CHAPTER 17 The in-situ bypass best exemplifies the IC-IC bypass, requiring no scalp artery, no anticipatory graft harvest, no additional incisions or tunnels, and hardly any extra effort to recruit participants. The in-situ bypass requires that its donor and recipient run parallel and in close proximity to one another, distal to, but not necessarily right at, the site of the aneurysm. This communicating bypass adapts and conforms to a wide variety of upstream aneurysms of different sizes, shapes, and etiologies. Communicating flow across a side-to-side anastomosis is governed by deliberate arterial occlusions, pressure gradients, and physiological demands that transcend simple bypass construction, almost endowing the in-situ bypass with self-control. Whereas donor–recipient proximity and mobility are prerequisites for reimplantation, donor–recipient proximity and parallelism are prerequisites for the in-situ bypass. In fact, parallelism and the “one way up” side-to-side anastomosis eliminate the need for mobility. This anastomosis is more challenging than other anastomoses because of its first suture line, requiring entrance and exit stitches, management of four arterial walls at once, and intimate contact with sensitive endothelium. Like reimplantation, the in-situ bypass implicates a bystander artery not involved with the pathology as a donor, and then subjects its territory to cross-clamping, possible ischemic injury, and complications if the anastomosis occludes. These factors give the in-situ bypass a certain elegance and mystique, and make it my favorite of the third generation of bypasses. Arterial parallelism is hard to find in the cerebral circulation, occurring at only four locations. The MCA and its many parallel branches give it the greatest in-situ bypass potential (Fig. 17.1). The superior and inferior trunks from the MCA bifurcation, and their subsequent branches into the M2 stem arteries, course in parallel through the Sylvian triangle to the frontal, temporal, and parietal lobes. The ATA and TPA originate from the M1 MCA and also parallel the M2 stem arteries. These arteries drape the short and long gyri of the insular cortex at the depths of the Sylvian triangle. After migrating either temporally or frontally, the M3 arteries turn laterally to exit the Sylvian fissure through the operculum where they again become parallel and in close contact. The anatomy is most favorable for the in-situ bypass inside the Sylvian triangle near the limen insula with proximal M2 arteries, and above the Sylvian triangle at the lips of the operculum with the distal M3 arteries, before the temporal and frontal M4 arteries diverge on the cortical surface. The distal ACA, with its parallel partner on the other side of the midline in intimate contact, has the perfect anatomy for the in-situ bypass (Fig. 17.2). These preconditions for the in-situ bypass are maintained all the way around the corpus callosum, from the rostrum to the splenium. In-situ bypasses can be performed anywhere along this stretch of arteries, but the anatomy favors the segments at the top of the rostrum (the distal A2 ACA) and at the genu (the A3 ACA) because of their large arterial caliber and ease of access through the interhemispheric fissure. The falco-frontal triangle opens widely for this bypass, with or without gravity retraction of the frontal lobe. The ACA in-situ bypasses are indicated for ACoA aneurysms, and simultaneous aneurysm exposure shifts the bypass to more proximal segments. The left and right PcaAs can also be joined more distally, and the left and right CmaA can be joined more superficially, for distal ACA or PcaA aneurysms. Although the left and right PcaAs remain parallel and in close contact throughout their course to the splenium, the CmaAs have variable distal branching patterns that may separate them in the sagittal plane. Superior–inferior bypasses are possible with reimplantation techniques because the PcaA and CmaA can be aggressively mobilized to connect on the same side (see Chapter 16, Case 16.4); superior–inferior bypasses are not possible with the in-situ techniques because, although the PcaA and CmaA run in parallel, their lack of proximity precludes side-to-side anastomosis. The only parallelism contained within the tentorial-oculomotor triangle and the only site for the in-situ bypass for basilar apex aneurysms is between the P2A PCA and the s1 SCA in the crural cistern, behind CN III (Fig. 17.3). Critical perforators must be excluded from temporary clipping or temporarily clipped when performing this anastomosis. The PedPs arising from the P2 segment pass directly into the cerebral peduncle and supply the corticospinal and corticobulbar tracts, the substantia nigra, the red nucleus, and the tegmentum. The CirPs arising from the P1 and P2 PCA segments medial to the parent PCA encircle the brainstem before penetrating at variable distances (short and long). A short CirP penetrates at or before the geniculate bodies, and a long CirP travels to the quadrigeminal cistern to supply the superior colliculus. The PedP typically supplies the anterior peduncle, whereas the CirP supplies the lateral peduncle. The ThGenP originating from the P2 PCA at the P2A-P2B junction beneath the lateral thalamus ascend to the geniculate bodies, the posterior half of the lateral thalamus, the posterior limb of the internal capsule, and the optic tract, but these are distal to the anastomotic site. The mPChA also arises from the proximal P2A PCA on its medial wall, encircling the midbrain parallel to the PCA trunk before curling forward at the superior colliculus and ascending next to the pineal gland to the roof of the third ventricle and velum interpositum, where it supplies the choroid plexus. The mPChA may be difficult to differentiate from the CirPs, and it sends branches along its course to the cerebral peduncle, tegmentum, geniculate bodies, colliculi, pulvinar, pineal gland, and medial thalamus. Similarly, the only site for an in-situ bypass for PICA aneurysms is between the left and right p3 PICA segments as they meet behind the medulla in the cisterna magna underneath the cerebellar tonsils (Fig. 17.4). The tonsillomedullary segment begins where the PICA passes through the rootlets of CN IX-X-XI at the lateral edge of the olive, descends to the inferior pole of the cerebellar tonsil, and reverses course in the caudal or infratonsillar loop. As it ascends along the medial tonsil, it meets its counterpart from the opposite side and they course in parallel in the midline to the roof of the fourth ventricle. This short vertical stretch provides the site for the PICA-PICA bypass, outside of the vago-accessory triangle and remote from the lower cranial nerves. These early ascending portions of p3 are optimal for the side-to-side bypass because they lie inferior to the tonsils. The vallecula between the tonsils deepens superiorly and makes the late ascending portions of p3 more difficult to anastomose. Perforators along the early ascending p3 segment might supply the gracile fasciculus and tubercle medially, and the cuneate fasciculus and tubercle laterally. These perforators must be excluded from temporary clipping. The distal PICA branches are non-eloquent and supply the roof of the fourth ventricle and choroid plexus. In-situ bypasses are so-named because the donor and recipient have limited mobility and are joined side to side in their resting place. However, a successful in-situ bypass requires some microsurgical coaxing to establish the parallel relationship between the pair. Midline arteries (ACAs and PICAs) align naturally without much additional dissection. Both ACAs are completely linear, and the afferent limbs, anastomotic segments, and efferent limbs come together effortlessly. The afferent limbs of the PICA encircle the medulla en route to the midline and only achieve parallelism at the anastomotic segment. This arterial pair may need some arachnoidal dissection to join in the anastomosis without tension. Non-midline arteries (MCAs and the PCA/SCA) typically need more dissection to elicit parallel alignment. Fissures and cisterns are widely decompartmentalized, and adhesions, trabeculations, and tethering twigs may need to be released. Arterial partners should run side by side not only at the anastomotic site but also along the afferent and efferent limbs. Afferent and efferent limbs that enter or exit the anastomosis at sharp angles tense the anchoring stitches, which can tear through the arterial wall, kink or occlude the arteries, and compromise the bypass. Arteries in easy parallelism produce a better in-situ bypass that is easier to sew and more likely to work. Without fish-mouth arteriotomies, long linear arteriotomies that are three times the arterial diameter are needed to enlarge the anastomotic area and enhance the communicating cross-flow. The donor’s caliber should be equal to or larger than that of the recipient. Unfavorable asymmetry may exist with dominant and nondominant MCA trunks, a small ATA donor, asymmetrical PICAs, and the normal mismatch between the PCA and the SCA. After completing the anastomosis, the in-situ bypass will supply the distal territory of the recipient anterograde and the proximal territory of the recipient retrograde, back to the occluded aneurysm. As a result, the side-to-side anastomosis does not need to be adjacent to the aneurysm. MCA and ACA parallelism occurs over longer and more variable segments, which allows some flexibility in choosing the anastomosis site. PCA/SCA and PICA parallelism occurs over short segments, and anastomotic sites are limited to the prescribed sites. Three in-situ bypasses are possible with the MCA’s many parallel branches: the M2 MCA-M2 MCA bypass, the ATA-M2 MCA bypass, and the M3 MCA-M3 MCA bypass. MCA bifurcation trunks and insular M2 segments meet the requirements for the in-situ bypass, but their anastomosis requires simultaneous temporary occlusion of two M2 segments and ischemia to a large part the MCA territory. The STA-MCA bypass was frequently preferred over the M2 MCA-M2 MCA in-situ bypass and accounts for the low number of these bypasses (Table 17.1). The ATA-M2 MCA bypass capitalizes on an ATA donor that is rarely involved in MCA aneurysms, and its M1 origin makes it immune to occlusive maneuvers at the MCA bifurcation. The recipient M2 trunk may dwarf the ATA, but a wide anastomosis with high demand on the bypass will enlarge and remodel it over time. The M2 MCAM2 MCA and ATA-M2 MCA in-situ bypasses are performed in the Sylvian triangle, whereas the M3 MCA-M3 MCA in-situ bypass is performed superficially as opercular M3 segments transition to M4 segments on the cortical surface (Cases 17.1 and 17.2). Participants in an M3 MCA-M3 MCA bypass must be chosen carefully when the efferent recipient artery is hidden in the insular recess or not obvious among the MCA’s many cortical branches. Techniques like flash fluorescence decipher which arteries are branches of the efferent artery and therefore appropriate bypass recipients, and which arteries are unrelated to the aneurysm and therefore appropriate donors. Table 17.1 Summary of Clinical Experience with IC-IC In-Situ Bypasses
In-Situ Bypass
In-Situ Bypass
Microsurgical Anatomy Sylvian Triangle
Falco-Frontal Triangle
Tentorial-Oculomotor Triangle
Vago-Accessory Triangle
Donor and Recipient Dissection
In-Situ Bypass Technique
MCA In-Situ Bypass
Bypass Type | N | % |
MCA in-situ bypasses |
|
|
M2 MCA-M2 MCA | 2 | 8 |
ATA-M2 MCA | 1 | 4 |
M3 MCA-M3 MCA | 2 | 8 |
ACA in-situ bypasses | ||
L A3 ACA-R A3 ACA | 5 | 20 |
L CmaA-R CmaA | 1 | 4 |
L PcaA-R PcaA | 1 | 4 |
PCA/SCA in-situ bypasses |
|
|
s1 SCA-P2 PCA | 1 | 4 |
V4 VA-a3 AICA | 1 | 4 |
PICA in-situ bypass |
|
|
L p3 PICA-R p3 PICA | 11 | 44 |
Total | 25 |
|
