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  • If atherosclerosis of the aneurysm neck or the parent artery can make clipping dangerous or impossible;
  • If recurrent aneurysms after endovascular coil embolization may be unclippable.


Bypass types

  • Type I bypass - Interposition vein grafts
  • Type II bypass - Extracranial-to-intracranial bypass with a saphenous vein or radial artery graft
  • Type III bypass - Scalp artery (STA or occipital) extracranial-to-intracranial bypass
  • Type IV bypass - Direct intracranial revascularization


Type I bypass

Type I bypass involves an interposition graft from the parent artery proximal to the site of occlusion to the point immediately distal to the parent artery.
The primary example is the purely intracranial petrous carotid-to-supraclinoid carotid saphenous vein interposition graft.

  • The most important disadvantage is that it requires prolonged occlusion of ICA;
  • Being technically complex and requiring a lengthy procedure are also disadvantages;
  • It is associated with a significant complication rate related to graft occlusion and perioperative ischemic brain injury.

Type II bypass

Type II bypass consists of extracranial-to-intracranial bypass with a saphenous vein graft or radial artery graft. This procedure is used when major arterial trunk must be occluded to treat tumor or giant aneurysm and distal collateral circulation is inadequate.

  • A vein graft generally has lower long-term patency, higher risk of kinking, and caliber mismatch between the larger vein and smaller intracranial vessels;
  • A type II bypass can be a substitute for an STA-MCA bypass when the scalp artery is hypoplastic or occluded.

Type III bypass

It is extracranial-to-intracranial bypass via scalp artery (superficial temporal or occipital).
This procedure is performed when a giant aneurysm requires occlusion of a single, crucial arterial branch and when carotid occlusion is required and the circle of Willis is only marginally inadequate.

  • Pedicled scalp artery is used as the donor vessel;
  • The STA is used to revascularize the MCA territory, as well as the distal posterior circulation via the superior cerebellar artery or PCA;
  • The occipital artery is most commonly used for bypass to the PICA, but it can also be used to revascularize the AICA as well;
  • STA-MCA bypass includes a craniotomy centered 6 cm above the external auditory meatus (Chater’s point), where several large MCA branches emerge from the distal sylvian fissure.

Type IV bypass

It's direct intracranial revascularization and involves an anastomosis between two adjacent cerebral arteries.

  • This type of procedure can involve end-to-end primary reanastomosis after excision of an aneurysm;
  • Examples include PICA-PICA, pericallosal-pericallosal, MCA-MCA, and PICA-AICA anastomoses;
  • This type of procedure can involve end-to-end primary reanastomosis after excision of an aneurysm, side-to-side anastomosis of two adjacent intracranial arteries, or an end-to side anastomosis between two cerebral arteries;
  • Pericallosal-to-pericallosal bypass can be used to treat fusiform aneurysms of the proximal pericallosal artery or for giant anterior communicating artery aneurysms that require trapping, this side-to-side anastomosis serves as a new communicating artery;
  • A PICA-to-PICA anastomosis can be used when the occipital artery is small or has been damaged during a previous surgical procedure.


  • Treatment options include clip occlusion of the aneurysm along with the branching vessel or vessels, parent artery Hunterian ligation, and trapping;
  • Not all proximal vessel occlusion often results in ischemia, and proximal vessel occlusion (ICA or vertebral artery) may be tolerated without bypass;
  • Occlusion of branches such as the MCA, AICA, and PICA often result in ischemia, and therefore bypass should be considered;
  • Preoperative evaluation by angiography and balloon test occlusion (with or without hypotensive provocative testing or cerebral blood flow measurements) may aid in the identification of patients who can tolerate carotid sacrifice;
  • Even if the patient passes a balloon test occlusion, there remains up to a 20 % chance of stroke with complete occlusion without a bypass.



  • Normal blood flow of the MCA is about 250 mL/min;
  • Blood flow of saphenous vein graft is 70–140 mL/min;
  • Blood flow of radial artery graft is 40 and 70 mL/min;
  • Blood flow of STA graft is 15–30 mL/min;
  • Blood flow through a saphenous vein graft, which averages about 4–5 mm in diameter, is high enough to support the circulation in an entire major arterial territory at a level well above the ischemic threshold;
  • Blood flow of saphenous vein graft typically ranges from 70 to 140 mL/ min and can exceed 250 mL/min;
  • The Radial artery graft can be used, which has a smaller diameter than saphenous vein graft (about 3.5 mm);
  • In saphenous vein graft, it is important to pass the saphenous vein so that the end that was proximal in the leg is the end used for the cranial, distal anastomosis (because of the unidirectional valve arrangement).



  • The most serious acute complication is early graft occlusion;
  • If there is any doubt about the integrity of the graft, intraoperative angiography should be considered;
  • Rate of ischemic complications and graft occlusion typically exceeds 10 %;
  • A normal flow signal should be confirmed with intraoperative Doppler assessment;
  • Subdural or epidural hematomas (or both) developed postoperatively, some of these hematomas were small and asymptomatic.




Hoz, Samer S. Vascular Neurosurgery. 1st ed. Springer, 2017




Samer S. Hoz, MD

Neurosurgeon Consultant
"Neurosurgery Teaching Hospital"
Baghdad (Iraq)