Minimally Invasive Approaches to Aneurysms

Patient Selection

The minimally invasive approach for aneurysms means small and strategically placed craniotomies ( ▶ Fig. 5.1), avoiding brain retraction and minimizing tissue damage during dissection. Acceptable minimally invasive approaches must provide sufficient control for ruptured as well as unruptured aneurysms. We do not encourage using very small openings for unruptured aneurysms that only allow clipping at the neck of the aneurysm and do not permit adequate proximal and distal control. In the event of an intraoperative rupture, such minimal-access procedures are a setup for an adverse outcome. Also, the suspected difficulty of clipping the aneurysm needs to be considered. Features such as larger aneurysm size, calcification and thrombus in the parent artery or aneurysm, and perceived need for bypass or other complex reconstruction techniques will weight against employing a minimally invasive craniotomy.

Fig. 5.1 Minimal invasive keyhole craniotomies to the typical aneurysm sites. The specific minimally invasive craniotomies are designed to provide adequate exposure and control for the specific aneurysms instead of a universal aneurysm craniotomy, such as a generous pterional craniotomy. ACA, orbitocranial opening for anterior communicating artery aneurysms; BA, subtemporal keyhole approach to basilar bifurcation and basilar superior cerebellar artery aneurysms; ICA, minipterional craniotomy for internal carotid artery aneurysms; MCA, sylvian craniotomy for middle cerebral artery aneurysms.

The critical aspect of these craniotomies is to use three-dimensional imaging to define the relation between the skull (entry point) and the position of the aneurysm (target point) and to select the appropriate craniotomy based on this plan ( ▶ Fig. 5.2 and ▶ Fig. 5.3).

Fig. 5.2 Three-dimensional rotational digital subtraction angiography is helpful to plan minimally invasive approaches to specific aneurysms. The reconstruction allows planning the approach and clip application as well as clip selection. (a) 3D overview of a right middle cerebral artery aneurysm. (b) Higher magnification of the right MCA bifurcation. Proximal control of the sphenoidal segment (M1) of the middle cerebral artery is achieved either between the middle cerebral branches (M2) or medial to the superior trunk, depending on the course of the M1 and M2 branches.

Fig. 5.3 Recent technological innovation has made aneurysm surgery easier, safer, and more precise. (a, b) Neuronavigation is helpful for middle cerebral artery (MCA), pericallosal artery, and peripheral aneurysms. (c) Indocyanine green videoangiography has become an accepted tool for intraoperative assessment of parent artery patency, (d) Quantified measurement of fluorescence intensity (Flow 800, Zeiss) with differential analysis of transit times in arteries, parenchyma, and veins still must be considered experimental.

5.2 Preoperative Preparation

Prior to surgery, a master plan of the craniotomy and approach to the aneurysm should be made. The most direct route to the aneurysm that requires minimal retraction and microsurgical dissection should be identified. We distinguish aneurysms accessible through basal approaches (aneurysms of the carotid artery) and those accessible through hemispheric approaches (middle cerebral artery [MCA] and pericallosal artery aneurysms). Anatomical landmarks are adequate for placement of the craniotomy for aneurysms near the skull base, whereas image guidance is useful for hemispheric approaches.

Brain retraction should be minimized by achieving brain relaxation with cisternal, ventricular, or spinal drainage of cerebrospinal fluid and by administering mannitol if necessary. We use a spinal or ventricular catheter for all operations in the acute stage after subarachnoid hemorrhage (SAH). Lumbar drainage is preferred for good-grade patients and ventricular drainage for the worse grades.

5.3 Operative Procedure

5.3.1 Keyhole Approach to Middle Cerebral Artery Aneurysms

Craniotomies for MCA aneurysms can be divided into frontolateral and temporal approaches and then by whether one approaches the aneurysm by following the proximal MCA (M1) from the internal carotid artery (ICA) bifurcation or inward from the sylvian fissure. Controlling M1 from the ICA bifurcation appears safer, but accessing M1 close to the ICA bifurcation may require considerable retraction of the fronto-orbital cortex and also maneuvering around the aneurysm. Also, particularly in cases with a long M1, this artery tends to course high in its middle segment before turning downward again at the bifurcation. We prefer to control M1 by following the superior trunk (M2) of the MCA proximally to the M1. Depending on the exact anatomy of the bifurcation and aneurysm, M1 is controlled either between the MCA main branches posteriorly or on the frontal side of the superior M2 ( ▶ Fig. 5.2). In cases with a temporal intracerebral hematoma, we initially evacuate the hematoma via a corticotomy in the superior temporal gyrus. We then expose the MCA bifurcation through the hematoma cavity or through the sylvian fissure.

For most MCA aneurysms, the head is positioned with a 45-degree rotation to the contralateral side. A limited hairline incision is fashioned that allows sufficient exposure down to the orbital rim. The temporal muscle is incised and the anterior portion of the muscle is mobilized together with the skin flap. A round craniotomy 3 cm in diameter is placed on the sylvian fissure, immediately behind the orbital rim ( ▶ Fig. 5.1). Two-thirds of the craniotomy should lie above the sylvian fissure and one-third below. The dura is opened as a flap with an anterolateral base.

The sylvian vein is identified. The lumbar or ventricular catheter is opened to drain cerebrospinal fluid and relax the brain. If relaxation is insufficient, mannitol is given (1 g/kg body weight). If there is a large intracerebral hematoma, sufficient relaxation is achieved by evacuation of the hematoma. The sylvian fissure is split on the frontal side of the sylvian vein. Venous branches crossing the fissure must usually be coagulated and divided. The superior trunk of the MCA is then identified in the depth of the sylvian fissure and followed proximally to the MCA bifurcation. For proximal control, it is important to have the course of the M1 and the projection of the aneurysm in mind and to dissect along the side opposite the projection of the aneurysm ( ▶ Fig. 5.2).

We do not use brain retractors for MCA aneurysms unless the brain remains full despite sufficient brain relaxation. Clipping the aneurysm does not differ with the limited approach. We use temporary clipping when the neck of the aneurysm is wider than the diameter of the M1. We keep the blood pressure at a normal level and do not use pharmacological neuroprotection. Closure of the craniotomy is unremarkable. We do not use wound drains.

5.3.2 Transorbital Keyhole Approach to Anterior Communicating Artery Aneurysms

We introduced the transorbital keyhole approach as a more ventral means of access to the anterior communicating artery complex, which avoids retraction of the orbital cortex and resection of the gyrus rectus ( ▶ Fig. 5.4). The goals are to have a sufficiently wide exposure to allow for unconfined manipulation and control of the relevant parent arteries but with a minimal size. The approach is from the side of the dominant precommunicating anterior cerebral artery (A1). The head is positioned in 45-degree rotation to the contralateral side and with 10-degree hyperextension to allow the orbital cortex to fall away from the skull base.

Fig. 5.4 Transorbital keyhole craniotomy for anterior communicating artery aneurysm. This approach requires minimal brain retraction and allows approaching the aneurysm through the ventral interhemispheric fissure. (a) Orbital craniotomy including the orbital rim and orbital roof. (b) Dissection of the ventral interhemispheric fissure and exposure of the aneurysm.

(Reproduced with permission from Steiger HJ, Schmid-Elsaesser R, Stummer W, Uhl E. Transorbital keyhole approach to anterior communicating artery aneurysms. Neurosurgery 2001;48:347–351.)

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