Key points

Introduction

Supraclinoid aneurysms are typically defined as intradural aneurysms that arise from the internal carotid artery (ICA) distal to the distal dural ring, or roof of the cavernous sinus, to the carotid terminus. Being intradural, these aneurysms pose a risk for subarachnoid hemorrhage and are, therefore, generally treated depending on the size of the aneurysm, patients’ age, and comorbidities. The paraclinoid region is composed of complex osseous anatomy and dural attachments and is in close proximity to the optic nerve, thereby making microsurgical clipping of some aneurysms in this region technically challenging. Endovascular treatment with balloon- or stent-assisted coil embolization and, more recently, flow diversion technology is being used more frequently for treatment of paraclinoid as well as supraclinoid aneurysms.

Relevant anatomy and pathophysiology

The anterior clinoid process projects from the posteromedial border of the lesser wing of the sphenoid bone and forms a roof over the proximal portion of the ICA. The roof of the optic canal (anterior root) and optic strut (posterior root) communicate with the body of the sphenoid bone at the medial aspect of the anterior clinoid process. The optic strut also borders the lower margin of the anterior clinoid process to the body of the sphenoid bone. The anterior clinoid process harbors attachments of the falciform ligament, the anteromedial part of the tentorium, the petroclinoid (anterior and posterior) ligaments, and the interclinoid dural folds.

The clinoidal segment of the ICA lies between the proximal and distal dural rings. The clinoidal ICA traverses the distal dural ring and enters the subarachnoid space. The distal dural ring is thicker at its lateral margin and tightly adheres the artery to adjacent structures. However, medially it may be incompletely attached allowing a small potential subarachnoid space, or carotid cave, abutting inferiorly and medially to the ICA. Proximal to the proximal dural ring, the ICA becomes intracavernous. From the distal dural ring to the carotid terminus, or anterior and middle cerebral artery bifurcation, is the supraclinoid region. The supraclinoid ICA courses medial to the anterior clinoid process below the optic nerve and superior/posterior to the lateral aspect of the optic chiasm. It terminates below the anterior perforated substance at the medial edge of the Sylvian fissure.

Typically, paraclinoid and cavernous aneurysms are differentiated based on their relationship to the origin of the ophthalmic artery. This distinction is important for treatment versus no treatment decision making, as their natural history and risk of subarachnoid hemorrhage is significantly different. There is variability in the origin of the ophthalmic artery off the ICA. For instance, Horiuchi and colleagues reported a prevalence of intradural origin of the ophthalmic artery of 85.7% and an extradural origin of 7.6%. They reported an interdural origin (between the 2 dural rings) of 6.7%. Among the supraclinoid aneurysms, the ophthalmic and superior hypophyseal aneurysms can be angiographically differentiated based on their projection off the ICA. The former typically projects superiorly off the parent vessel and the latter inferiorly or inferomedially.

For purposes of this article, endovascular treatment of ophthalmic, superior hypophyseal, dorsal wall blister, posterior communicating, anterior choroidal, and carotid terminus aneurysms are discussed.

Introduction

Supraclinoid aneurysms are typically defined as intradural aneurysms that arise from the internal carotid artery (ICA) distal to the distal dural ring, or roof of the cavernous sinus, to the carotid terminus. Being intradural, these aneurysms pose a risk for subarachnoid hemorrhage and are, therefore, generally treated depending on the size of the aneurysm, patients’ age, and comorbidities. The paraclinoid region is composed of complex osseous anatomy and dural attachments and is in close proximity to the optic nerve, thereby making microsurgical clipping of some aneurysms in this region technically challenging. Endovascular treatment with balloon- or stent-assisted coil embolization and, more recently, flow diversion technology is being used more frequently for treatment of paraclinoid as well as supraclinoid aneurysms.

Relevant anatomy and pathophysiology

The anterior clinoid process projects from the posteromedial border of the lesser wing of the sphenoid bone and forms a roof over the proximal portion of the ICA. The roof of the optic canal (anterior root) and optic strut (posterior root) communicate with the body of the sphenoid bone at the medial aspect of the anterior clinoid process. The optic strut also borders the lower margin of the anterior clinoid process to the body of the sphenoid bone. The anterior clinoid process harbors attachments of the falciform ligament, the anteromedial part of the tentorium, the petroclinoid (anterior and posterior) ligaments, and the interclinoid dural folds.

The clinoidal segment of the ICA lies between the proximal and distal dural rings. The clinoidal ICA traverses the distal dural ring and enters the subarachnoid space. The distal dural ring is thicker at its lateral margin and tightly adheres the artery to adjacent structures. However, medially it may be incompletely attached allowing a small potential subarachnoid space, or carotid cave, abutting inferiorly and medially to the ICA. Proximal to the proximal dural ring, the ICA becomes intracavernous. From the distal dural ring to the carotid terminus, or anterior and middle cerebral artery bifurcation, is the supraclinoid region. The supraclinoid ICA courses medial to the anterior clinoid process below the optic nerve and superior/posterior to the lateral aspect of the optic chiasm. It terminates below the anterior perforated substance at the medial edge of the Sylvian fissure.

Typically, paraclinoid and cavernous aneurysms are differentiated based on their relationship to the origin of the ophthalmic artery. This distinction is important for treatment versus no treatment decision making, as their natural history and risk of subarachnoid hemorrhage is significantly different. There is variability in the origin of the ophthalmic artery off the ICA. For instance, Horiuchi and colleagues reported a prevalence of intradural origin of the ophthalmic artery of 85.7% and an extradural origin of 7.6%. They reported an interdural origin (between the 2 dural rings) of 6.7%. Among the supraclinoid aneurysms, the ophthalmic and superior hypophyseal aneurysms can be angiographically differentiated based on their projection off the ICA. The former typically projects superiorly off the parent vessel and the latter inferiorly or inferomedially.

For purposes of this article, endovascular treatment of ophthalmic, superior hypophyseal, dorsal wall blister, posterior communicating, anterior choroidal, and carotid terminus aneurysms are discussed.

Natural history, clinical presentation, and indications for treatment

Various risk factors have been associated with higher prevalence of intracranial aneurysms, including female sex, family history, cigarette smoking, excessive alcohol use, hypertension, ischemic heart disease, hyperlipidemia, autosomal dominant polycystic kidney disease, type IV Ehlers-Danlos syndrome, pituitary tumors, aortic coarctation, Graves disease, Marfan syndrome, neurofibromatosis type 1, cerebral arteriovenous malformations (flow-related aneurysms), and oral contraceptive use.

There is no debate that ruptured intracranial aneurysms should be treatment promptly as it carries a high mortality rate of approximately 50% to 60%. However, the decision of treatment versus no treatment is not as clear when the aneurysm is unruptured and found incidentally. In these cases, risk stratification must take into account patients’ age, aneurysm size, location, and morphology.

Multiple previous large trials as well as numerous case series have made efforts to quantify risk of intracranial aneurysms. For instance, the International Study of Unruptured Intracranial Aneurysms (ISUIA) trial concluded the 5-year cumulative rupture rates for patients with aneurysms involving the anterior circulation and who did not have subarachnoid hemorrhage to be 0% for less than 7 mm, 2.6% for 7 to 12 mm, 14.5% for 13 to 24 mm, and 40.0% for greater than or equal to 25 mm. In ISUIA, Posterior communicating artery (Pcomm) aneurysms were grouped with the posterior circulation aneurysms with respect to risk of rupture. The posterior circulation and posterior communicating artery aneurysms of the same size were shown to have higher risk of rupture: 2.5%, 14.5%, 18.4%, and 50.0%, respectively. A systemic review by Rinkel and colleagues reported an overall risk of aneurysm rupture of 1.9% per year (0.7% for unruptured aneurysms ≤10 mm and 4.0% for those >10 mm). A meta-analysis by Clark and colleagues of patients with unruptured aneurysms found that when the ISUIA cohort was excluded, the rate of rupture for Pcomm aneurysms (0.46% annually) mimics that of anterior circulation aneurysms (0.49% annually) rather than posterior circulation aneurysms (1.9% annually). The authors’ of this article tend to treat Pcomm aneurysms more aggressively.

In addition to the risk of rupture and subarachnoid hemorrhage, large or giant aneurysms, particularly in the paraclinoid/supraclinoid regions, can cause mass effect on adjacent cranial nerves resulting in visual disturbances, thereby necessitating treatment. Endovascular treatment of large or giant aneurysms with coil embolization, and more recently flow diversion, has been shown to reduce mass effect (on cranial nerve) as the aneurysm thromboses, contracts, and diminishes in pulsatility, often with resolution of associated cranial neuropathy.

Posterior communicating artery aneurysms are relatively common, accounting for 15% to 25% of all intracranial aneurysms. Pcomm aneurysms arise at the origin of the respective vessel off the posterolateral aspect of the ICA. The laterally projecting Pcomm aneurysms can compress the oculomotor nerve against the tentorium at its superomedial aspect causing pupil-sparing cranial nerve III palsy. The Pcomm harbors multiple small arterial branches supplying the optic chiasm, oculomotor nerve, ventral thalamus, mammillary bodies, tuber cinereum, hypothalamus, and internal capsule. Hence, of course, treatment strategies should focus on maintaining antegrade flow so as to reduce the risk of ischemic injury to the diencephalon to avoid significant morbidity. In some cases, however, retrograde flow can be adequate enough to maintain perfusion of this territory, which should be confirmed with angiography or intraoperative indocyanine green (ICG) during clipping.

Anterior choroidal artery (ACh) aneurysms arise at the origin of the respective vessel off the posterolateral aspect of the ICA. Although the ACh artery is relatively small in caliber, it supplies eloquent territories, including the posterior two-thirds of the posterior limb internal capsule, optic tract, cerebral peduncle, uncus, optic radiations, lateral geniculate body, amygdala, anterior hippocampus, fornix, pulvinar, and globus pallidus. The preservation of this vessel (or vessels, if duplicate variant anatomy) during microsurgical clipping or endovascular treatment of an aneurysm is important and may be challenging in certain circumstances, with morbidity reported at 6% to 33% and mortality at 10% to 29%. Morbidity can be as severe as ACh syndrome, which presents with contralateral hemiplegia, dysarthria, lethargy, and occasionally sensory and vision loss.

Blister, or dorsal wall, aneurysms that arise from nonbranching sites of the supraclinoid ICA are rare but devastating causes of a subarachnoid hemorrhage. These aneurysms are most often tenuous because of their small size, very broad necks, and fragile walls. Pathology studies have shown blister aneurysms to be more like pseudoaneurysms than saccular aneurysms. They have been shown to have focal wall laceration causing ulceration and separation of the internal elastic lamina and media. The focal defect is covered with thin fibrous tissue and clot, lacking a collagen layer, making them much more prone to intraoperative and perioperative rupture.

Although aneurysm size is a main factor based on previous trials, other factors, such as the aspect ratio, presence of daughter sac, and psychosocial issues, also contribute to the decision to treat. In addition to the aforementioned factors, the authors’ also take into account their individual philosophies and experiences in the treatment decision-making process.

Diagnosis and treatment planning

Because of the complex anatomy in this region, cross-sectional imaging, such as magnetic resonance angiography (MRA) or computed tomographic angiography (CTA), is often limited in classifying aneurysms, especially large or giant variants. Digital subtraction angiography (DSA) including 3-dimensional (3D) rotational dual-volume angiography with multiplanar reformations remains the gold standard modality and is widely used to classify intradural versus extradural aneurysms as well as to evaluate the size, morphology, and bony relationship of the aneurysm as part of treatment (open skull base surgery or endovascular) planning. Intraprocedural use of flat detector CT (FDCT) may be beneficial to evaluate the anatomy of the device and its relationship to the vasculature (see Fig. 2 ).

CTA with 3D reconstructions can also be useful to examine the aneurysm relationship to the bony skull base anatomy, which is particularly helpful for open surgical approaches. CTA also allows for making accurate measurements of aneurysms and parent vessels. For treatment planning and virtual surgery, the authors’ usually start with CTA of the head and neck to assess the endovascular route, aneurysm morphology, and measurements of the aneurysm and parent vessel.

MR imaging (MRI) and MRA are useful in evaluating the true size of thrombosed aneurysms as well as evaluating potential mass effect on adjacent structures and aneurysm wall calcification. Additionally, MRI is useful preemptively for ventriculoperitoneal shunt placement in patients with hydrocephalus with transependymal cerebrospinal fluid flow in the setting of subarachnoid hemorrhage, especially before stenting or flow diverter placement requiring dual antiplatelet therapy.

Endovascular considerations

Until the advent of endovascular strategies for treatment of intracranial aneurysms in the paraclinoid/supraclinoid regions, open surgery was the mainstay treatment option and still may remain the primary option depending on the patients’ age, aneurysm location, size, and morphology.

The complex anatomy surrounding the paraclinoid ICA can make microsurgical treatment of aneurysms in this region technically challenging, requiring clinoid drilling and difficulty acquiring proximal control, which adds risk to clipping procedures. Endovascular treatment is increasingly used because of technological advancements. However, in certain supraclinoid ICA aneurysms, surgical clipping can be a more straightforward option.

There have been significant advances in technology for endovascular management of intracranial aneurysms over the decades, whether it be softer coils, improved stents and stent deployment systems, parent vessel flow diversion technology, or endosaccular flow diversion technology (not yet available in the United States). Advances in guide catheter technology allow operators to negotiate difficult arch types, tortuous great vessels, cervical tonsillar loops, and so forth.

The International Subarachnoid Aneurysm Trial Collaborative Group published the first prospective randomized trial comparing surgical clipping versus endovascular coiling in 2143 patients with ruptured intracranial aneurysms. The results demonstrated the following rates of dependency or death at 1 year: 23.7% (endovascular arm) versus 30.6% (surgical arm), with relative and absolute risk reduction of 22.6% and 6.9%, respectively.

In regard to unruptured aneurysms, prospective ISUIA data demonstrated the 1-year combined morbidity and mortality rate for group 1 (those without a history of subarachnoid hemorrhage (SAH) from a different aneurysm) to be 12.6% (surgical arm) versus 9.8% (endovascular arm), with a 22.3% relative risk reduction. For group 2 (those with a history of SAH from a different aneurysm that was repaired), the results were 10.1% (surgical arm) versus 7.1% (endovascular arm), with a 29.7% relative risk reduction.

Ophthalmic ICA/Superior Hypophyseal Aneurysms

As previously mentioned, because of their intricate intracranial location, paraclinoid aneurysms are often treated by endovascular means.

However, because of their relationship with the tortuosity of internal carotid artery, ophthalmic ICA and superior hypophyseal aneurysms can be challenging to treat by endovascular coil embolization. Microcatheterization of such aneurysms require navigation around the anterior genu of the ICA followed immediately by an acute turn into the aneurysm neck. For the same reason, stenting of this segment can also be technically challenging and result in kinking around turns. Nevertheless, with improvement in current access devices, these tasks are quite feasible.

A significant drawback of coil embolization is insufficient aneurysm packing resulting in a higher risk of recurrence rates compared with surgical clipping. Although with the use of adjunctive techniques, such as balloon- or stent-assisted coil embolization, recurrence rates can be otherwise lowered. In the setting of subarachnoid hemorrhage, the latter is avoided as it requires dual antiplatelet therapy, which can complicate external ventricular drain placement, if clinically required ( Fig. 1 ).

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