Contents of This Section:

1. What is a brain bypass?

A brain bypass is the brain's equivalent of a heart bypass. It involves rerouting blood flow around a blocked or damaged or abnormal blood vessel so that the region of brain involved can continue to get blood supply. If the bypass is from outside of the head to inside of the head, it is referred to as an extracranial-to-intracranial (EC-IC) bypass (see Figure 1, below). This is an important, technically challenging and potentially very helpful procedure in select patients with appropriate clinical circumstances. Brain bypass operations are carried out by a relatively few neurosurgeons with experience in this area of neurovascular surgery.

2. Why is a brain bypass done?

Brain bypass is carried out for two main sets of reasons:

1. Symptomatic blockage or occlusion or traumatic injury of a major brain artery such as the internal carotid artery or the main trunk of the middle cerebral artery. The occlusion may be due to moya moya disease, atherosclerosis, penetrating or intraoperative injury, dissection and so forth ( take me to the section on Moya Moya Disease or Carotid Artery Disease now).
2. A brain aneurysm that cannot be successfully obliterated using a clip or coil, but instead its parent artery requires to be sacrificed for the aneurysm to be effectively treated.

3. How is a patient evaluated for brain bypass?

The radiological workup in patients being considered for brain bypass may include one or more of the following:

1. Cerebral angiogram with or without trial balloon occlusion (TBO): For TBO, during the angiography, a balloon deployed from the tip of the catheter is inflated to see if the patient becomes symptomatic from poor brain blood flow during the test. The angiogram is studied meticulously by the neurosurgeon who will note the caliber of the vessels involved, the state of the circle of Willis and collateral pathways, and the presence of any vessel abnormalities. Intentional and temporary compression of the neck's carotid artery may also be carried out during vertebral angiography (Allcock maneuver) to look at the state of the circle of Willis ( take me to the section on Brain Arteries now).
2. PET scan: A safe dose of a radioisotope is administered intravenously to see the regional differences in blood flow and metabolism between the left and right hemispheres of the brain. The oxygen extraction fraction (OEF) is calculated. If this number is high on one side of the brain, it means that part of the brain is trying to extract as much oxygen as it can because it is in an abnormally high state of need. Sometimes a medication known as Diamox (acetazolamide) is administered during PET scanning to see if the brain has any "circulatory reserve".
3. Other studies: SPECT and CT-perfusion scans can also be done as alternatives to PET scan. So can quantitative MR angiography (qMRA or NOVA).

4. How is a brain bypass done?

Brain bypass is a very complicated and intricate procedure. It involves many meticulous and time-consuming steps. It will involve a craniotomy, and may involve exposure of the carotid artery in the neck. It may also involve the harvest of a vascular graft from a remote or relatively distant site in the patient such as the arm (radial artery bypass graft, RABG) or leg (saphenous vein bypass graft, SVBG). A donor artery (which is not a graft) may also be harvested locally in some patients from an artery in the scalp flap itself. Such donor arteries include the superficial temporal artery (STA), and the occipital artery (OA). The recipient vessel is the artery in the brain thought to be suitable by the neurosurgeon, and into which the graft or the donor vessel can be successfully attached or anastomosed. It may be a branch of the middle cerebral artery (MCA), posterior cerebral artery (PCA), posterior-inferior cerebellar artery (PICA), and so forth. There are many types of bypass configurations. Surgeons refer to these as end-to-side, side-to-side, end-to-end, and so forth.

Sometimes, there is an onlay of tissue involved as part of the operation, particularly for patients with moya moya disease. Here, the onlay involves local vascular muscle and vascular dural tissue that is layed onto the brain surface in addition to the STA-MCA bypass procedure ( take me to the section on Moya Moya Disease now).

The actual joining of vessels during a bypass is done using microinstruments, microsuture, and an operating microscope. A new laser technique is also being used by some surgeons as an alternative to suture material for certain brain bypasses.

Brain Bypass:

Figure 1 shows a schematic of a EC-IC bypass. In this patient, the internal carotid artery in the neck is occluded or completely blocked (O) owing to a dissection, thereby requiring a brain bypass to preserve flow to the brain. The donor artery (D) in this case is the external carotid artery at its origin in the neck. The recipient artery (R) is a large branch of the middle cerebral artery in the brain. The bridge between the donor and recipient arteries is the graft itself (G), in this case a saphenous vein graft harvested from the patient's leg. In the brain (circle inset), the graft is suture-anastomosed (A) to the recipient artery. This complex operation requires a craniotomy, neck dissection, and vein harvest.

5. Images of a brain bypass operation

A young patient presented to our hospital with symptoms of poor blood flow to the right half of her brain and was diagnosed with symptomatic spontaneous right internal carotid artery dissection. ( take me to the section on Carotid Artery Disease now). She underwent a brain bypass procedure carried out to enhance flow to her right brain "hemisphere". Doctor Khurana's team at The Canberra Hospital took ("harvested") an artery in her arm (the radial artery) and grafted it between her neck's right external carotid artery and her brain's right middle cerebral artery branch. Owing to the extra and much needed blood flowing to the right half of the patient's brain via the radial artery brain bypass, the patient no longer has any symptoms of her carotid dissection, and has returned to a normal and independent life. Here are some of her images:

Image 1 (above). The above left figure shows an magnetic resonance angiogram (MRA) image of the patient's spontaneously dissected internal carotid artery (the normal one with good flow is shown with green arrow heads, the abnormal one with poor or "severely compromised" flow is shown with red arrow heads). The good calibre external carotid artery is shown by the large yellow arrow. This vessel will represent the "donor" artery in the operation. The above right-upper figure is a cerebral angiogram showing the dissected artery (red cirle). Notice how it's calibre dramatically thins indicating the dissection, the thin column of blood (red arrow heads) passing through this artery to the brain's right hemisphere is very inadequate compared with its normal "pefusion". The above right-lower figure shows a vascular surgery team and a neurosurgery team at The Canberra Hospital operating simultaneously during this brain bypass procedure.

Image 2 (above). Above left-upper panel shows the "graft" artery, namely the radial artery (RA) meticulously harvested by our vascular surgeons from the left (nondominant) forearm of this patient. A temporary blue "vascular loop" is gently placed around the radial artery. The above right-upper panel shows the "donor" artery for this operation, namely the normal right external carotid artery (ECA). This is where one end of the radial artery graft will be sewn in to, in the patient's neck. Temporary red "vascular loops" are shown around the external carotid artery, and the main or common carotid artery in this panel. The above left-lower panel shows the "recipient" artery for this operation, namely the right middle cerebral artery (MCA) branch. This is where the other end of the radial artery graft will be sewn in to, in the patient's brain or right hemisphere. A green "rubber dam" has been placed under this vessel to aid visualisation during the operation, which involves tiny suturing thread that is almost invisible to the eye, requiring strong magnification by the operating microscope to see and physically handle. The above right-lower panel shows the radial artery graft (RAG) after it has been sewn by Dr Khurana into the right middle cerebral artery. Temporary micro-Clips (TC) made of titanium are shown in place during this part of the operation; these will be removed at the conclusion of the grafting procedure.

Image 3 (above). The bypass operation has now been completed. The above image shows the radial artery graft (now connecting normal neck and brain arteries in this patient) under special fluorescence imaging of our Leica OH4 operating microscope. There is excellent flow in the graft as indicated by the bright fluorescence in it. A special, safe intravenous contrast-dye called indocyanine green or ICG has been injected into the patient in order to assess the success of the operation visually. Our team also measures the amount of blood flow in such situations using a new microvascular ultrasound probe from Transonic Systems. ( take me to a page on our partner Site with more information on these technologies now).