Look, Listen, and Feel

Clipping an aneurysm can be such a relief that it is tempting to consider the operation over. However, post-clipping inspection is an important part of aneurysm dissection. Clipped aneurysms can be aggressively de-tethered, mobilized, and deflated. The added visualization from these maneuvers enables mistakes to be caught.

Inspection checks seven points: (1) aneurysm occlusion; patency of the (2) parent artery, (3) efferent branches, (4) perforating arteries, and (5) adjacent arteries; (6) no neck remnant beneath the clips; and (7) surgical blind spots. The inspection sequence is ordered. A dome should not be mobilized to explore a blind spot until complete aneurysm occlusion is confirmed; additional mini-clips should not be applied to a small neck remnant if primary clips need an adjustment to open an occluded branch.

Visual inspection is most informative: swirling blood is no longer seen through the aneurysm wall; intraluminal red blood cells settle to blanch the aneurysm; afferent and efferent arteries are red and pulsating; the entire path of each clip blade is free of perforators; and clip tips pass beyond the deep portion of the neck. Palpation with a Rhoton No. 6 dissector (Codman; Raynham, MA) on the aneurysm confirms complete clipping when the instrument is still and persistent filling when the instrument pulsates. Auscultation with a Doppler flow probe assesses the patency of critical arteries.

Other adjuncts take post-clipping inspection to another level of sophistication. Indocyanine green (ICG) videography is quick and easy, and it addresses aneurysm occlusion and arterial patency; however, it can only check what is visible through the microscope, and will miss errors that lie outside the field of view. Intraoperative angiography checks arterial anatomy beyond what is visible through the microscope, but it is more complicated and time-consuming than ICG videography, and consequently its use is decreasing.

Neurophysiologic monitoring can detect problems with blood flow in large branch arteries and small perforating arteries to motor and sensory tracts. Somatosensory evoked potentials (SSEPs) measure conduction along the dorsal columns, brainstem, thalamus, and primary sensory cortex. Motor evoked potentials (MEPs) measure conduction along the corticospinal tract, the anterior horn cells, peripheral nerves, and muscle after cortical stimulation. Ischemia resulting from imperfect clip application may produce elec-trophysiologic changes that lead to the identification of a technical error not detected by inspection. Imperfect clip application can be avoided with most anterior circulation aneurysms because operative exposure is excellent. However, complex aneurysms at the basilar artery apex often do not permit panoramic inspection. Perforators are at greatest risk of occlusion and they hide so well behind the distal neck. Even intraoperative angiography fails to visualize these perforators because of their small size. Missed perforators can cause devastating infarctions in the thalamus, internal capsule, and midbrain, which make neurophysiologic monitoring especially beneficial with basilar artery aneurysms.

A well-clipped aneurysm that passes inspection is punctured away from the neck, just in case it bleeds and an additional clip needs to be stacked. Deflation facilitates the exploration of surgical blind spots and the search for technical errors.

Persistent Aneurysm Filling

Persistent aneurysm filling is the most common problem, and incomplete distal neck occlusion is the most common reason. This spot is difficult to see because it is furthest from the neurosurgeon and the view is tangential. When applying the clip, the microscope is oriented to bring both sides of the neck and both clip blades into one panoramic view down the blades. The relationship between the blade’s tips and the end of the aneurysm neck is difficult to assess from this tangential perspective. An oblique perspective is better, but it sacrifices the view of the other side of the neck, which is unacceptable during clip application. However, this perspective is acquired easily and safely during inspection.

Aneurysms that continue to fill despite well-placed clips that pass beyond the distal neck may not be completely closed at the tips. The weakest part of a straight clip is its tip. In addition, aneurysm tissue between the proximal blades can splay the distal blades, in the same way that a foot in the doorway on the hinge side opens the door wider than a foot in the doorway on the knob side. Splaying is difficult to appreciate visually because the problem is often due to atherosclerosis, calcifications, thrombus, or other intraluminal irregularities. Tandem clipping fixes this problem. A fenestrated clip encircles tissue at the near neck that might splay the tips, and it maintains high closing forces at the blades’ tips. After securing the distal neck, the proximal neck is closed with additional clips.

Persistent aneurysm filling can result from incomplete occlusion of the proximal neck with a fenestrated clip. Simple fenestrated clipping is used when an efferent artery lies between the neurosurgeon and the near neck. There is a delicate balance between completely closing the proximal neck with the heel of the blades and compromising the caliber of the artery in the fenestration. This spot can be difficult to see, and imperfections in the clip application might allow aneurysm filling. A simple straight clip might be stacked across this opening, working on either side of the efferent artery. Alternatively, a stacked fenestrated clip with the heel of the blades pulled back will close the leak in the proximal neck, with the fenestrations forming a tube that transmits the efferent artery. This antegrade fenestration tube is useful with adherent arteries that cannot be freed from the aneurysm wall, sparing the neurosurgeon dangerous dissection along a fragile aneurysm wall and difficult repairs when this wall is torn.

The booster clip is an additional clip stacked above the initial clip that reinforces the neck closure at a site of persistent filling. It is often a fenestrated clip that closes strongly at its tips, frequently at the distal neck when the initial clip cannot be advanced. Booster clips can be applied at the interface between two adjacent clips to overlap a site of persistent filling.

Branch Occlusions

Unintended branch artery occlusions can cause devastating strokes. Clips should ride high on the neck and generously reconstruct the efferent arteries, particularly with atherosclerotic aneurysms and those with aberrant branch angles. These aneurysms have thickened walls and branches that appear to be patent after clipping when viewed externally, but may be narrowed internally with compromised blood flow. ICG videoangiography is particularly useful with these aneurysms.

Clips applied to broad-based, dolichoectatic, and giant aneurysms can slide down the neck and occlude parent and branch arteries. Intraluminal thrombus and coils can hold aneurysm walls apart and transform the neck into a dangerous wedge than can also slide clips down the neck and occlude arteries. A poorly placed clip or one that has migrated can be used as a “tentative clip.” The natural reaction to a poorly applied clip occluding the parent artery is to remove it immediately. However, by leaving it on, it can serve as a scaffold that guides the stacking of permanent clips above it, keeping them off the parent artery. After these permanent clips are secured, the tentative clip is removed and the parent artery reopens. A tentative clip can be used for vascular control when the usual proximal or distal control is inaccessible and the aneurysm needs to be deflated or mobilized. If branch arteries remain occluded, permanent clips should retreat progressively from the neck until patency of the trunks is restored. Aneurysms that continue to slide permanent clips down the neck may require suction decompression, thrombectomy, coil mobilization, or transection to simplify the neck for stable clipping.