Persisting carotid duct and proximal external carotid artery agenesis in an adult.

The carotid duct (CD) is a transient embryological structure connecting the 3rd and 4th aortic arches. We found a persisting CD in an adult female case, by studying the computed tomography angiogram. On the left side, the proximal external carotid artery (ECA) agenesis was noted. The CD was inserted into the left subclavian artery and continued upwards to reach the level of the atlas, and then it descended to connect to a normally configured segment of that ECA. It could be speculated that the CD-to-ECA connection was possible via unregressed 1st and/or 2nd aortic arches. The segmental ECA agenesis is extremely rare, while its supply via a persisting patent CD was not reported previously to the authors' knowledge. The variants are extremely important during neck surgery because damaging the CD could determine hemorrhage, as well as ischemia in the ECA territory.