Aneurysm Anatomic Analysis and Indications for Surgical Intervention

The gold standard for aneurysm diagnosis continues to be a complete transfemoral digital subtraction angiogram. The catheter angiogram will also show any associated vascular abnormalities (i.e., arteriosclerosis with luminal narrowing) that may influence the treatment method. The size and shape of the aneurysm, its direction of projection, and its appearance in relation to the size of the lesion on computed tomography (CT) or magnetic resonance imaging (MRI) should be noted. ¹ A lesion on CT or MRI larger than one seen on angiography suggests intra-aneurysmal thrombosis. CT angiography in multiple planes can help discern whether the neck of the aneurysm is intra-, inter-, or extradural. Calcification best noted on the CT scan can make clipping more difficult. ²

6.1.1 Anatomy of the Ophthalmic Segment

The ophthalmic segment (OphSeg) is that portion of the internal carotid artery (ICA) that extends from the point the ICA enters the subarachnoid space to the origin of, but not including, the posterior communicating artery (PComArt). The OphSeg is preceded by the cavernous and clinoidal segments of the ICA. ³ The cavernous segment (CavSeg) lies within the dural confines and venous lumen of the cavernous sinus, outside the subarachnoid space. As the CavSeg ascends just beyond its anterior genu, it pierces the carotid-oculomotor membrane and becomes the clinoidal segment (ClinSeg). ⁴ The ClinSeg is covered laterally by the anterior clinoid process (ACP) and ascends and passes through the dura at the dural ring (DR) to enter the subarachnoid space, at which point it becomes the OphSeg ( ▶ Fig. 6.1 and ▶ Fig. 6.2). ^{5,​ 6}

OphSeg aneurysms are defined by their relation to the two named branches of the segment, which are the ophthalmic and superior hypophyseal arteries. ⁶ The ophthalmic artery (OphArt) arises from the dorsal or dorsomedial aspect of the ICA just distal to the DR and courses anteriorly through the optic canal underneath the lateral aspect of the optic nerve. ⁷ The first of often multiple superior hypophyseal arteries (SupHypArt) originates just distal to the DR as one or several perforators from the medial or inferomedial aspect of the ICA. ⁸

6.1.2 Classification of Ophthalmic Segment Aneurysms

Three aneurysm variants arise with the OphSeg ( ▶ Fig. 6.3). ^{9,​ 10} OphArt aneurysms arise from the ICA just distal to the origin of the OphArt and project dorsally or dorsomedially toward the lateral aspect of the optic nerve. SupHypArt aneurysms arise along the medial surface of the ICA in association with one or several perforators from the medial or inferomedial ICA wall that supply the optic chiasm and pituitary stalk. Some project inferomedially toward the sella turcica and burrow into a diverticulum of the subarachnoid space medial to the ICA (the carotid cave), and are termed the parasellar variant. ¹¹ Others extend early in their growth directly into the suprasellar space and represent the suprasellar variant. ⁹ Both variants are more difficult to treat surgically than OphArt types because of their medial projection and proximal origins. ^{12,​ 13} The much less common dorsal variant OphSeg aneurysm arises several millimeters distal to the origin of the OphArt and projects superiorly. Some may be typical saccular aneurysms, but others appear as “blisters” on the dorsal surface of the ICA. ¹⁴

6.1.3 Indications and Contraindications for Surgery

Small (<1 cm) asymptomatic OphSeg aneurysms, especially those that project into the carotid cave (parasellar variant), are much less likely to rupture compared to lesions more distal in the subarachnoid space. ¹³ The isolated, incidentally found lesion in an older patient should usually be managed conservatively. Treatment is indicated for virtually all symptomatic aneurysms and for those exceeding 1 cm in size. ⁹ Patients presenting with visual loss should be treated urgently, ideally with surgery if the patient’s risk factors and the experience of the operating team are reasonable. For subarachnoid hemorrhage (SAH) patients, surgery is relatively contraindicated in poor-grade moribund cases with intracranial pressure elevations that cannot be controlled adequately to allow proper brain relaxation and skull base exposures to repair the aneurysm.

6.1.4 Timing of Surgery

Ruptured aneurysms presenting with SAH or epistaxis are treated urgently. ^{15,​ 16} The surgery can be technically challenging and is usually done during daylight hours. Unruptured but symptomatic aneurysms should also be treated with a sense of urgency, depending on the rapidity of symptom onset and severity of neurologic signs. Elective treatment is reasonable for asymptomatic lesions. ¹⁷

6.1.5 Alternatives to Surgery

Alternative treatments include observation or endovascular treatment. Endovascular techniques include both deconstructive and reconstructive procedures. Reconstruction of the ICA (or repair of the aneurysm) can be done by coiling, stenting, endoluminal flow-diverting devices, or some combination of these. ¹⁸ The most appropriate mode of treatment is best determined by a neurosurgeon and an interventional surgeon with expertise in aneurysm treatment, after the patient is stabilized clinically and the radiological workup is complete (which may include a balloon test occlusion of the ICA). ¹⁹

6.1.6 Risks

Specific risks of surgery include death, inadvertent major arterial occlusion causing stroke, incomplete clipping or inability to clip the aneurysm, unilateral blindness and chiasmal distribution visual loss, neurological deficits as a consequence of the surgical exposure, cerebrospinal fluid rhinorrhea, and the systemic complications associated with general anesthesia and a craniotomy. The arteries at risk of occlusion include the ICA, ophthalmic, posterior communicating, and anterior choroidal as well as other perforating arteries. ^{17,​ 20} ICA occlusion has variable effects ranging from none to hemispheric stroke, brain swelling, and death.

6.2 Preoperative Preparation

Antibiotic prophylaxis, steroids, and mild hypothermia are standard adjuvants, as are a urinary catheter and radial arterial line. Continuous evoked potential and electroencephalographic monitoring are essential in our opinion. Brain relaxation is achieved with wide opening of the sylvian fissure to drain cerebrospinal fluid and with mannitol 0.5 g/kg given 20 minutes before dural opening in patients with SAH. A ventricular drain can be used in patients with SAH. We do not routinely use lumbar spinal drainage. If temporary ICA occlusion is necessary, mild hypertension is induced and intravenous barbiturates are administered to achieve burst suppression on the electroencephalogram.

6.3 Operative Procedure

6.3.1 Positioning and Draping

The patient is placed supine with a shoulder roll underneath the ipsilateral shoulder and the head elevated above the heart to promote venous return. The head is rotated 45 to 60 degrees toward the contralateral side, with the vertex lowered ( ▶ Fig. 6.4). The maxilla is at the highest point to allow gravitational retraction of the frontal and temporal lobes. Exposure of the cervical ICA may be useful in gaining proximal ICA control. For small and simpler aneurysms, the neck is prepared and draped out without further exposure. For large or complicated aneurysms, especially those presenting with hemorrhage, the cervical ICA should be exposed before the aneurysm is dissected.

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