CHAPTER 14

Extracranial-Intracranial Bypass

Extracranial-Intracranial Bypass

Although reduced in neurosurgical textbooks to something flapped or peeled away en route to better parts of the case intracranially, the scalp is important to EC-IC bypass because it harbors donor arteries. The scalp has five layers, captured by the mnemonic “SCALP”: skin, dense connective tissue, galea aponeurosis, loose connective tissue, and pericranium (Fig. 14.1). The layers are appreciated best at the vertex where there are no muscles and the galea courses between the frontalis and occipitalis muscles. The layers are less relevant laterally and posteriorly in the regions of the STA and OA, respectively. Scalp arteries reside in the subcutaneous tissue beneath the skin and above the galea aponeurotica, and periosteum in these regions is replaced by muscular fascia and muscle.

Fig. 14.1 (A) The STA resides in the dense connective tissue layer beneath the skin and above the galea aponeurotica (cadaveric specimen, left side). A thin layer of loose connective tissue lies between the galea and the temporalis fascia. Branches of the facial nerve lie anterior to the STA. (B) Elevation of the superficial temporalis fascia (purple sutures) and windows in the deep temporalis fascia and temporalis muscle reveal all layers of the lateral scalp.

Scalp Arteries from ICA

Five paired arteries supply the scalp: supratrochlear, supraorbital, superficial temporal, posterior auricular, and occipital arteries (Fig. 14.2). The supratrochlear and supraorbital arteries are part of the internal carotid arterial system, and originate as branches of the OphA to supply the skin, muscles, and pericranium of the midline and lateral forehead, respectively. The supraorbital artery arises from OphA as it enters the orbit, runs along the superior rectus and levator palpebrae superioris muscles, and exits the supraorbital foramen. The supratrochlear artery is a terminal branch of the OphA that exits from the medial orbit. These two anterior scalp arteries are not used in EC-IC bypasses because of their small caliber and unfavorable location.

Fig. 14.2 (A) Supratrochlear and supraorbital arteries are terminal branches of the OphA that exit from the medial orbit and supraorbital foramen, respectively (cadaveric specimen, right side). (B) Supraorbital artery arises from the OphA as it enters the orbit and runs along the superior rectus and levator palpebrae superioris muscles, whereas the supratrochlear artery runs along the superior oblique muscle.

Scalp Arteries from ECA: Superficial Temporal Artery

The superficial temporal artery, posterior auricular artery, and occipital artery form the external carotid arterial system and are the major suppliers to the scalp. The STA is the main supplier of the scalp and the smaller of the two terminal branches of the ECA (the other one being the internal maxillary artery). It originates within the parotid gland deep to the facial nerve and ascends over the zygomatic root, anterior to the external auditory canal (Fig. 14.3). It lies in the dense connective tissue of the scalp below the skin and above the galea. The STA gives rise to a small zygomatico-orbital branch at the level of the zygomatic root that courses anteriorly to the orbit, and then divides into frontal and parietal (anterior and posterior, respectively) branches typically 4 to 5 cm above its origin, although this bifurcation is highly variable in its location and symmetry. Terminal branches of the STA anastomose freely with each other and with counterparts on the contralateral side to supply the scalp over the frontal and parietal convexities, underlying muscles, and pericranium, whereas its proximal branches supply the masseter muscles. The STA typically runs in parallel with the superficial temporal vein, which is larger in caliber and straighter, lacking the serpentine morphology of the STA.

Fig. 14.3 (A) The STA is the main supplier of the scalp, originating within the parotid gland deep to the facial nerve and ascending over the zygomatic root, anterior to the external auditory canal (cadaveric specimen, right side). It divides into frontal and parietal branches 4 to 5 cm above its origin. (B) The PAA originates from the ECA distal to the OA and divides into auricular and stylomastoid branches. The OA passes through the occipital groove medial to the posterior belly of the digastric muscle and lateral to the superior oblique muscle.

Scalp Arteries from ECA: Occipital Artery

The OA arises from the ECA just opposite the origin of the facial artery and just proximal to the ECA’s termination into the STA and the internal maxillary artery (Fig. 14.4). It transmits meningeal branches that pass through the hypoglossal canal, jugular foramen, and mastoid canal to supply the posterior fossa dura and anastomose with the middle meningeal artery. The OA courses medial to the mastoid process of the temporal bone and medial to the posterior belly of the digastric muscle. It passes through a dedicated sulcus or occipital groove in the temporal bone located medial to the mastoid sulcus or groove for the digastric muscle, and lateral to the mastoid foramen for an emissary vein from the sigmoid sinus. The OA continues lateral to the superior oblique muscle (obliquus capitis superior) and medial to the longissimus capitis muscle, and then travels beneath the splenius capitis muscle laterally and over the semispinalis capitis muscle medially. The OA can sometimes run under the semispinalis and penetrate this muscle. When the OA reaches the fascia of the cranial attachment of the trapezius, just inferior to the superior nuchal line, it ascends to the scalp suboccipitally, where it supplies the skin and the sternocleidomastoid muscles.

Fig. 14.4 (A) A posterior overview of neurovascular structures in the neck with the suboccipital muscles removed demonstrates the close relationship between the ECA branches and the vertebral arteries (cadaveric specimen, posterior view). (B) The PAA and the OA originate from the ECA before its terminal bifurcation into the STA and the IMA. (C) After coursing medial to the posterior belly of digastric muscle, the OA runs over the deep/third muscular layer forming the suboccipital triangle (superior oblique capitis, inferior oblique capitis, rectus capitis posterior major, and rectus capitis posterior minor). (D) The V3 VA lies inside the suboccipital triangle in the sulcus arteriosus of the C1 lamina, surrounded by venous plexus. (E) The superficial/first muscular layer of the neck is formed by the sternocleidomastoid muscle laterally and the trapezius muscle medially, both sharing a common tendon inserting at the superior nuchal line. The intermediate/second layer of muscles is formed by the splenius capitis, semispinalis capitis, and longissimus capitis. The distal or posterior intermuscular course of the OA runs under the splenius and emerges at its medial border between the splenius and the semispinalis. (F) The OA is always just below the periosteum or inner edge of the myocutaneous flap, enabling a transperiosteal “inside-out” harvest that begins at the occipital groove, follows the OA distally to the edge of the flap, and dissects superficially along the artery as it ascends through the three muscle layers.

Dissection Technique: Superficial Temporal Artery Bypasses

The STA-MCA bypass can be performed most simply through a linear incision over the course of the parietal branch of the STA that begins at the zygomatic arch and ends just above the superior temporal line (Fig. 14.5). This incision exposes an adequate length of donor artery and adequate room for a craniotomy over the Sylvian fissure, which is approximately 6 cm above the zygoma. This simple incision is modified depending on the STA anatomy, the underlying pathology, and the planned bypass. A curvilinear incision, beginning as a straight line over the parietal branch and then arcing anteriorly at the superior temporal line, is used when the parietal branch is small and the larger frontal branch is needed, or when both branches are needed for a double-barrel bypass. The scalp flap folds anteriorly with this incision to follow the frontal STA branch to the forehead. A larger incision extending anteriorly to the hairline in the midline is used when standard pterional or orbitozygomatic craniotomies are needed for an aneurysm or other pathology.

The secret to a fast, safe STA harvest is to use the microscope for better visualization. The skin is incised with a No. 15 blade through the dermal layer into the subcutaneous fat, and bleeding from both skin edges is controlled with cautery kept on high. An incision directly over the STA, followed by some spreading dissection with a fine tenotomy scissors, exposes it immediately, and upward traction on the scalp with a toothed forceps separates the skin and connective tissues from the STA. The length of the STA is exposed first by extending the skin incision superiorly to the superior temporal line, cauterizing the scalp bleeders, and spreading apart the dense connective tissue layer. With the STA in full view, the dissection deepens around its sides to divide its tiny branches and detach it from the surrounding connective tissues. The STA has a snaking course with anterior branches that originate at the anterior-most point on its curves, posterior branches that originate at the posterior-most point on its curves, and no branches on its medial wall against the temporalis fascia (see Chapter 5, Figure 5.4). These branches are cauterized several millimeters from the trunk and cut with microscissors or torn with the bipolar forceps (bipolar cutting technique; see Figure 5.7). A protective connective tissue cuff is left around the STA to protect the donor from dissection injury and preserve its supportive adventitia. This cuff maintains the STA’s serpentine morphology; its removal straightens and lengthens the donor artery to reach the recipient in a deep bypass.

The superficial temporal vein can be mistaken for the STA because it parallels the artery in the connective tissue, but it has larger caliber, thinner walls, darker color, and a straighter course. The vein may need to be separated from the STA to follow the artery. Harvest to the superior temporal line yields 8 cm of donor artery. The distal STA is left in continuity beyond the superior temporal line to preserve flow to the vertex until ready for anastomosis.

When the STA’s frontal branch is also needed for a double-barrel bypass or for a larger donor, the curvilinear scalp incision extends anteriorly, the scalp flap reflects forward, and the STA is dissected down from the flap, again to the superior temporal line at or near the keyhole. As its small branches are divided, the frontal branch mobilizes away from the scalp flap and a 6- to 8-cm length can be harvested (Fig. 14.5). The distal STA is left in continuity beyond the superior temporal line to preserve flow to the forehead until it is ready for anastomosis. It will come to lie underneath the reflected temporalis muscle, which will require some disassembly of the surgical field later to release this retracted muscle, access the frontal branch, transect it distally, and reroute it into the field.

The frontal branch of the STA can be harvested through the linear incision, without the anterior arc of the curvilinear incision, when only a short (3 to 5 cm) segment of donor artery is needed. The anterior scalp edge is elevated with toothed forceps and the artery is dissected down from the scalp with limited visualization. The harvested length of the STA is shorter than with a curvilinear incision, but usually reaches a cortical artery on the temporal side of the Sylvian fissure. Unlike the standard harvest that leaves the distal artery in continuity until it is ready for anastomosis, the frontal branch must be occluded with a temporary clip, transected, and pulled out from under the flap. This frees the STA trunk and allows it to mobilize posteriorly during the remainder of the dissection.

Harvesting the STA to the superior temporal line provides 6 to 8 cm of donor artery. The STA-M4 MCA bypasses typically require 5 to 6 cm of donor artery, because part of the distal artery is sacrificed when cleaning the connective tissue cuff and fish-mouthing the donor. Shorter lengths of STA may be limited to recipients on the temporal side of the Sylvian fissure. Bypasses to M2 and M3 recipients deeper in the Sylvian fissure require 6 to 8 cm of donor STA, and the STA-P2 PCA and STA-s2 SCA bypasses in the crural and ambient cisterns require more than 8 cm of donor STA. Bypasses to the ACA in the interhemispheric fissure require dissection all the way to the vertex and some stripping of the connective tissue cuff to use the artery’s full serpentine length, and even then may still require an interposition graft from another scalp branch. The STA branching beyond the superior temporal line markedly diminishes the STA’s caliber.

Fig. 14.5 Skin incisions for STA-MCA bypasses. (A) A linear incision over the parietal branch (black line) and a frontotemporal craniotomy (blue circle) are used for a simple STA-M4 MCA bypass. (B) The linear incision extending from the zygomatic arch to the superior temporal line exposes an adequate length of donor artery and room for a craniotomy over the Sylvian fissure. (C) A curvilinear incision (black line) and pterional craniotomy (pink oval) are used for a double-barrel STA-M4 MCA bypass, or when the parietal branch is diminutive. (D) The curvilinear incision beginning over the parietal branch and arcing anteriorly at the superior temporal line elevates a scalp flap that folds anteriorly to follow the frontal STA branch to the forehead.

Temporalis Muscle Dissection

Temporalis muscle dissection depends on the skin incision. With a linear incision, the temporalis is incised vertically, or in a line parallel to its muscle fibers and the course of the STA (Fig. 14.6). The muscle is then released superiorly with a transverse incision along its insertion at the superior temporal line, allowing the muscle to retract anteriorly and posteriorly. Two fishhook retractor systems hold the muscle flat against the head, whereas self-retaining retractors deepen the surgical field and prevent the hands from resting comfortably. With curvilinear and standard pterional incisions, temporalis muscle is incised vertically along the course of the STA, and then transversely along its insertion at the superior temporal line to the keyhole. A single fishhook retractor system pulls the muscle anteriorly. The temporalis is exposed as an intact muscle belly for use as an onlay graft to augment the direct STA-MCA bypass, if necessary.

Working on the temporalis muscle deep to the STA places the donor artery in harm’s way, particularly with a high or distal bifurcation or when both the frontal and parietal branches are prepared. The STA is most secure when the smaller of its two branches is transected and the larger one is kept in continuity, wrapped in a rubber sleeve, bathed in topical papaverine, and pulled off the field. The parietal branch is usually dominant, and the frontal branch is temporary clipped and cut. When the frontal branch is the selected donor, it is kept in continuity, the temporalis muscle is reflected on top of it, and it is retrieved when needed later. After craniotomy, dural opening, and preparation of the recipient artery, the dural and temporalis flaps must be rolled back to access this frontal branch, transect it, and bring it into the field. Dural retention sutures and muscle retractors should be easily readjustable, and fishhooks are usually used on the muscle and dura to shorten this interruption. After freeing the STA’s frontal branch, the temporalis muscle and dura are reset with the fishhooks and the bypass can proceed.

Fig. 14.6 Temporalis muscle incisions for STA-MCA bypasses. (A) With a linear skin incision, temporalis muscle is incised parallel to its fibers, then released superiorly with a transverse “T” incision along its insertion at the superior temporal line (black lines), which allows the muscle to retract anteriorly and posteriorly. (B) With a curvilinear skin incision, temporalis muscle is incised along its fibers, then anteriorly along its insertion at the superior temporal line to the keyhole (black lines). A single fishhook retractor system pulls the muscle anteriorly.

Craniotomy

Four different craniotomies are used with STA bypasses, depending on the skin incision and anastomotic site: frontotemporal, mini pterional, standard pterional, and orbitozygomatic craniotomy (Fig. 14.7). The frontotemporal craniotomy is used with the linear incision; it centers on the Sylvian fissure, and accesses the M4 MCA branches emerging from the fissure. The linear incision exposes the pterion minimally, which makes the craniotomy a convexity opening. The craniotomy is moved as anterior as the skin incision will allow, exposing larger recipients in the proximal Sylvian fissure. This craniotomy need not extend above the superior temporal line for routine STA-M4 MCA bypasses.

The mini-pterional craniotomy requires the curvilinear incision and is centered on the pterion. The craniotomy does not extend above the superior temporal line. This craniotomy flap lies directly over the Sylvian fissure for access to the M2, M3, and M4 recipients. The standard pterional craniotomy extending above the superior temporal line is used with a pterional incision and with pathology that requires more than just a bypass, such as an MCA aneurysm that must be exposed and occluded after the bypass.

The orbitozygomatic craniotomy is used with posterior circulation bypasses to the SCA and PCA for increased exposure, illumination, and maneuverability. With this craniotomy, STA dissection can continue inferior to the zygoma, down to the capsule of the parotid gland, which frees more donor length and exposes a larger-caliber proximal segment for an interpositional bypass, if necessary. The hairpin turn of the STA’s infrazygomatic segment should be appreciated to avoid injury, particularly when making the osteotomy cuts through the zygomatic root. In addition, facial nerve injury is avoided by not incising below the midlevel of the tragus. The dissection should remain above the galea, which acts as a protective anatomic barrier to facial nerve branches running in the subgaleal plane (especially the most posterior [temporal] branch).

Fig. 14.7 (A) The temporalis muscle can be used as an onlay graft that forms synangiotic connections between the deep temporal artery and cortical MCA branches. Four different craniotomies are used with STA bypasses. The frontotemporal craniotomy is used with a linear incision crossing the Sylvian fissure for access to the M4 MCA branches emerging from the fissure. (B) The mini-pterional craniotomy requires a curvilinear incision, does not extend above the superior temporal line, and is centered on the pterion for access to the M2, M3, and M4 recipients. (C) The standard pterional craniotomy extends above the superior temporal line and is used with pathology that requires more than just a bypass, such as a complex MCA aneurysm. (D) The orbitozygomatic craniotomy is used with posterior circulation bypasses for increased exposure, illumination, and maneuverability.

The STA is prepared for the bypass after the recipient artery is selected. The STA is temporary clipped and transected. Releasing the temporary clip and bleeding the donor artery are done to check patency and cut flow. In order for the STA to reach the recipient without tension, the recipient site is selected in advance and the STA is cut 1 to 2 cm beyond this length. Extra length slackens the donor for mobility around the anastomosis. A donor cut too short cannot be transposed from side to side for visualization, and when sewn, pulls on the anastomosis. Extra length is also needed to excise its connective tissue cuff, strip its adventitia, and fish-mouth its end. The cuff is thick, often more than twice the thickness of the STA itself. Precise suturing requires working with just the scalp artery’s intima and media. The adventitia and surrounding connective tissue make it more difficult to read the anatomic layers and take clean bites. Furthermore, this adventitial layer is thrombogenic and can be pulled by the needle into the suture line. The cuff is trimmed first with microscissors to expose the adventitia, and the adventitia is then stripped (Fig. 14.8). The arterial end (media and intima) is grasped firmly with one microforceps and pulled distally, while the adventitia is grasped with another microforceps and peeled back proximally. This retracts the STA’s adventitia, which is excised to leave a clean, white vessel. The retracting forces on the adventitia should be gentle enough not to tear the donor artery, whereas the grasping forces on the arterial end should be firm enough to resist retraction, which destroys the arterial tissues in their grasp. Therefore, the artery is held right at its end to minimize the loss and these tissues are trimmed from the fish-mouth arteriotomy.

Fig. 14.8 (A) Preparation of scalp arteries. (B) The connective tissue cuff at the transected end is trimmed with microscissors to expose the scalp artery. (C) The artery is grasped firmly with one microforceps on the transected end and the adventitia is grasped firmly with another microforceps on the opposite end. (D) The adventitia and connective tissue cuff are peeled proximally with counter-traction on the artery distally, thereby stripping the adventitia from the media. (E) Retracted adventitia and connective tissue cuff are excised to clean the arterial end. (F) Grasping the arterial end traumatizes the tissues and they are trimmed with the fish-mouth arteriotomy.

The cleaned STA is then arteriotomized to maximize anastomotic area. A fish-mouth arteriotomy is routinely used, with the donor transected obliquely at a 60-degree angle and the arteriotomy extended along the long axis of the donor artery a distance equal to the length of the transection. Adventitial cleaning should be completed before making the arteriotomy to minimize handling of the arteriotomized ends of the STA. Any additional cleaning is done sharply with microscissors to avoid pinching or damaging the end of the donor artery.

The EC-IC bypasses are the most common bypasses in my experience (59%), and the STA-MCA bypass is the workhorse (96% of all EC-IC bypasses) (Table 14.1). Most are performed for ischemic indications: moyamoya disease (48%), cervical carotid occlusion (18%), middle cerebral artery occlusion (14%), and supraclinoid internal carotid artery occlusion (8%) (Table 14.2). Adult patients with atherosclerotic occlusive disease have large donor and recipient arteries with thickened walls and occasionally plaque, calcification, and separation of the wall’s layers (Case 14.1). In contrast, the STA-M4 MCA bypass in children with moyamoya disease have recipients with sub-millimeter diameters, thin walls, and fragile tissues (Case 14.2).

Table 14.1 Summary of Clinical Experience with EC-IC Bypasses

Table 14.2 Indications for the STA-MCA Bypass

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