Accessory middle cerebral artery
The accessory middle cerebral artery is a vessel arising from the anterior cerebral artery, paralleling the course of the MCA, and replacing some of the MCA territory. The accessory MCA typically supplies the anterior frontal lobe (as in this case).
Early suggestions that the anomaly arises from a hypertrophied recurrent artery of Heubner are now thought unlikely, as RAH often coexists, and the territory of supply is different. The vessel may be an anomalous early branch of the MCA. Accessory MCA may arise from either the proximal or distal portion of the A1 segment of the ACA. There are classification systems which reflect this (e.g. Teal and Manelfe).
This case arises from the proximal portion - Teal type 1, Manelfe type 2. Manelfe type 1 is equivalent to a duplicated MCA (see below).
Other common embryological variants of the MCA include duplicated middle cerebral artery (vessel arising from the ICA bifurcation, and typically supplying the anterior temporal lobe), and fenestrated middle cerebral artery. There is an association between MCA variants and intracranial aneurysms. It is important to accurately document MCA anomalies as they have implications for operative or endovascular management.