Endoscopic Approaches to Intracranial Aneurysms

The most important step in applying the endoscope to aneurysm surgery is the design of a small, short, lightweight scope and camera ( ▶ Fig. 19.1). The Storz Image 1 camera body has been combined with an 11-cm long rigid glass scope with a lens diameter of 4 mm. Permanently attached to the camera body is a lightweight cable, which is connected to the camera box. A removable light cable plugs into the camera body just below the electronics cable. It is easily removed for sterilization. The total weight the surgeon holds is just 219 g and includes the camera, cords, scope, and suction. The camera body has, at the end opposite the cables, a bayonet mount for attaching the endoscope. There is a focusing wheel on the body and three push buttons for normal camera functions such as white balance. By adding to this endoscope an adjustable length suction of various diameters, which slides over the endoscope shaft, the surgeon can perform the surgery with two hands very much as he normally would work with a suction in one hand and an instrument in the other. The value of a high-definition and stereoscopic picture is overridden by placing the endoscope close to the anatomy of interest. Because this endoscope system is so lightweight and the attached cables do not drag, there is no significant hand fatigue from holding the endoscope.

Fig. 19.1 Storz Frazee II Mini-endoscope. (a) Image 1 camera (8.4-cm long, 1.9-cm wide, and 3.3-cm tall, 98 g). (b) Cable from light source to camera (16 g). (c) 0-degree endoscope (4-mm diameter, 10.6-cm long) (d) Suction tips in three sizes (1.9-mm, 2.4-mm, and 3.3-mm in diameter). The length of each suction tip is adjustable (from 1.9 to 10 mm beyond the tip of the scope). (e) Fully assembled Frazee II Mini-endoscope (total weight 219 g).

Given its minimally invasive nature, endoscopic aneurysm surgery combines the advantages of definitive open surgery with the limited morbidity of endovascular treatment. Traditional endoscopic surgery (usually performed in the ventricular system) uses an endoscope sheath through which irrigation and instruments are passed. However, in endoscope-directed surgery, the surgeon’s primary operative instrument is passed alongside or parallel to the camera. In these cases, there is no role for the intraoperative microscope as visualization is as good or better with the hand-held endoscope. This technique is distinguished from endoscope-assisted surgery where the camera is merely used as an adjunctive tool to the microscope for close examination of the surgical bed or to look behind the aneurysm in areas obscured from the direct view of the operator.

There are many clear advantages to endoscopically directed surgery including smaller craniotomies (as small as 15 mm), minimal brain retraction, reduced manipulation of the aneurysm before clipping, better visualization of perforating arteries, and improved assessment of the surgical clip placement. Compared with the intraoperative microscope, the neuroendoscope offers superior brightness, increased depth of field, up to 25% greater magnification, a much closer view of the anatomy, and better maneuverability. These advantages highlight many of the shortcomings of the microscope including the need for generous surgical openings to permit direct lines of sight to all important anatomic structures relevant to the particular procedure.

19.2 Patient Selection

Endoscopic techniques can be used for both ruptured and unruptured aneurysms. In cases of acute rupture, most neurosurgeons advocate treatment within the first 24 hours to reduce the risk of rerupture. Given the increasing role of endovascular therapy, the treatment strategy for each aneurysm is formulated on a case-by-case basis after discussion with the interventional neuroradiology team.

19.3 Preoperative Preparation

After the decision is made to operate on a patient, consideration is then given to the potential role of the endoscope. Initially, the surgical team should evaluate if the aneurysm is approachable and treatable with the endoscope alone. If it is determined that the aneurysm cannot be wholly treated in an endoscopic fashion, then the endoscope may be used to assess the aneurysm anatomy and the regional vascular anatomy and to verify accurate clip placement (endoscopically assisted surgery). Many aneurysms, particularly posterior communicating, anterior communicating, middle cerebral bifurcation, and internal carotid artery bifurcation aneurysms, can be treated using the endoscope as the only means of viewing and completing the surgery (endoscopically directed surgery). Even basilar tip and superior cerebellar artery aneurysms can be treated in this manner. Distal aneurysms of the middle cerebral artery are generally not candidates for endoscopically directed surgery at present. However, pericallosal aneurysms can be approached with the endoscope with or without the use of image guidance.

In most cases, computed tomography angiography alone is the only preoperative vascular imaging necessary. It provides three-dimensional imaging and a fly-through tour from outside the skull through the cranial opening to the aneurysm. In some cases, digital subtraction angiography is necessary to evaluate aneurysm morphology or critical vascular anatomy, when the computed tomography angiography is of poor quality or the aneurysm anatomy requires time-resolved imaging.

19.4 Operative Procedure

19.4.1 Preparation and Positioning

All patients undergoing surgical treatment of aneurysms should be placed under general anesthesia with endotracheal intubation. At the start of induction, mannitol (1 g/kg) can be administered intravenously to enhance brain relaxation, thereby reducing the need for brain retraction. The timing of mannitol administration in endoscopically directed surgery is important and this should be communicated to the anesthesia team. Because smaller craniotomies can be used, it often takes 15 minutes or less from skin incision to dural opening and mannitol often is given sooner than would be expected for traditional open procedures. The authors administer antiepileptic medication at the start of the operation. Placement of an external ventricular drain is performed before surgery in most cases of ruptured aneurysms, but is usually unnecessary when treating unruptured aneurysms. If an external ventricular drain is not placed, further brain relaxation can be achieved by opening the cerebrospinal fluid (CSF) cisterns as well as by fenestrating the lamina terminalis. This technique cannot be used prior to clipping in cases of anterior communicating artery aneurysms. In these cases, if the brain seems tight, an external ventricular drain can be placed before clipping.

Many aneurysms of the anterior circulation can be approached through a small eyebrow incision ( ▶ Fig. 19.2). The patient’s head is placed in a neutral position facing the ceiling. Then, the head is slightly extended to allow the frontal lobes to fall away from the anterior fossa floor. The supraorbital notch should be palpated and the skin incision marked approximately 2.5 cm laterally to this point to avoid injuring the exiting supraorbital nerve. In order to improve cosmesis, the incision can be placed just under the lower margin of the eyebrow and the hair left unclipped.

Fig. 19.2 Approach to anterior communicating artery aneurysm. An eyebrow incision and a subfrontal approach along the floor of the anterior fossa can be used to surgically access most anterior circulation aneurysms.

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