Nonbifurcating Cervical Carotid Artery Diagnosed by MR Angiography

Discussion

We diagnosed 6 cases of nonbifurcating cervical carotid artery among 2866 patients. The incidence of 0.21% in our study was low, but it was almost the same as the reported 0.2% incidence of the persistent trigeminal artery, the most common type of carotid-vertebrobasilar anastomosis.¹⁰ To our knowledge, ours is the first report of the incidence of nonbifurcating cervical carotid artery detected by MRA.

We found an additional 11 cases of this anomaly in the English language literature (Table 2) and thus analyzed 17 total cases (12 men, 5 women; 7 cases, right side; 10, left). Two of the 11 cases were diagnosed during postmortem examination and 9 cases, by angiography. In 10 patients, the branching order from proximal to distal was the LA, FA or F-L trunk, distal trunk of the ECA, and OA. In 5 of the 17 patients, the OA did not arise from the carotid artery. In 2 of our patients, the FA arose from the distal trunk of the ECA, resembling high bifurcation of the carotid artery, and in 1 of these 2, the OA arose from the VA (Fig 2).

Morimoto et al³ first used the term “nonbifurcating cervical carotid artery” in 1990. Normally, the LA, FA, and OA arise from the proximal main trunk of the ECA. As shown above, these branches rarely arise separately from a single large vessel, so there is no bifurcation of the carotid artery. Two hypotheses are proposed to explain the formation of this anomaly—agenesis of either the proximal main trunk of the ECA or the most proximal ICA.³ Embryologically, the ICA is formed from the third aortic arch and dorsal aorta during early gestation. Abnormal regression of the third aortic arch might result in the persistence of the hyoid artery, which derives from the second aortic arch, and might produce high bifurcation of the carotid artery with an anomalous branching pattern¹¹ or a nonbifurcating carotid artery.

Although the proximal main trunk of the ECA is apparently absent, we support the hypothesis that this anomaly results from agenesis of the most proximal ICA because the affected artery does not have physiologic dilation of the origin of ICA and the OA arises most distally. Thus, the distal ICA seems to originate from the proximal OA or APA. The APA rarely arises from the proximal ICA, and in the case of acquired occlusion of the ICA at its origin, the APA serves as an important collateral pathway from the VA.¹² Lasjaunias et al¹³ reported that the OA is formed from the proatlantal artery and arises from the primitive ICA, ECA, and VA at points determined by the sites of regression. Thus, the OA rarely arises from the ICA¹⁴ or VA, as in our case 6. We suggest that in the 4 previously reported patients in whom the OA was not visualized, the OA may have arisen from the VA. Figure 3 schematically illustrates our proposed hypothesis regarding this anomaly.

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Fig 3.

Schematic illustration of our hypothesis of a nonbifurcating cervical carotid artery in a lateral projection. A, Normal development. B, Agenesis of the most proximal ICA. C, Final configuration of this anomaly. The proximal OA or APA probably plays an important role in forming the nonbifurcating cervical carotid artery.

Right-sided predominance of both congenital total absence of the ICA and an aberrant course of the ICA caused by segmental agenesis of the petrous ICA is well known. In contrast, the nonbifurcating cervical carotid artery shows no significant lateral predominance. The reason for this difference is unknown.

In our institution, patients with or suspected of having ischemic cerebrovascular diseases routinely undergo MRA of intra- and extracranial regions that include the carotid bifurcation. We suggest that many cases of this anomaly have been overlooked or misdiagnosed as high bifurcation of the carotid artery during angiography and that a nonbifurcating cervical carotid artery cannot be detected in routine cerebral MRA, which is usually performed to exclude the carotid bifurcation. Correct diagnosis is necessary to avoid complications, such as embolic cerebral infarctions, during interventional radiologic procedures^8,9 or head and neck surgeries.

Our study is limited because it is a retrospective clinical study, and only 1 neuroradiologist analyzed the MRAs. Also, because most of the examined patients had or were suspected of having ischemic cerebrovascular diseases, there may have been some selection bias in our study materials and results.