The Carotid Siphon: A Historic Radiographic Sign, Not an Anatomic Classification

BackgroundAfter the term carotid siphon was introduced by Moniz in 1927 to describe the radiographic appearance of the intracranial internal carotid artery (ICA), the concept gained popularity in decades following in both the anatomic and the medical literature. However, as conflicting definitions persist in the delineation of proximal and distal sites, does the term carotid siphon provide the precision needed for current anatomic and clinical studies?MethodsA PubMed search of “carotid siphon” detected >400 articles from the anatomic and medical literature during the past 6 decades. Moniz's text and figures in his original Lancet article and a compilation of other seminal historical articles and references were reviewed to trace the use of the term carotid siphon during this period.ResultsViewing the radiographic silhouette of a normal ICA, Moniz defined the carotid siphon as the series of bends and curves; an additional curvature was identified as a double siphon. Throughout Moniz's works, in text and figures, the boundaries of the carotid siphon were never delineated. Authors who followed attempted to correlate his original description of this two-dimensional radiographic projection with anatomic documentation.ConclusionsTracing the origin and usage of the term carotid siphon during 6 decades in the medical literature shows continued discrepancy rather than consensus. The term carotid siphon is historically relevant but can now be supplanted by definitive ICA classification systems, which continue to evolve in contemporary medical and anatomic communications.