22 OA–MCA or OA–PCA Bypass
Abstract
When the parietal and frontal superficial temporal arteries are not available as a direct donor to the middle cerebral artery (MCA) territory, the occipital artery (OA) can often be utilized. Additionally, due to significant leptomeningeal collateralization from the posterior circulation to the anterior circulation, progressive steno-occlusion of the posterior cerebral artery (PCA) in advanced moyamoya disease (MMD) can result in inadequate collateralization of the MCA or PCA territory. The OA is also an appealing donor choice for revascularization of these territories due to its close proximity to the target recipient vessels; however, there are challenges associated with this strategy, such as tortuosity, multiple branches, and thick adherent investing tissue, which can result in inadvertent OA injury during vessel harvest. In this chapter, we highlight the steps and nuances in performing the challenging OA–MCA or OA–PCA bypass, the techniques used to troubleshoot inadvertent OA occlusion or injury, and illustrate with appropriate case examples. We also include the preoperative workup, intraoperative patient positioning, a step-by-step description of key procedures, and the long-term clinical and radiological postoperative care and surveillance, all of which contribute to achieving maximal benefits of direct bypass to the posterior MCA or PCA in select patients with MMD vasculopathy.
22.1 Background
Moyamoya disease (MMD) is a progressive angiopathy that, over time, can involve the posterior circulation. Because the posterior circulation provides significant collateralization, especially to the anterior circulation, stenosis or occlusion of the posterior cerebral artery (PCA) can also affect the anterior parts of the brain. When the target recipient vessel for bypass is the posterior middle cerebral artery (MCA), or posterior circulation, the occipital artery (OA) is a particularly appealing donor choice.
22.1.1 History
1970—Yasargil and colleagues reported the first successful extracranial to intracranial (ECIC) bypass.
1976—Khodadad reported the first successful OA–posterior interior cerebellar artery (PICA) bypass for a 58-year-old man with posterior circulation ischemic symptoms and a postoperative angiogram confirming bypass graft patency.
Apart from ischemic neurological disease involving the posterior circulation, OA grafts are frequently used for PICA bypasses in the cases of PICA aneurysms that required parent vessel sacrifice.
Recently, OA grafts have increasingly been used for posterior MCA or PCA revascularization in MMD.
22.2 Indication
Repeat or initial revascularization for MMD patients with inadequate collateralization of the MCA or PCA territory, when no suitable superficial temporal artery (STA) donor is available.
Key Principles
The harvest of OA is challenging due to its tortuosity, multiple branches, and thick investing fascia.
Harvest an adequate length of OA to reach the territory to be revascularized.
Harvest the main OA trunk, as smaller secondary tributaries are not usually of adequate size for direct bypass.
SWOT Analysis
Strength
Good donor option in the absence of suitable STA, when posterior MCA or PCA territories are to be revascularized.
Weakness
OA is tortuous, has multiple branches, and thick adherent investing tissue.
Opportunity
Stereotactic CT angiography of OA to accurately locate and harvest the OA branch to be utilized.
Threat
Inadvertent injury resulting in occlusion of OA donor during dissection.
Contraindications
Poor patient medical or neurologic condition excluding patients as candidates for cranial bypass.
OA with extensive preexisting ECIC collateralization.
22.5.1 Relative Contraindications
Patients with recent cerebral infarct should undergo diffusion-weighted imaging (DWI) on magnetic resonance imaging (MRI) of brain (within 1–2 weeks), as the risk of perioperative strokes is significantly increased.
22.6 Special Considerations
On angiogram, carefully study the OA tributaries and sizes as well as determine the branch to be harvested.
Assess existing ECIC collateralization provided by OA on cerebral angiograms so as to determine if harvesting OA as a donor graft might lead to more ischemic brain injury.
Optimize patient positioning for the brain territories to be revascularized, and the OA vessel to be harvested.
22.7 Risk Assessment—Stanford Experience
From 1991 to 2016 we have performed 1,440 ECIC bypasses for MMD, of which 1,252 (87%) were direct bypasses. OA–MCA or OA–PCA bypasses were used for repeat revascularization in eight cases.
Age of patients ranged from 8 to 60 years, with a mean follow-up of 6 years (range: 1 to 15 years).
None of the eight patients experienced perioperative infarction. One patient developed a postoperative transient neurologic deficit (TND) that resolved within 10 days.
At last follow-up, all patients had preoperative symptom resolution, with follow-up cerebral angiogram confirming patent bypass grafts of all OA–MCA or OA–PCA revascularizations.
22.8 Preoperative Workup
Preoperatively, patients underwent a thorough medical, cardiac, and anesthetic assessment with routine preoperative labs and the relevant diagnostic imaging, which includes 6-vessel cerebral angiogram, MRI brain, and cerebral perfusion imaging with and without Diamox (positron emission tomography, MR perfusion, TCDs). At our institution, we perform MR perfusion with and without Diamox and patients demonstrating poor cerebrovascular reserve or steal (indicating that the affected vascular territory is already maximally vasodilated to promote flow) are considered especially at high risk for ongoing ischemia without treatment. These patients are also at higher risk for perioperative ischemic complications, thus particular care is taken to avoid hypotension perioperatively and during the recovery period. Intraoperatively, the patient’s blood pressure is maintained at or above the preoperative baseline at all times.
22.8.1 Specific Consideration with Anticoagulation
For patients with mechanical heart valves or recent venous thromboembolism, we restart anticoagulation at 2 to 4 weeks postoperatively after CT head confirms no significant hemorrhage.
Aspirin is continued through the day before surgery, withheld on the day of surgery, and restarted on the first postoperative day.
22.9 Patient Preparation
22.9.1 Patient Position with Skin Incision
Depending on the vascular territory to be revascularized, positioning options are: (1) prone (for occipital region, PCA territory; Fig. 22‑1 a) or (2) supine, head turned with the parietotemporal uppermost (posterior MCA territory).
A Mayfield head holder secures the head position.
A Doppler probe is used to map out the course of the OA (Fig. 22‑1 b)
Skin incision options include: (1) horseshoe flap (Fig. 22‑1 b) or (2) linear incision over the vessel to be harvested.
Fig. 22.1 (a) Prone position for occipital artery–posterior cerebral artery (OA–PCA) bypass, or alternatively supine, head turned so that the parietotemporal is uppermost to vascularize posterior middle cerebral artery territory. (b) Doppler is used to map out the course of OA, and horseshoe flap for the OA harvest. Alternatively, linear incision over the main trunk of OA to be harvested can also be used.
22.10 Surgical Steps
22.10.1 Key Procedural Step 1: OA Harvest
Use microscopic guidance.
For a linear incision, make the superficial skin incision through the epidermis and partial thickness of the dermis starting above the suboccipital muscle over the vessel.
To avoid damage to the OA, dissect through the remaining dermis and subcutaneous tissue using blunt-tip, fine curve scissors.
Once the OA is visualized, carefully dissect along the main tributary with meticulous hemostasis to harvest enough length of the vessel to reach the recipient territory.
For dissection of the OA using a horseshoe flap, the vessel is dissected from the underside of the scalp flap (Fig. 22‑2 a).
Fig. 22.2 (a, b) Using a horseshoe flap, the occipital artery (OA) vessel is dissected from the underside of the scalp flap. Note the thick scalp tissue overlying OA vessel, and the underlying pericranium to be dissected for the craniotomy. (c) Craniotomy performed over the parietotemporal or occipital region depending on whether the posterior middle cerebral artery or posterior cerebral artery is being revascularized. It is important to be mindful of the superior sagittal sinus and transverse sinus when raising the bone flap.
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