PICA Bypass Strategy

PICA aneurysms account for 4% of my overall aneurysm experience. Although relatively few in number, they have an unusually high incidence of nonsaccular or fusiform morphology, making them less likely to be clippable and more likely to require bypass. In addition, PICA aneurysms are frequently located distally on the parent artery, rather than arising from its branch site on the parent artery, which differs significantly from aneurysms at other sites. In our review of nearly 1700 aneurysms treated microsurgically, the incidence of distal aneurysms on the PICA was five times that of distal aneurysms on cerebral arteries: 35% of PICA aneurysms versus only 6.5% of all cerebral artery aneurysms. This same review identified the p2 and p3 segments of the PICA as the most common sites of distal PICA aneurysms, segments notable for their curvature. Furthermore, PICA aneurysms have nearly twice the frequency of complex features such as intraluminal thrombosis, giant size, and coil recurrence. The combination of unclippable pathology and complex features makes revascularization techniques an essential part of the treatment of PICA aneurysms. Overall, 27% of treated PICA aneurysms required bypass, and 23% of all bypasses for aneurysms involved PICA aneurysms.

No aneurysm epitomizes the evolution in bypass surgery from using scalp arteries and remote donor sites in the neck toward a more local and reconstructive approach better than the PICA aneurysm. When we found no difference in bypass patency rates, aneurysm obliteration rates, and outcomes between patients treated with IC-IC bypass and patients treated with EC-IC bypass, I adopted a practice that utilizes IC-IC bypass preferentially when revascularization is needed for a complex aneurysm. Forty IC-IC bypasses have been performed as part of PICA aneurysm management, with no EC-IC bypasses. With this practice, all aneurysms were completely or nearly completely obliterated, 94% of bypasses were patent, 77% of patients were improved or unchanged after treatment, and 76% of patients had good outcomes (mRS ≤ 2). IC-IC bypasses match the spectrum of aneurysm pathology in this region, and the use of intracranial donor arteries spares the neurosurgeon the tedious dissection required for OA harvest.

The algorithm for PICA aneurysm management depends on the segmental location of the aneurysm on the anterior medullary (p1), lateral medullary (p2), tonsillomedullary (p3), or telovelotonsillar (p4) segments (Figs. 24.1 and 24.2).

Anterior Medullary PICA Aneurysms (p1 Segment)

Unclippable aneurysms arising from the PICA origin or the p1 segment are trapped and revascularized with an L p3 PICA-R p3 PICA bypass, reserving the p1 PICA-V4 VA reimplantation as an alternative (Figs. 24.3 and 24.4). Reimplantation of the PICA onto the V4 VA is most favorable with these proximal PICA aneurysms because the gap bridged by reimplantation is the smallest of all PICA aneurysms. However, reimplantation may be the hardest of the IC-IC bypasses. Although opened by the far lateral craniotomy’s aggressive condylectomy, the surgical corridor is deep and surrounded by sensitive lower cranial nerves that form the infrahypoglossal portion of the vagoaccessory triangle. Consequently, the risk of postoperative cranial neuropathy is greatest with this technique. Other technical factors make this technique more difficult, such as a VA wall that is more layered than that of the PICA and frequently atherosclerotic, creating mismatches in wall thickness as well as in caliber. The end-to-side anastomosis requires a long arteriotomy in the V4 VA, and bites of suture must be full thickness with good eversion of the edges. The V4 VA reimplantation site may be short or crowded, which is not always cleared by placing the temporary clip on the V3 VA extradurally or lifting the accessory nerve. Although the V4 VA temporary occlusion for the reimplantation is well tolerated when the contralateral VA supplies the basilar circulation, it can compromise basilar circulation with a terminal PICA or a hypoplastic distal V4 VA on the contralateral side.

The L p3 PICA-R p3 PICA bypass is considered the first choice to avoid the risk of lower cranial nerve morbidity associated with reimplantation, and to avoid the tissue irregularities of the VA. The L p3 PICA-R p3 PICA bypass is the easiest of the IC-IC bypasses. It is performed in the midline through a far lateral craniotomy, and can be done through a standard suboccipital craniotomy when aneurysm access is not needed. Visibility and maneuverability are excellent, and lower cranial nerves are not involved. The caudal loops of PICA are generously mobilized to bring their ascending segments together naturally, and only rarely is there tension on the sutures or kinking of the arteries. A long arteriotomy three times the diameter of the PICA ensures communicating flow across the anastomosis. The caliber of the contralateral donor PICA should be equal to or larger than the ipsilateral recipient.

Included in p1 segment PICA aneurysms is the V4 VA aneurysm that incorporates the PICA’s origin. These V4 VA aneurysms are often arterial dissections that present with SAH and must be excluded aggressively to prevent rehemorrhage. In most cases of V4 VA aneurysms located proximal or distal to the PICA’s origin, the VA can be sacrificed endovascularly without revascularization because the contralateral VA collateralizes the basilar circulation and the PICA’s origin is spared. With proximal V4 VA aneurysms, this arterial occlusion elicits retrograde filling of the distal V4 VA from the contralateral VA, which then supplies the PICA ipsilaterally. With distal V4 VA aneurysms, this arterial occlusion maintains anterograde filling of the proximal V4 VA and the PICA. Coil occlusion may block the ipsilateral origin of the anterior spinal artery, but its contralateral origin can compensate.