An aberrant internal carotid artery (ICA) is thought to be caused by regression of the cervical (C1) ICA during embryogenesis resulting in an absent vertical petrous (C2) segment and aplasia of the ascending carotid canal.
Blood then takes an aberrant course to get from the common carotid artery to the horizontal petrous (C2) segment of the ICA. The inferior tympanic artery and the hyoid/caroticotympanic arteries are involved in this process (see Lo et al below for a great diagram). The inferior tympanic artery normally arises from the ascending pharyngeal artery (branch of the proximal external carotid artery), travels through the inferior tympanic canaliculus, and anastomoses with the caroticotympanic artery (a remnant of the embryologic hyoid artery). In patients with an aberrant ICA, the inferior tympanic artery and inferior tympanic canaliculus enlarge. Blood is redirected through the hyoid artery into the horizontal petrous segment of the ICA. This aberrant course gives rise to the "7" or reversed "7" appearance seen on angiography.
A lateralized (also known as laterally displaced and dehiscent) ICA, on the other hand, has a normal course but a focally dehiscent lateral wall of the carotid canal (pink arrows compared to the normal side indicated by the blue arrow), usually near the basal turn of the cochlea. In contrast to patients with an aberrant ICA, the vertical carotid canal is present and the inferior tympanic canaliculus is not enlarged. The genu of the petrous segment lies more laterally and posteriorly.
References
- Lo WW, Solti-Bohman LG, McElveen JT Jr. Aberrant carotid artery: radiologic diagnosis with emphasis on high-resolution computed tomography. Radiographics. 1985 Nov;5(6):985-93.
- Vattoth S, Shah R, Curé JK. A compartment-based approach for the imaging evaluation of tinnitus. AJNR Am J Neuroradiol. 2010 Feb;31(2):211-8.
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