Paraclinoid Aneurysms

Paraclinoid aneurysms are common lesions. Although most are now detected at a small size, these aneurysms have a tendency to reach large and giant proportions, particularly in women. In addition, mirror image lesions are common in this location, at times complicating treatment paradigms.

From a surgical perspective, paraclinoid aneurysms can be difficult because of their intimate relationship to the anterior clinoid process, which can interfere with early establishment of proximal control ( Fig. 3.1 ). These aneurysms are exposed through a standard pterional craniotomy, as described in Chapter 1, with aggressive extradural removal of the sphenoid wing. When treating a small lesion, minimal opening of the proximal Sylvian fissure, to avoid tension on the brain and bridging Sylvian veins, is usually adequate. For larger lesions, a wider opening of the fissure is helpful, and an orbitocranial (OC) approach may be useful when treating large or giant aneurysms in this location.

In our experience, in almost all cases, the anterior clinoid process must be removed at least partially and the falciform ligament opened to mobilize the optic nerve safely prior to clip placement . The amount ofclinoid removal varies depending on the local anatomy. In some cases, only a small amount of bone removal is necessary, and often the bone has been thinned already by local pressure from the aneurysm ( Figs. 3.2, 3.3 ). In other instances, particularly in the setting of a short optic nerve (prefixed chiasm), we have found that a wide opening of the optic canal, at times all the way to the posterior orbit, has been necessary to allow for adequate mobilization of the optic nerve to expose the aneurysm properly without placing undue tension on the nerve itself ( Figs. 3.4, 3.5 ).

As a matter of personal preference, we have favored intradural rather than extradural removal of the clinoid. In the past, we used a high-speed diamond drill for clinoidectomy. Several gently curved, fine drill bits are available that optimize the surgeon’s view of the local anatomy during the drilling. For the past 5 years, we have used an ultrasonic bone-cutting device in all cases to avoid the use of a high-speed drill in close proximity to a thin-walled aneurysm. As the clinoid is removed, there is often a small amount of venous bleeding from the bone, which is readily controlled with bone wax applied using a microinstrument. Occasionally, the clinoid is highly pneumatized, and this may pose a risk for delayed cerebrospinal fluid (CSF) fistula development. In such cases, one should aggressively wax the bony opening, and we have sometimes used a small amount of muscle or fat to fill the opening at the conclusion of the procedure to avoid a delayed CSF fistula.

After removal of the clinoid, the falciform ligament is opened, and the optic nerve is freed. The aneurysm neck is then explored and the ophthalmic artery identified. On occasion, we have found the optic nerve to be densely adherent to the aneurysm dome. In such cases, the nerve can be released from under the falciform ligament and left adherent to the aneurysm dome as long as a clip can be worked across the aneurysm neck below the nerve. When treating an ophthalmic region aneurysm, visual impairment can result from overly aggressive manipulation of the optic apparatus with inadequate opening of the falciform ligament and optic canal, ischemic injury to the nerve or chiasm, occlusion of the ophthalmic artery, or thermal injury from drilling near the nerve. Following clip application, the clip itself may, on occasion, put pressure on the optic nerve or chiasm. In such cases, one can gently interpose a piece of collagen sponge material between the clip and the nerve, or a microsuture can be utilized to reposition the clip away from the optic apparatus ( Fig. 3.6 ).

Paraclinoid aneurysms include a variety of anatomically distinct aneurysm subtypes. True ophthalmic aneurysms are the simplest to treat, although they require the most manipulation of the optic apparatus. Carotid cave and superior hypophyseal aneurysms require facility with skull base dissection to enable adequate exposure and clipping. It may be difficult to visualize the proximal neck of a posteriorly directed paraclinoid aneurysm, and the surgeon requires a good intuitive sense of the three-dimensional anatomy in such cases ( Fig. 3.7 ) . In all these lesions, the aneurysm dome can become intimately adherent to the basal dural reflections, which can significantly complicate the final dissection of the aneurysm. In addition, it may be difficult to predict which aneurysms are partially extradural, and complete clipping may require opening of the proximal and distal dural rings of the cavernous sinus. Bleeding from the cavernous sinus can be bothersome, and surgeons treating these lesions should be comfortable with this possibility.

Large and giant aneurysms in this location are more difficult because of their distortion and compression of the optic apparatus ( Fig. 3.8 ). Options for proximal control include exposure of the cervical internal carotid artery (ICA) through a neck incision, the use of a balloon microcatheter positioned endovascularly in the cervical ICA, or exposure of the petrocavernous carotid through a complex skull base dissection. When treating giant paraclinoid aneurysms, we generally expose the cervical ICA for possible suction decompression of the aneurysm. The endovascular placement of a balloon in the cervical internal carotid artery represents a reasonable alternative, although we have found it faster and more reliable to primarily expose the cervical carotid artery. The use of an endovascular balloon for any period of time carries a thromboembolic risk, and repeated inflation and deflation of the balloon can result in a carotid dissection. In addition, we have occasionally had trouble achieving adequate decompression with a balloon. When using suction decompression, it is important to apply a clip distal to the aneurysm across the supraclinoid internal carotid artery prior to beginning the suction to avoid stealing important collateral flow from the ipsilateral hemisphere. Despite temporary occlusion and suction decompression, giant paraclinoid lesions can be very challenging to clip when densely adherent to the basal dura or the optic apparatus. Examples of giant paraclinoid aneurysms treated with direct clip reconstruction and with bypass and occlusion are included in Chapter 9.

When treating a ruptured paraclinoid aneurysm, the issue of proximal control becomes even more critical. In the setting of a severe, unexpected intraoperative rupture of a carotid ophthalmic aneurysm, we have used an intravenous infusion of adenosine to induce transient cardiac standstill, providing temporary flow arrest when proximal control could not be achieved easily. Note that bilateral ophthalmic aneurysms, when small, can often be clipped via a unilateral approach, dissecting and clipping the contralateral aneurysm by working underneath the opposite optic nerve ( Fig. 3.9 ) ( Table 3.1 ).