Bypass surgery allows blood to enter the brain through a surgically created alternate route. This is similar to how heart bypass surgery works. The alternate pathways can either be from arteries that normally supply blood to the scalp, a connection between the arteries of the neck to brain arteries with a length of vein taken from the leg or from muscle over the head.
Why may bypass surgery be necessary?
Bypass surgery for the brain is sometimes necessary when blood flow needs to be increased to prevent stroke or where blood flow needs to be interrupted to remove an aneurysm or tumour.
How is bypass surgery performed?
Bypass surgery is microsurgery. It requires experience in joining small vessels together under the microscope. Two types of surgery will be described below: in the case where this surgery involves taking the artery from the scalp to join onto the brain artery, the most common of these operations is called superficial temporal to middle cerebral artery bypass; in the case where vein is used between the carotid artery in the neck to the internal carotid artery inside the skull, the operation is called common carotid to intracranial internal carotid interposition saphenous vein graft.
Superficial temporal to middle cerebral artery bypass:
scalp artery entering skull after surgery
This surgery commences by making a cut in the skin directly overlying a scalp artery. After enough length of artery has been exposed a small disk of bone is removed from the skull overlying the brain artery that is to be joined. With the assistance of magnification through a microscope, the end of the scalp artery is joined to the side of the brain artery. During this time the blood flow in the brain artery is temporarily stopped for about 20 minutes. The bone is then replaced with a divot in the bone disk to accommodate the path of the scalp artery entering the skull.
Common carotid to intracranial internal carotid interposition saphenous vein graft:
This surgery commences by making a cut in the skin overlying the carotid artery in the neck. After the carotid artery in the neck is exposed an incision is made in the scalp from in front of the ear to arc across the head behind the hair-line crossing the midline by 2 cm. A disc of bone from the skull is then removed from above and behind the eye and temporal region. The main valleys of the brain are then opened with the microscope to expose the large arteries of the brain – the internal carotid artery and the middle cerebral artery. Once this is done, vein is removed from the leg (usually below the knee) and placed in a tunnel under the skin between the artery of the brain and the carotid artery in the neck. The end of the vein is then joined to the side of the middle cerebral artery or the divided end of the internal carotid artery (that goes to the brain). After successfully joining the brain end, the vein is then joined to the side of the carotid artery in the neck. The period of time that may be required in blocking flow to the brain during this surgery averages 40 minutes.
vein in blue between neck and brain artery
joining the vein to the brain artery
CTA of bypass with vein
Choosing the bypass type.
Bypass surgery of any kind is only recommended when no other course of action to reduce stroke is reasonable. The choice of bypass is made based upon how much blood flow is required and the risks, expectations and alternatives of such surgery are considered. Our group has performed over 400 of these bypasses of which more than 150 are common carotid to intracranial internal carotid interposition saphenous vein grafts (vein bypass). The early risks of stroke or blockage associated with superficial temporal to middle cerebral artery bypass (scalp artery bypass) is of the order of 5% of cases as compared with 10% for vein bypass. However, the scalp artery bypass cannot carry as much blood flow as the vein bypass and is not used where it is known or suspected that very high flows are required (e.g. when the internal carotid artery needs to be removed to treat a tumour or an aneurysm). Sometimes, it is possible to test how much blood flow is required by doing a test balloon occlusion during angiography. This test involves the radiologist doing an angiogram and placing a balloon in the carotid artery that temporarily stops blood flowing in this artery. You will be monitored during this test to closely observe changes that indicate your ability to cope with reduced blood flow in this artery. At the end of the test it may be possible to decide that no bypass is required, that bypass with the scalp artery will be sufficient or that a vein bypass is required. What will I be like immediately after the bypass surgery?
Following superficial temporal to middle cerebral artery bypass you will feel back to your preoperative self within hours. You will have pain and some swelling over the scalp incision but this will not be present for long. You will be discharged from hospital within seven days of surgery and be capable of doing most activities that you were performing before surgery. You will almost certainly require to be taking aspirin. Following vein bypass surgery you will be kept in induced coma for a short period as the length of the temporary blockage of blood flow during your surgery is much longer than for superficial temporal to middle cerebral artery bypass. In addition, you will need medication to reduce the chance of a reactive narrowing of the blood vessels occurring about seven days after surgery. This requires you to stay in intensive care for more than a week following surgery. Because of the extent of the surgery you will have more swelling of the face and eye that takes 2 weeks to settle. The length of stay for this surgery is often more than 14 days. You will almost certainly require to be taking aspirin and warfarin.
How long do the bypasses last?
Our experience suggests that both the vein bypass and the superficial temporal to middle cerebral artery bypass last many years and do not have a tendency to narrow with time.
