Introduction

Transitional internal carotid artery (ICA) aneurysms incorporate the cavernous, clinoidal, and supraclinoidal segments of the ICA as portions of the neck of the aneurysm. In this chapter, we review the natural history, clinical course, our series, and surgical techniques/nuances for the treatment of these aneurysms.

Pure cavernous ICA aneurysms are a separate category of aneurysms, which do not extend beyond the dural ring of the ICA. In general, in the neurosurgical literature these aneurysms are considered benign, even in larger sizes.¹ However, in our experience it is possible that when these aneurysms get significantly large in size, they can cause rupture through the middle fossa dura, causing hemorrhage and potential mortality (Figure 23–1).

Figure 23–1 A, A 55-year-old presented with new onset diplopia. T1 axial MRI shows cavernous sinus aneurysm. B, Digital subtraction angiogram (DSA) showing a giant cavernous ICA aneurysm (does not extend into the supraclinoidal segment). Following balloon test occlusion in hospital, the patient had an acute headache, and became fixed and dilated. The patient subsequently died within 2 hours. C, Autopsy revealed a rupture through the middle fossa dura into the temporal lobe.

Natural history of transitional segment of ICA aneurysms

Unruptured intracranial aneurysms are being diagnosed more often than in the past and these lesions pose management problems. The first series of giant aneurysms was presented in 1969 by Morley and Barr, who described giant aneurysms as >2.5 cm.² Giant aneurysms are relatively rare, comprising between 5% and 7% of aneurysms³ in most series. Giant aneurysms have a female predominance with ratios as high as 3:1 in some series⁴ and most commonly present in the middle decades. Giant aneurysms are most frequently located on the ICA, specifically, involving the paraclinoid segment 21% of the time.⁵

The natural history of giant aneurysms is one of a poor prognosis. Some have compared the fate of unruptured giant aneurysms to that of subarachnoid hemorrhage (SAH).⁶ Patients often present with symptoms consistent with SAH or mass effect. The Italian cooperative study has shown a 28% mortality rate for untreated giant aneurysms compared to a 14% mortality rate for patients who received treatment.⁷ In the same series, morbidity was seen in 48% of cases due to expansion. Giant aneurysms, not unlike other aneurysms, continue to enlarge over time. Expansion is thought to be from growth of the lumen or thrombus formation within the sac. This progressive enlargement often causes symptoms of mass effect.⁸

Historically, it was thought that giant aneurysms were less prone to rupture; however, contemporary evidence suggests that this is inaccurate. Drake found that 62 of 174 cases did rupture.⁹ Aneurysms sized ≥25 mm had a relative risk of rupture of 59% compared to aneurysms <10 mm.¹⁰ Giant aneurysms had a 6% rupture rate in the first year compared to <1% for aneurysms <10 mm.¹⁰ Patients with a giant aneurysm, but without previous SAH, were at a significantly increased risk for rupture based on size. The rupture rate was not dependent on the location of the aneurysm; all anterior circulation giant aneurysms were at increased risk of rupture.³ Risk factors for aneurysm rupture include hypertension, current or former tobacco use, alcohol use greater than five drinks per day, and use of oral contraceptives.¹⁰ It was thought that partially or completely thrombosed aneurysms would be less likely to rupture, thus not requiring treatment; however, in Drake’s large series, this was not the case.⁹

The ISUIA (International Study of Unruptured Intracranial Aneurysms Investigators) trial has shown that un-ruptured aneurysms overall do not pose the risk they once were thought to; and that treating aneurysms, specifically small ones, puts the patient at more risk than surveillance. However, this advice should not be applied to the special case of giant aneurysms, where “the natural history is ominous.”⁶ Morley and Barr’s original series demonstrated similar findings, with an 80% mortality rate for untreated aneurysms. Other studies have shown high 1-year mortality rates.¹¹ Peerless et al. found that 85% of patients with giant intracranial aneurysms were dead at 5 years.¹² Additionally, the ISUIA II found that the cumulative rupture rates for anterior circulation giant aneurysms with no previous history of SAH were 40%. Patients who present with SAH are at increased risk of mortality compared to patients without SAH.¹³ We present a case of a ruptured giant aneurysm without previous history of SAH 6 months after initial diagnosis (Figure 23–2).

Figure 23–2 A, A patient was diagnosed with this aneurysm at an outside institution and was recommend conservative treatment. B, Patient was transferred to our institution 6 months after initial diagnosis with acute SAH and IVH, and underwent an angiogram. C, During angiography, the aneurysm reruptured (see extravagation of blood). D, Postangiography CT scan showing blood completely filling the lateral ventricles. The patient subsequently expired.

The Dilemma of Deliberate ICA Occlusion

Traditional neurosurgical literature and practice have held a liberal view of ICA occlusion for the treatment of giant ICA aneurysms. Historically, this has been accompanied by a relatively high risk of stroke following the ICA occlusion (Table 23–1). With recent improvements in the balloon test occlusion (BTO) paradigms, the risk is smaller but clearly not absent.

Table 23–1 ICA Sacrifice.

Source	Surgery	Outcomes
Sahs and Locksley, 196914	ICA ligation	Stroke 30% Mortality 20.7%
Roski et al., 198115	ICA ligation	TIA 16.6% Stroke 16.6% SAH 5.5% Mortality 5.5%
Linskey et al., 199416	Intravascular ICA occlusion	Stroke 12.9% Mortality 3.2%
Larson et al., 199517	Intravascular ICA occlusion	TIA 10.9% Stroke 3.6% SAH 3.6% Mortality 5.4%

ICA, internal carotid artery; SAH, subarachnoid hemorrhage; TIA, transient ischemic attack.

For the treatment of carotid occlusive disease, traditional neurosurgical literature and practice have strongly recommended preservation of the ICA. High-grade stenosis (i.e., 99% stenosis) is considered a relative neurosurgical emergency, requiring an endarterectomy or stenting. It is interesting that most patients who present with cervical carotid occlusive disease are usually older and generally have multiple medical comorbidities; yet performing a procedure to open the carotid artery has remained standard practice. Most cervical carotid occlusive disease symptomatology is due to embolic disease rather than hemodynamic compromise. Therefore, in the majority of cervical carotid occlusive disease, a deliberate occlusion of the ICA, in the opinion of the senior author, could be performed with a relatively low risk of morbidity, and most likely less risk of morbidity and mortality, when compared to carotid endarterectomy or stenting. However, this would be considered a complete departure from our current standards of care (Figure 23–3).

Figure 23–3 ICA sacrifice versus ICA preservation.

On the other hand, patients with giant ICA aneurysms are usually younger and have relatively less medical comorbidities when compared to patients with cervical carotid occlusive disease. Yet, in these cases, it is considered acceptable in neurosurgical practice to sacrifice the ICA. Long-term effects of deliberate carotid occlusion are not well studied. The risks of forming additional aneurysms, developing hemodynamic ischemic disease, and developing potential cognitive disorders have to be taken into account when this decision is made.

Therefore, it has been the practice of the senior author (SIA) to preserve flow to the hemisphere in the treatment of giant aneurysms, especially in younger patients.

Experience at St. Louis University Hospital in Treatment of Giant ICA Aneurysms

We performed a retrospective review of 55 consecutive patients treated at St. Louis University Hospital from June 1999 to June 2007. All 55 patients had transitional ICA aneurysms. The mean aneurysm size was 34.7 mm (range 25 to 70 mm). The mean age was 46 years (range 30 to 64 years). The mean follow-up period was 34.7 months (range 1 to 76 months). The objectives of this study were to assess the use of high-flow EC-IC bypass for the treatment of giant transitional ICA aneurysms, delineate surgical mortality and morbidity, and determine the long-term efficacy of the treatment and functional outcomes of these patients. The presenting symptoms were cranial neuropathy (42%), headache or incidental finding (40%), and SAH (18%) (Figure 23–4). The World Federation of Neurological Surgeons (WFNS) grade was 0 in 81%, 1 in 15%, 2 in 2%, and 3 in 2% (Figure 23–5). All 55 patients underwent a high-flow EC-IC bypass procedure.

Figure 23–4 Percentage of patients initially presenting with headache, cranial neuropathy, and SAH.

Figure 23–5 Initial WFNS grade at presentation.

Surgical technique

Craniotomy: A skull base approach, usually a cranio-orbital or a cranio-orbital-zygomatic access, is usually needed. We prefer a single-piece craniotomy, which entails an osteotomy of the orbital roof and lateral margins. A high-speed drill is used to thin the zygomatic arch posteriorly, which allows the radial artery to lie without kinking at the zygomatic area (Figure 23–6A).

Figure 23–6 A, Operative photograph shows simultaneous planning for the craniotomy, cervical incision, and radial artery harvest. B, Operative photograph shows exposure of the radial artery with superficial marking. C, Trans-sylvian preparation for anastomosis. M3 recipient branch with temporary clips. Radial artery donor graft is shown. D, Completed anastomosis is shown. E, Anastomosis of the radial graft to the ECA in the neck and temporary clips on ECA are shown. ECA, external carotid artery.

(From Abdulrauf SI: Extracranial-to-intracranial bypass using radial artery grafting for complex skull base tumors: technical note, Skull Base 2005;15:207-213, with permission.)

Exposure of the carotid artery in the neck: A standard incision anterior to the sternocleidomastoid muscle to expose the common, external, and internal carotid arteries (CCA, ECA, and ICA) is used. Vessel loops are applied around the CCA, ECA, and ICA. We recommend the use of the ECA as the site for the proximal anastomosis.

Radial artery harvest: