# Extracranial-Intracranial Bypass Surgery

## Overview and Historical Context

Extracranial-intracranial (EC-IC) bypass involves creating an anastomosis between an extracranial donor artery and an intracranial recipient artery to augment cerebral blood flow. The technique was first performed by Yasargil and Donaghy in 1967 using a superficial temporal artery (STA) to middle cerebral artery (MCA) anastomosis. The 1985 EC-IC Bypass Study, a landmark randomized controlled trial, showed no benefit for atherosclerotic ICA or MCA occlusion, dramatically reducing the procedure's popularity. However, resurgence of interest has been driven by Moyamoya disease, the hemodynamic subgroup concept, and improved patient selection tools.

## Indications

### Hemodynamic Compromise

EC-IC bypass is considered for symptomatic ICA or MCA occlusion with documented hemodynamic failure (stage II hemodynamic failure), recurrent ischemic events despite maximal medical therapy, and cerebrovascular reserve testing showing impaired vasoreactivity (increased oxygen extraction fraction on PET).

### Moyamoya Disease

Moyamoya disease involves progressive stenosis and occlusion of the distal ICA and proximal ACA/MCA. It is the most widely accepted indication for bypass in the modern era. Bilateral disease is common and staged procedures are recommended.

### Complex Aneurysms and Skull Base Tumors

When parent artery sacrifice is required (trapping, Hunterian ligation), bypass may be needed to maintain flow. Preoperative balloon test occlusion with hemodynamic assessment guides the need for bypass. High-flow bypass using a saphenous vein or radial artery graft is appropriate for large-caliber vessel sacrifice.

## Types of Bypass

### Low-Flow Bypass

The STA-MCA anastomosis is the most common bypass. The donor vessel is the superficial temporal artery (frontal or parietal branch) and the recipient is an M4 cortical branch of the MCA. Flow through the anastomosis is approximately 15-25 mL/min. The technique involves an end-to-side anastomosis with 10-0 nylon under microscopic magnification.

### High-Flow Bypass

This involves an external carotid artery to M2/M3 or P2 anastomosis using an interposition graft. Graft options include the saphenous vein or radial artery, providing flow of approximately 70-120 mL/min. High-flow bypass is required when large-caliber flow replacement is needed, such as after ICA sacrifice.

### Indirect Revascularization

Indirect techniques include encephalo-duro-arterio-synangiosis (EDAS), encephalo-myo-synangiosis (EMS), and multiple burr holes with dural inversion. These rely on neoangiogenesis from vascularized tissue laid on the brain surface. They are preferred in pediatric Moyamoya and can be combined with direct bypass in adults.

| Bypass Type | Donor | Recipient | Flow Rate | Primary Indication |
|-------------|-------|-----------|-----------|-------------------|
| Low-flow (STA-MCA) | STA (frontal or parietal branch) | M4 cortical branch | 15-25 mL/min | Moyamoya, hemodynamic failure |
| High-flow (interposition graft) | ECA via saphenous vein or radial artery | M2/M3 or P2 | 70-120 mL/min | Parent vessel sacrifice (aneurysm, tumor) |
| Indirect (EDAS, EMS, burr holes) | Vascularized tissue on cortex | Neoangiogenesis | Variable (delayed) | Pediatric Moyamoya |

## Surgical Technique: STA-MCA Bypass

### Preoperative Planning

Planning involves CTA or DSA to identify donor (STA) and recipient (M4) vessels, Doppler ultrasound to map the STA course and mark it on the skin, general anesthesia with normotension while avoiding hypotension, and continuation of antiplatelet therapy perioperatively.

### Steps

The patient is positioned supine with the head turned contralateral in Mayfield fixation. The STA branch is harvested with a cuff of surrounding tissue and the end is spatulated. A small craniotomy (approximately 3 cm) is performed centered over a suitable M4 branch. The dura is opened and a cortical recipient artery identified. Temporary clips are placed on the recipient vessel and an arteriotomy matching the donor caliber is made. An end-to-side anastomosis is performed with interrupted or running 10-0 nylon (8-12 sutures). Clips are released and patency is confirmed with micro-Doppler and ICG fluorescence angiography. The dura is closed loosely around the pedicle and the bone flap is replaced with a groove for the STA.

### Postoperative Care

Patients are monitored in the ICU for 24-48 hours. Hypotension is avoided (maintaining SBP above 120 mmHg) and blood pressure swings are minimized. Serial neurological examinations are performed. Doppler or CTA confirms graft patency within 24 hours. Antiplatelet therapy continues postoperatively while anticoagulation is held.

## Key Trials and Evidence

### EC-IC Bypass Study (1985)

This trial enrolled 1,377 patients with symptomatic atherosclerotic ICA or MCA disease, randomizing them to STA-MCA bypass plus medical therapy versus medical therapy alone. There was no benefit of surgery for stroke prevention or mortality. The study was criticized for lacking hemodynamic selection criteria.

### Carotid Occlusion Surgery Study (COSS, 2011)

COSS selected patients with symptomatic ICA occlusion and increased oxygen extraction fraction on PET (stage II hemodynamic failure), randomizing them to STA-MCA bypass versus medical therapy. The trial was terminated early for futility (21% versus 22.7% ipsilateral stroke at 2 years). The perioperative stroke rate was 15% in the surgical arm. The results suggest that even hemodynamically selected patients may not benefit, though the medical therapy arm improved more than expected.

### Japanese EC-IC Bypass Trial (JET, 2002)

This non-randomized study showed benefit in hemodynamically compromised Japanese patients but was criticized for methodological limitations. It supports bypass in Moyamoya (where evidence is strongest though largely observational).

## Moyamoya-Specific Considerations

Moyamoya is staged using the Suzuki system (I-VI) based on angiographic progression. Direct bypass (STA-MCA) is the gold standard in adults. Indirect procedures (EDAS, EMS) are preferred in children under age 3-4 due to small vessel caliber. Combined direct plus indirect procedures are increasingly advocated in adults. Perioperative management must avoid hyperventilation (which worsens ischemia), maintaining normocapnia and euvolemia.

## Complications

Perioperative stroke (ischemic or hemorrhagic) occurs in 3-15% depending on the series. Hyperperfusion syndrome after bypass manifests as headache, seizures, or hemorrhage. Graft thrombosis or occlusion may occur. Scalp wound healing problems are seen especially after STA harvest. Subdural hematoma or hygroma can develop.

<image>Intraoperative photograph of a superficial temporal artery to middle cerebral artery (STA-MCA) end-to-side anastomosis performed under high-power microsurgical magnification, showing the spatulated STA donor vessel sutured to an M4 cortical branch with 10-0 nylon sutures, with temporary clips visible on the recipient artery</image>

<image>Cerebral digital subtraction angiography (DSA) in a patient with Moyamoya disease, lateral projection, demonstrating bilateral distal internal carotid artery stenosis with extensive basal collateral networks giving the characteristic puff-of-smoke appearance on contrast injection</image>

<image>Diagram illustrating the different types of extracranial-intracranial bypass: low-flow STA-MCA direct anastomosis, high-flow ECA-to-MCA saphenous vein interposition graft, and indirect revascularization techniques including EDAS with the superficial temporal artery laid on the cortical surface beneath the arachnoid</image>

<image>PET scan showing increased oxygen extraction fraction (OEF) in the left MCA territory of a patient with symptomatic left ICA occlusion, indicating stage II hemodynamic failure with misery perfusion, compared to a normal contralateral hemisphere</image>

## Clinical Pearls

The strongest indication for EC-IC bypass today is Moyamoya disease, not atherosclerotic occlusion. Intraoperative patency assessment with ICG videoangiography or micro-Doppler should always be performed. Hyperperfusion syndrome after bypass can mimic stroke; maintaining close blood pressure control postoperatively is essential. In patients requiring parent vessel sacrifice, balloon test occlusion with hypotensive challenge and perfusion imaging should guide the need for bypass. The failure of COSS does not negate bypass utility for Moyamoya or tumor-related vessel sacrifice; trial results apply specifically to atherosclerotic ICA occlusion. For pediatric Moyamoya, indirect revascularization alone often provides excellent long-term results due to robust angiogenic capacity.

## References
- EC/IC Bypass Study Group. "Failure of Extracranial-Intracranial Arterial Bypass to Reduce the Risk of Ischemic Stroke." *NEJM*. 1985;313(19):1191-1200.
- Powers WJ, et al. "Extracranial-Intracranial Bypass Surgery for Stroke Prevention in Hemodynamic Cerebral Ischemia: The Carotid Occlusion Surgery Study Randomized Trial." *JAMA*. 2011;306(18):1983-1992.
- Scott RM, Smith ER. "Moyamoya Disease and Moyamoya Syndrome." *NEJM*. 2009;360(12):1226-1237.
- Yasargil MG. *Microsurgery Applied to Neurosurgery*. Stuttgart: Georg Thieme Verlag; 1969.
- Sekhar LN, Kalavakonda C. "Cerebral Revascularization for Aneurysms and Tumors." *Neurosurgery*. 2002;50(2):321-331.