resection in certain cases.³⁸ ICA sacrifice comes with the inherent risk of stroke, however, and has led many authors to advocate that resection with revascularization should proceed only in patients with unsuccessful results from a balloon test occlusion (radiographically or clinically). The test has significant drawbacks, however, including a high false-negative rate, often leading to a high rate of perioperative stroke despite a successful balloon occlusion test. It is difficult to predict immediate and delayed postoperative ischemic events. Consequently, we advocate universal revascularization in all patients with cranial base tumors that involve the carotid artery. Other groups have suggested that in severe metastatic disease, aggressive surgery including carotid sacrifice should be performed only if there is a chance of complete cure of the tumor. More prospective studies are needed to determine a specific set of guidelines on the controversy (Fig. 66-5).

FIGURE 66-5 Imaging studies obtained in a 66-year-old woman. A and B, Axial T1-weighted MR images without (A) and with contrast material (B) showing recurrent squamous cell carcinoma on the left side. Lesion infiltrates left internal carotid artery (ICA) and cavernous sinus (arrow in B). Note the absence of flow void in left cavernous ICA. These images were obtained after clip occlusion of left ICA in the neck and supraclinoid process. C, Angiogram showing ICA-to-middle cerebral artery bypass with saphenous vein placed before tumor was resected. D, Axial T1-weighted MR image obtained after tumor resection and left orbital exenteration. Skull base was reconstructed with free tissue transfer by using rectus abdominis muscle flap.

(From Feiz-Erfan I, Han PP, Spetzler RF, et al: Salvage of advanced squamous cell carcinomas of the head and neck: Internal carotid artery sacrifice and extracranial-intracranial revascularization. Neurosurg Focus 14(3):e6, 2003. Used with permission from Journal of Neurosurgery.)

The concept of a contralateral extracranial–to–ipsilateral intracranial bypass was originally introduced in 1978 by the senior author (R.F.S.) using a saphenous vein as an interposition graft, the so-called bonnet bypass (Fig. 66-6).³⁹ Since then, based on our experience, we have found that the radial artery as an interposition graft can be used as a bonnet bypass and works with a high technical success rate with minimal complications. Bonnet bypass should be considered if the ipsilateral carotid artery is unavailable because of previous surgery or radiation, or if a radical neck dissection is planned. The ipsilateral subclavian artery also can be used. In this case, however, the bypass would pass through the surgical field. Experience from this group has shown that the contralateral superficial temporal artery is an excellent donor vessel to perfuse the ipsilateral hemisphere after carotid sacrifice.³⁸ A radial artery interposition graft is positioned between the contralateral superficial temporal artery and the ipsilateral distal middle cerebral artery.