Various procedures for indirect revascularization surgery (encephalomyosynangiosis, EDAS, EGS, and EDAMS) can be adapted according to the tissues inserted into the cranial cavity. Two representative procedures, the STA EDAS and the bifrontal EGS are described below. Direct revascularization by a superficial temporal artery to middle cerebral artery anastomosis will not be discussed due to its relatively infrequent use in children.
The STA and a surrounding cuff of galeal tissue are inserted into the subdural space and placed over the cerebral convexity of middle cerebral artery territory (43).
- Anesthesia: During the perioperative period, hyperventilation, hypoventilation, hypovolemia, anemia, hypotension and hyperthermia should be avoided. Among these, continuous hyperventilation is especially injurious to the brain. Common practice of hyperventilation in general brain surgery frequently leads to perioperative cerebral infarction. The surgeons should repeatedly be aware of the status of anesthesia, especially the level of PaCO2, during operations (particularly if Moyamoya Disease surgery is not frequently performed or anesthesiologists (residents) are frequently changed).
Patient Positioning EDAS
The head is positioned according to the operation site. Often two sites, such as unilateral STA area and bifrontal area, or unilateral STA area and occipital artery area of the same side, are treated concurrently in the same field. Careful positioning with soft pads is necessary. Focal high pressure should be avoided at the previously operated areas where scalp arteries supply brain tissue.
Surgical Approach EDAS
- Location of STA mapped out: After induction of anesthesia, the course of parietal branch of STA is palpated or traced with Doppler device and marked. When STA cannot be palpated or traced, insertion of galeal layer only is a good alternative in pediatric Moyamoya Disease cases. The STA area is prepared for surgery.
- Scalp incision: To minimize postoperative cosmetic problem, the incision can be made with an ‘S’ shape. Even with partial coverage of the scar line with hair, a linear incision may not be satisfactory cosmetically in male patients. The lower part of an ‘S’ shaped incision goes posteriorly to minimize scar exposure through hair.
- Open to galeal layer: The scalp layer is superficially cut and opened to the layer of galeal tissue where STA is located.
- Harvest STA with galeal cuff: The galeal tissue is exposed and harvested with anterior and posterior galeal incisions parallel to the STA as wide as to be inserted on to the brain. The continuity of STA should be preserved. Even in case where the STA is injured, neovascularization is frequently satisfactory in pediatric cases.
- Handle adjacent scalp carefully: During the surgery, scalp tissue should be dealt with care. The scalp layer in young children is thin and the vascularized layers are inserted on the brain and only the less vascularized portion is left at the original site. Therefore, the remained scalp tissue is vulnerable to ischemic necrosis. Reflection of the scalp with an acute angle should be avoided, coagulation and retraction be minimized, and the tissue should be kept moisturized.
- Incise muscles and periosteum: These tissues are incised parallel to the STA.
- Craniotomy: During the creation of the craniotomy, a massive venous bleeding may be encountered. In Moyamoya Disease patients, profuse collaterals can be formed through the cranium. They are cut during the procedure of craniotomy. Rapid craniotomy and bone waxing are the way to minimize blood loss.
- Avoid middle meningeal artery during dural opening: On dural opening, middle meningeal artery is saved. On gross examination, it is difficult to differentiate dural arteries from veins. Preoperative angiography is useful to understand how to best save key branches of the middle meningeal artery (52). The base of dural flaps is anterior or posterior margins of the craniotomy. After dural incision, the dural flaps are reflected inward and inserted into the subdural space adjacent to the craniotomy. This maneuver increases the area of contact between the vascularized surface of dura and brain.
- Arachnoid membrane: Although not unanimous, many surgeons recommend incision of arachnoid membrane to promote neovascularization from STA and galeal tissue to the ischemic brain. The effect is not clear. Under the surgical microscope, the arachnoid membrane is incised to expose the cerebrospinal fluid (CSF) in the subarachnoid space. Sulcal spaces, especially spaces beside the vessels, are good places to incise the arachnoid membrane. Bleedings can be easily controlled with cottonoids or small pieces of Surgicel.
- Galeal cuff STA sutured to dural edge +/- cut arachnoid: After arachnoid incision, the galeal flap is sutured to the anterior and posterior margins of dural opening. The adventitia of STA may be sutured to the pial membrane (50). Pial synangiosis may be valuable especially when the brain is atrophic and the gap between the donor tissue and the brain surface is of significant distance.
- Packing gaps dural closure: To minimize CSF leak through the area around the proximal and distal ends of STA, pieces of Gelfoam are used for packing the gaps.
- Bone flap repositioned with care: The bone flap is replaced. Attention is paid to prevent compression of the proximal and distal ends of STA and galeal tissue by the bone margins. When the brain is atrophic, the bone flap may be left floated to promote contact between the donor tissue and brain surface. The depressed skull flap becomes less prominent as the child grows older. As described above, the profuse development of collaterals through the cranial bone makes oozing of blood in the subdural and epidural spaces. Meticulous hemostasis with dural tenting (including the central portion of the bone flap) and placement of a drainage catheter into the epi-fascial or epi-periosteal space is helpful.
- Standard scalp closure: Scalp layers are closed as usual fashion. To promote healthy healing of less vascularized portion of the remained scalp tissue, meticulous subcutaneous sutures are desirable.
For ischemia of the anterior cerebral artery territory a bifrontal EGS is recommended. This is frequently done in conjunction with a STA EDAS (28,29). It may prevent progression of ischemia on the contralateral cerebral hemisphere to the side receiving the STA EDAS even though it is limited to the anterior cerebral artery perfusion area.
Anesthesia: same as used for a STA EDAS
Patient Positioning EGS
See EDAS for precautions.
Surgical Approach EGS
- Mapping of STA not required: For bifrontal EGS, arterial branches of STA are not used. Galeal tissue with or without periosteum is inserted into each side of frontal paramedian area.
- Scalp incision: A curvilinear scalp incision and bifrontal craniotomy crossing midline is designed. If the operation is combined with a STA EDAS, the space between the two scalp incisions used for the EDAS and the EGS should be wide enough (>3cm) to avoid postoperative scalp necrosis.
- Multiple burr holes if adequate STA supply: Often the patient already have collaterals from the frontal branch of STA or from the meningeal arteries through the frontal dura. In these cases, craniotomy and dural opening may injure these collaterals. Making multiple burr holes avoiding collaterals with inserting pieces of adjacent galeal-periosteal tissue through the burr holes is a good alternative.
- Galeal harvested for EGS: The scalp layer is superficially cut and opened to the layer of galeal tissue. The galeal tissue is exposed and harvested with an anterior, posterior and midline galeal incision of sufficient width as to be inserted onto the brain. The galeal flap for each side is reflected laterally. Some reflect and insert the periosteum with galeal tissue as one layer while others insert galeal tissue only and leave the periosteum in situ. For very young children, the scalp is thin, and the vascularized part is inserted on the brain surface leaving only the less vascularized part for the scalp layer there is a high possibility for postoperative ischemic scalp necrosis. When the periosteum is absent between the remnant scalp tissue and skull bone, the scalp is more vulnerable to ischemic necrosis. Therefore, leaving the vascularized periosteum in situ may be helpful for avoiding scalp necrosis.
- Midline craniotomy: The posterior margin of craniotomy is placed at the coronal sutures and the anterior margin located posterior to the hairline. A posteriorly placed craniotomy may be associated with profuse bleeding during the craniotomy or dural incision. This is because of the formation of collateral circulation from the scalp to the brain in Moyamoya Disease and the proximity of the surgical field to parasagittal draining veins. Inspection of the arterial and venous phases of the preoperative angiography is helpful for avoidance of large parasagittal draining veins. Rarely dural incision is not possible and not infrequently the area of dural incision is limited because of prominent venous lakes with associated draining veins at the site of dural incision. Dural flaps are reflected into the subdural space of lateral frontal convexity. Some surgeons insert the medial dural flap into the interhemispheric fissure with arachnoid membrane incision at the same region, but it is sometimes not feasible because of parasagittal veins. Others do not recommend such medial frontal procedures because of tedious nature and debatable evidence of superior outcome of the procedure.
- Arachnoidal dissection and dural closure: Generous arachnoid incision, and galea-dural suture are performed as described in the session of STA EDAS.
Same as for EDAS
Multiple Burr Hole Trephination
When collateral arterial branches of STA are seen to be supplying the brain at the area of craniotomy and dural incision, multiple burr hole trephination with insertion of galeo-periosteal flaps is a potential alternative to the EGS (8).