An unrecognized accessory middle cerebral artery mimicking a patent middle cerebral artery in internal carotid artery terminus occlusion: a diagnostic pitfall during mechanical thrombectomy

Mechanical thrombectomy is an established treatment for acute ischemic stroke caused by large vessel occlusion; however, unrecognized cerebrovascular anatomical variants can complicate diagnosis and endovascular management. The accessory middle cerebral artery (AMCA) is a rare vascular variant arising from the A1 segment of the anterior cerebral artery that runs parallel to the main middle cerebral artery (MCA). Although usually clinically silent, the AMCA may provide limited collateral flow and obscure the vascular anatomy on angiography, particularly in cases of internal carotid artery (ICA) terminus occlusion. We report a case of a 69-year-old woman who developed acute cardioembolic stroke due to ICA terminus occlusion shortly after surgery for a brachial artery embolism. Neuroimaging revealed early ischemic changes in the right MCA territory, and mechanical thrombectomy was performed without preceding intravenous thrombolysis. During angiography, an MCA-like vessel supplied via the anterior communicating artery was observed, which masked the presence of the AMCA, leading to initial misinterpretation of the vascular anatomy. A stent retriever deployed at the ICA terminus achieved complete recanalization (Thrombolysis in Cerebral Infarction, grade 3) on the first pass. Subsequent angiography revealed the previously unrecognized AMCA. The patient was discharged with a modified Rankin Scale score of 1. This case highlights the diagnostic pitfall of collateral flow-mediated AMCA visualization and underscores the need for anatomical awareness and judicious thrombectomy technique selection.