Anatomical variations of the circle of Willis and their surgical implications

The Circle of Willis (CoW) is a critical arterial network at the base of the brain that maintains cerebral blood flow during vascular compromise. Although classically described as a complete ring, anatomical and imaging studies show variations in approximately 40–68% of individuals, which can significantly affect neurosurgical and cerebrovascular procedures.A 2025 classification system identifies four main CoW configurations: Type I (complete, ~60%), Type II (posterior communicating artery hypoplasia or aplasia, ~20%), Type III (anterior communicating or anterior cerebral artery hypoplasia or aplasia, ~10%), and Type IV (fetal-type posterior cerebral artery, ~10%). Meta-analyses confirm that posterior circulation variants are most common, with notable population differences, including higher prevalence in South Asian cohorts.Clinically, these variations limit collateral circulation and increase the risk of ischemic complications during surgery. Advances reported in 2025—such as robotic surgical planning, intraoperative 4D flow MRI, artificial intelligence–based risk prediction, and adaptive deep brain stimulation—support safer, anatomy-guided interventions. Routine preoperative vascular imaging and individualized surgical planning are essential to optimize neurovascular outcomes.