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AJNR - American Journal of Neuroradiology

B, DSA, right CCA injection, unsubtracted lateral view. Because of its intrathoracic location, the carotid bifurcation could not be properly imaged on a lateral view. Standard lateral neck view shows unusual length of ICA and ECA. ICA is kinked at C2–C3. Note fusion of the C2 and C3 vertebral bodies.

C, DSA, left CCA injection, unsubtracted AP view. Left carotid bifurcation projects over posterior arch of the second rib (star). Here also, note medial position of ICA distal to the bifurcation (arrow).

D, DSA, right VA, unsubtracted AP view. See impingement of right VA at C7–T1 level due to severe degenerative changes secondary to the Klippel-Feil anomaly, with lumen narrowing of approximately 50% (arrow).

E, DSA, left subclavian artery, AP subtracted view. VA is also narrowed by C7–T1 degenerative changes, with a lumen narrowing of approximately 70% (arrow).

A, Normal development of the carotid bifurcation. The third arch (III) constitutes the proximal part of the carotid axis, which is divided into the CCA and proximal ICA either by in situ sprouting of the ventral pharyngeal artery or by its migration from the aortic sac. The distal ICA is formed by the dorsal aorta distal to the ductus caroticus. The ductus caroticus is the segment of dorsal aorta located between the third (III) and fourth (IV) aortic arches. Its regression at the 12–14-mm stage allows the carotid axis to take its definitive appearance.

B, Separate origins for the ECA and ICA from the aorta occur when the third (III) aortic arch involutes instead of the ductus caroticus (illustrated on the left side). The same process may occur on the right side, the separate origins for the ECA and ICA arising in that case from the innominate artery.

C, Low carotid bifurcations are formed when, in addition to the situation described in B, there is migration of the ECA origin toward the ductus caroticus, and reconstitution of a short CCA.