CTA-based classification of occipital artery course variations at the atlanto-occipital interval, with surgical...

The occipital artery (OA) is routinely encountered during posterior fossa and far-lateral craniocervical exposures and serves as a donor vessel for cerebrovascular bypass procedures. Despite its surgical importance, there is no standardised imaging-based classification of the OA course within the atlanto-occipital interval. This study aimed to characterise OA topography using computed tomography angiography (CTA) and develop a clinically applicable classification system.

This retrospective observational study analysed 200 occipital arteries in 100 adult subjects using head-and-neck CTA. A dual-parameter classification was developed based on: (1) the primary type, describing the OA relationship to the occipital sulcus (Types 1-3), and (2) the vertical position relative to the atlas transverse process, including mastoid-transverse process overlap status.

Three primary types were identified based on the OA-sulcus relationship: Type 1 (intrasulcal), Type 2 (infrasulcal), and Type 3 (distanced). The vertical position was classified as supraatlantal, atlantal, or infraatlantal. Type 1 (intrasulcal) was most prevalent (54.0%), followed by Type 3 (distanced, 25.5%) and Type 2 (infrasulcal, 20.5%). Supraatlantal positioning predominated (61.5%). Type 1 arteries demonstrated significantly lower infraatlantal rates (5.6%) compared to Types 2 (26.8%) and 3 (25.5%) (p < 0.0001). Mastoid-transverse process overlap occurred in 15.5% and was independent of primary type. Bilateral type symmetry was present in 61.0% of subjects. Subset analysis (n = 30 subjects) revealed a mean OA luminal diameter of 1.29 ± 0.39 mm, with 88.3% of arteries demonstrating tortuosity.

This CTA-based classification provides a structured framework for preoperative assessment of OA topography. The association between intrasulcal course and supraatlantal positioning may inform surgical planning for far-lateral exposures and OA harvest for bypass procedures.