Donors and Recipients

Pairing

Planning a bypass entails simply pairing the recipient artery in need of revascularization with a donor artery that may be intracranial, extracranial, cervical, or beyond. The variety of donors makes the donor artery’s location a defining characteristic of the bypass and a defining aesthetic of the bypass surgeon. Donors for reimplantations, reanastomoses, and in-situ bypasses are limited to adjacent arteries, but interposition grafts increase donor options. Scalp arteries are the most common donors, and their mobility after harvesting enables them to reach diverse recipient sites. Although segmental sites of the recipient anastomosis vary, recipient arteries lie in four major territories: MCA, ACA, PCA/SCA, and PICA. The segmental address dictates the recipient’s caliber and the anastomosis’s depth (Table 5.1). Donor–recipient pairing is often a choice between what is easiest and more traditional on the one hand (i.e., the EC-IC bypasses), and what is innovative and more challenging on the other hand (i.e., the IC-IC bypasses).

Superficial Temporal Artery

The EC-IC bypass is an option in almost every bypass case and often involves the STA (Fig. 5.1). This versatile donor was used in over three quarters of my bypass cases and was usable in even more cases. It is an easy and almost universal donor because is lies along the skin incision of the pterional craniotomy (Fig. 5.2). One simple reality is that if a donor artery is harvested and ready for use, the chances of performing a bypass increase significantly; one corollary is that if a donor artery is not harvested and ready, a bypass is not likely to be performed. Therefore, STA should be mapped with the Doppler flow probe, harvested, and prepared in advance if the thought of performing a bypass even crosses the mind.

Fig. 5.1 (opposite) Anatomy of the scalp arteries for EC-IC bypass. (A) 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 scalp supplier and the smaller of the ECA’s two terminal branches, the other one being the internal maxillary artery. The STA originates within the parotid gland deep to the facial nerve and ascends over the zygomatic root, anterior to the external auditory canal. 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) branches. The PAA originates from the ECA distal to the OA and divides into auricular and stylomastoid branches, but its small caliber and unfavorable location limit its use in EC-IC bypasses. (B) 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 IMA. 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 the occipital groove, a dedicated sulcus in the temporal bone 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. It continues lateral to the superior oblique muscle 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 reaches the fascia of the cranial attachment of trapezius, just inferior to the superior nuchal line, and ascends to the scalp suboccipitally. (C) Superior view of the head showing four of the five arteries supplying the left scalp: supratrochlear, supraorbital, superficial temporal, and occipital arteries (PAA not shown). 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 the 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. 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.

Harvesting the STA should take about 20 minutes. Keys to a quick harvest include incising the skin directly over the posterior or parietal branch, dissecting the donor under the microscope to see branches and tissue planes, applying upward traction on the scalp with a toothed forceps to separate the dermal layer and subcutaneous fat from the STA (Fig. 5.3), and dialing the bipolar cautery high to control scalp bleeding and avoid using Raney clips. The initial cut-down to the artery exposes its 8-cm course from the zygoma to the superior temporal line, which is sufficient for all but the deep bypasses to the SCA/PCA. Next, the artery is freed from its connective tissue attachments. The STA has a serpentine morphology; branches running anteriorly originate at the anterior-most point on the serpentine curve, and those running posteriorly originate at the posterior-most point on the curve (Fig. 5.4). There are few branches originating from the lateral wall and none from the medial wall. This anatomy makes it easy to find, cauterize, and cut branches 1 to 2 mm from the trunk.

Fig. 5.2 The scalp has five layers, captured in the mnemonic “SCALP:” skin, dense connective tissue, galea aponeurosis, loose connective tissue, and pericranium. These layers are appreciated best at the vertex where the galea courses between the frontalis and occipitalis muscles, but are less relevant laterally and posteriorly in the regions of the STA and OA, respectively (cross-sectional view). Scalp arteries reside in the subcutaneous tissue beneath the skin and above the galea aponeurotica, and periosteum in these regions is replaced by muscle and muscular fascia.

Fig. 5.3 Keys to a quick harvest: incise the skin directly over STA; dissect under the microscope to see branches and tissue planes; apply upward traction on the scalp with a toothed forceps to separate the dermal layer and subcutaneous fat from the STA; and dial the bipolar cautery high to control scalp bleeding.

Leaving a protective cuff of connective tissue around the STA keeps the dissection away from the arterial wall and decreases the risk of donor injury. The superficial temporal vein typically parallels the STA and has larger caliber, thinner walls, darker color, and no serpentine morphology (Fig. 5.5). The vein may need to be separated from the STA to clearly follow the artery’s course. The STA is released from the scalp, but left in continuity with its distal connections beyond the superior temporal line to maintain flow until it is ready for anastomosis. When the anterior limb is also needed for a double-barrel bypass, the scalp incision is extended anteriorly and the artery is followed into the reflected scalp flap (Fig. 5.6). Again, with the division of small branches, 6 to 8 cm of artery mobilizes from the scalp flap and allows its rerouting into the field. Even with only a linear incision along the parietal limb of STA, dissection under the flap with upward traction on the scalp will expose 3 to 5 cm of artery, which may be enough for a bypass to a temporal recipient artery on the cortical surface.