Unruptured aneurysm at bifurcation of super short middle cerebral artery mimicking duplicated middle cerebral artery aneurysm

Background: Duplicated middle cerebral artery (DMCA) is a normal variation of the middle cerebral artery (MCA), and distinguishing DMCA from other arteries based only on the angiographical findings is sometimes difficult. Case Description: Preoperative angiography of a 60-year-old woman revealed two MCAs originating near the top of the right internal carotid artery and a 6-mm-sized aneurysm located between these two MCAs. The ipsilateral A1 segment of the anterior cerebral artery was not visualized due to hypoplasia. The patient was diagnosed with an unruptured aneurysm at the bifurcation between MCA and DMCA, preoperatively. However, during clipping surgery, the hypoplastic A1 segment was observed at approximately 2 mm proximal to the bifurcation of the MCA, indicating that these two MCAs were not “true” MCA and DMCA, but rather two normal MCA trunks bifurcated from the very short prebifurcation segment of M1. This difference in interpretation was due to the fact that the hypoplastic A1 was not visualized on preoperative examinations. The patient was discharged following the surgery without any neurological deficits. Conclusion: The distinction between DMCA and MCA trunks from the very short prebifurcation segment of MCA might be difficult, especially when the ipsilateral A1 segment is hypoplastic. Such an M1 with a length of 2–3 mm might be named super short M1. Thus, it is necessary to confirm the anatomy during aneurysm surgery.