however , both treatment modalities may be impossible in certain situations , such as giant or fusiform thrombosed aneurysm , in which efferent or adjacent arterial branches may be involved . under these situations , an alternative surgical strategy using revascularization may be required6,7 ) . although revascularization using harvested graft vessels has been used for high - flow replacement in proximal vessels , local techniques of revascularization without a graft ( in situ ) may be sometimes more useful for distally located aneurysm . we report our surgical experience using in situ bypass technique for the treatment of giant serpentine distal anterior cerebral artery ( daca ) aneurysm . a 49-year - old woman presented with loss of consciousness ( loc ) and history of headache . on initial neurologic examination , she had no specific symptoms or previous seizure history . a non - contrast computed tomography scan showed a 31.8 cm sized , rim calcified , and round shaped mass lesion in the interhemispheric area , along the anterior falx above the head of the corpus callosum ( fig . brain t2-weighted structural magnetic resonance image ( mri ) scan revealed partially thrombosed , signal voided aneurysm , without parenchymal edema ( fig . cerebral angiography demonstrated an irregular serpentine aneurysm , involving the entire a2 segment of daca , measuring 4.5 cm in the anteroposterior dimension and 1.8 cm in the transverse dimension ( the proximal portion of aneurysm , fig . the patient was placed in the supine position , with the head slightly tilted at 20 degrees in the 3 pin fixation . the right calf lesion was prepped for possible bypass , using a venous interposition graft from the superficial temporal artery to the distal end of the anterior cerebral artery . to secure the wider surgical field , we performed cerebrospinal fluid ( csf ) drainage via lumbar puncture and midline bifrontal interhemispheric craniotomy with the neuronavigation system ( medtronics ) . 44 cm sized bone flap was made , then dura mater was opened with small straight incision . the superior sagittal sinus was ligated , cut , and reflected up and downward bidirectionally ( fig . 2a ) . under the operating microscope , we advanced with dissection of the interhemispheric fissure and indentified the distal end of the fusiform aneurysm ( fig . after further dissection , the left pericallosal artery was indentified just adjacent to the aneurysm . thus , at this point , we considered that it might be possible to perform the in situ bypass between right a3 just distal aneurysm sac and left a3 . to see the proxymal portion of aneurysm and parent artery , we further dissected the interhemspheric fissure toward anterior direction and were very cautious not to injury the both olfactory nerves ( fig . 2c ) . this complication can be avoided by making wide , precautious dissection to minimize posterior and lateral brain retraction . therefore , we did not fully dissect the aneurysm from the brain , in order to avoid postoperative parenchymal contusion . blood flow was measured at approximately 20 cm / s , in both distal artery to the fusiform aneurysm and opposite left pericallosal artery . when we placed the temporary clip on the proximal parent artery of aneurysm , blood flow was not visible on the micro - doppler recording . thus , we finally decided to perform aneurysm trapping with a right end - to - left side in situ bypass , to preserve the flow to the right daca . before anastomosis , the patient was placed into burst suppression with propofol , and the mean arterial pressure was raised 25% above the patient 's baseline , in order to improve the collateral circulation . intraoperatively , both somatosensory and motor evoked potentials were also used for monitoring . at the time of the anastomosis , 2000 iu of heparin both arteries were dissected as movable as possible , and a rubber dam was passed under both vessels . after placing two temporary clips on the right side , the atherosclerotic distal artery of the aneurysm was clearly trimmed with jeweler forceps , and marked with marking pen . two temporary clips were then placed on the left side , and following heparin irrigation , a small arteriotomy was made equal to the opposite arterial opening , using a diamond knife ( fd 115d aesculap , pa , usa ) . a marking pen is usually used to color the borders of the arteriotomy for idenfication of the margin of the transparent thin vessels . after both toe and heel stitches were placed at the proximal and distal ends of the anastomosis with 10 - 0 nylon suture ( ethicon co. , nj , usa ) , the posterior wall was sutured in a continuous fashion and the anterior wall was sutured in an interrupted pattern . on completion of the end - to - side bypass , robust blood flow in the right daca was observed in the distal segment to the bypass site ( fig . permanent clips were applied proximal and distal to the aneurysm , without removal of the thrombosed aneurismal mass , because the aneurysm itself was severely stuck to the brain surface . instead of leaving the thrombosed aneurysm , we aspirated the blood clot from the aneurysm using 14-gauzed needle , to identify total obliteration of the aneurysm and to reduce the mass effect postoperatively ( fig . postoperative cerebral angiography on day 7 demonstrated no residual filling of the aneurysm and excellent flow through a patent bypass into both the daca vascular territories ( fig . follow up t2-weighted brain mri at 3 months after surgery showed remarkably reduced mass size in the interhemispheric area , along the genu portion of corpus callosum ( fig . the patient was placed in the supine position , with the head slightly tilted at 20 degrees in the 3 pin fixation . the right calf lesion was prepped for possible bypass , using a venous interposition graft from the superficial temporal artery to the distal end of the anterior cerebral artery . to secure the wider surgical field , we performed cerebrospinal fluid ( csf ) drainage via lumbar puncture and midline bifrontal interhemispheric craniotomy with the neuronavigation system ( medtronics ) . 44 cm sized bone flap was made , then dura mater was opened with small straight incision . the superior sagittal sinus was ligated , cut , and reflected up and downward bidirectionally ( fig . 2a ) . under the operating microscope , we advanced with dissection of the interhemispheric fissure and indentified the distal end of the fusiform aneurysm ( fig . after further dissection , the left pericallosal artery was indentified just adjacent to the aneurysm . thus , at this point , we considered that it might be possible to perform the in situ bypass between right a3 just distal aneurysm sac and left a3 . to see the proxymal portion of aneurysm and parent artery , we further dissected the interhemspheric fissure toward anterior direction and were very cautious not to injury the both olfactory nerves ( fig . 2c ) . this complication can be avoided by making wide , precautious dissection to minimize posterior and lateral brain retraction . therefore , we did not fully dissect the aneurysm from the brain , in order to avoid postoperative parenchymal contusion . blood flow was measured at approximately 20 cm / s , in both distal artery to the fusiform aneurysm and opposite left pericallosal artery . when we placed the temporary clip on the proximal parent artery of aneurysm , blood flow was not visible on the micro - doppler recording . thus , we finally decided to perform aneurysm trapping with a right end - to - left side in situ bypass , to preserve the flow to the right daca . before anastomosis , the patient was placed into burst suppression with propofol , and the mean arterial pressure was raised 25% above the patient 's baseline , in order to improve the collateral circulation . intraoperatively , both somatosensory and motor evoked potentials were also used for monitoring . at the time of the anastomosis , 2000 iu of heparin both arteries were dissected as movable as possible , and a rubber dam was passed under both vessels . after placing two temporary clips on the right side , the atherosclerotic distal artery of the aneurysm was clearly trimmed with jeweler forceps , and marked with marking pen . two temporary clips were then placed on the left side , and following heparin irrigation , a small arteriotomy was made equal to the opposite arterial opening , using a diamond knife ( fd 115d aesculap , pa , usa ) . a marking pen is usually used to color the borders of the arteriotomy for idenfication of the margin of the transparent thin vessels . after both toe and heel stitches were placed at the proximal and distal ends of the anastomosis with 10 - 0 nylon suture ( ethicon co. , nj , usa ) , the posterior wall was sutured in a continuous fashion and the anterior wall was sutured in an interrupted pattern . on completion of the end - to - side bypass , robust blood flow in the right daca was observed in the distal segment to the bypass site ( fig . permanent clips were applied proximal and distal to the aneurysm , without removal of the thrombosed aneurismal mass , because the aneurysm itself was severely stuck to the brain surface . instead of leaving the thrombosed aneurysm , we aspirated the blood clot from the aneurysm using 14-gauzed needle , to identify total obliteration of the aneurysm and to reduce the mass effect postoperatively ( fig . postoperative cerebral angiography on day 7 demonstrated no residual filling of the aneurysm and excellent flow through a patent bypass into both the daca vascular territories ( fig . follow up t2-weighted brain mri at 3 months after surgery showed remarkably reduced mass size in the interhemispheric area , along the genu portion of corpus callosum ( fig . since the first bypass reported in 1986 by ikeda , including the microvascular side - to - side anastomosis technique3 ) , the concepts and techniques for bypass have evolved over time and diverse surgical methods have been tried and evaluated for cerebral revascularization , in order to get the best surgical outcomes . the ideal surgical technique is to make an artificial conduit for blood flow between two adjacent vessels of similar caliber . this type of in situ bypass has been often applied for the treatment of complex brain aneurysms6,7 ) . the overall incidence of daca aneurysms has been known to be from 2% to 9% of all intracranial aneurysms4,5 ) ; however , only 0.6% were fusiform , and is still unclear whether these were giant aneurysms . dunn et al.1 ) were the first to demonstrate the technique as well as challenges of performing the a3 : a3 bypass to treat a large fusiform aneurysm located in the daca . thus , this presented case is only the second reported end - to - side in situ a3 : a3 bypass with aneurysm trapping . furthermore , to the best of our knowledge , giant serpentine aneurysm has not been reported previously in the literature in this daca location . herein , we described a successful surgical experience for the treatment of giant serpentine daca aneurysm . considering surgery in the interhemisheric lesion , the operation field is quite deep and narrow that we often have difficulty with performing surgery in this location . furthermore , daca aneurysm is sometimes adhesive to the cingulated gyrus and often has wide or atherosclerotic neck , with difficulty to confirm the parent artery and surrounding structures5 ) . therefore , it is important to secure wider surgical field and brain relaxation , using csf drainage and infusion of hyperosmolar fluid . although this type of bypass surgery may be technically challenging because of aforementioned obstacles , meticulous dissection and enough surgical space can lead to successful completion of the anastomosis . dunn et al.1 ) reported one bypass for large ( 17 mm ) fusiform a3 aneurysm , with resection of the aneurysm after end - to - side bypass . aneurysmal resection was impossible in our case , because the aneurysm was 4.5 cm in length and too large to resect . the goal of surgery was total obliteration of the aneurysm , with preservation of blood flow to the parent vessel territory and bypass revascularization in the end . therefore , we did not remove the trapped aneurysm , to avoid injury of the adhesive tissue between the brain and the aneurysm wall . 3b ) , the mass size was decreased and the patient was symptom free postoperatively . in a recent study , gelfenbeyn et al.2 ) suggested that venous graft should be prepared , if necessary , to be used as interposition graft between the superficial temporal artery and the distal artery of the resected aneurysm . if there is a large , broad neck a2-a3 aneurysm , it might be difficult to perform the end - to - end anastomosis after aneurysm resection . as previously suggested , we also prepared the venous interposition graft in the patient 's calf , but we did not use it , because this procedure should be tailored to the operative findings and availability of donor and recipient vessels . giant serpentine aneurysm in a2-a3 are very rare and can be difficult to treat with standard surgical methods , because of their atherosclerotic pathology and characteristically narrow surgical field . we describe an end - to - side in situ bypass technique combined with aneurysm trapping .