Head and Neck Imaging

Internal carotid artery, partial absence

During embryogenesis, an agenesis of the vertical portion of the internal carotid artery may occur. In such a case, the vascularization of the internal carotid artery system is rerouted through the hyoid artery via the ascending pharyngeal artery. As a consequence, the inferior tympanic artery, a branch of the ascending pharyngeal artery, hypertrophies and proceeds more cranially through an enlarged inferior tympanic canaliculus into the tympanic cavity. After its course through the tympanic cavity, the artery anastomoses with the horizontal portion of the carotid artery (Fig.1).

This vascular anomaly is sometimes wrongly referred to as 'aberrant course of the internal carotid artery'; this term is better reserved for an internal carotid artery protruding into the tympanic cavity through a dehiscence in the bony wall separating the carotid canal from the tympanic cavity. Both anomalies clinically mimic an intratympanic hypervascular tumour (see glomus tumour).

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Fig.1

Axial CT images through the temporal bones in a patient with left-sided tinnitus and clinical diagnosis of glomus tympanicum tumour. a. Absence of vertical part of left carotid canal and enlarged inferior tympanic canaliculus (arrow). b. The left carotid artery is reinjected in its horizontal part by the enlarged inferior tympanic artery, running from the inferior tympanic canaliculus over the promontorium (arrows) (from: Verbist B., Hermans R., Devlies F., De Foer B., Baert A.L.: Partial agenesis of the internal carotid artery. Eur Radiol 1996, 6: 939-940, with permission).
Internal carotid artery, partial absence, Fig.1 (a)
Internal carotid artery, partial absence, Fig.1 (b)