Internal carotid artery agenesis: a systematic review of prevalence, basal brain circulation variations, and associated pathologies.

The internal carotid artery (ICA) is a crucial vessel, and its congenital absence, known as ICA agenesis, is a rare vascular anomaly. This systematic review aimed to provide a comprehensive overview of the available evidence on this condition. We systematically searched multiple databases from inception until January 2024. Two authors independently screened titles and abstracts, and a third reviewer was involved if consensus could not be reached. Data extraction and quality assessment were performed using validated the AQUA tool and the JBI critical appraisal checklist tool. A total of 138 (7.8%) articles out of 1,778 met the inclusion criteria, reporting 2040 subjects from which 176 presented (8.6%) ICA agenesis. Most of the studies were case reports (96%), and the included studies originated from various countries, with the United States (17.39%), Japan (14.49%), Turkey (10.87%), and India (10.14%) contributing the most. Left-sided agenesis was the most common (67.39%), followed by right-sided (42.03%) and bilateral (22.46%) agenesis. The most reported clinical manifestations were Horner's syndrome, increased risk of intracranial aneurysms, and pituitary gland abnormalities. ICA agenesis is a rare vascular anomaly with a left-sided predominance. While some patients may develop associated clinical conditions, many remain asymptomatic due to the development of robust collateral circulation. Comprehensive preoperative evaluation and awareness of these anatomical variations are crucial to guide surgical planning and minimize the risk of complications. PROSPERO registration number: CRD42024592673.