We read with great interest and anticipation the paper by Patel et al (1) describing the second angiographically demonstrated case of a persistent otic artery. From our early training, all neuroradiologists have studied the embryonic anastomoses between the developing carotid arterial system and the longitudinal neural axis, the future basilar artery. We all have a few examples of trigeminal arteries—some also have the odd hypoglossal or proatlantal arteries—but which of us has seen an unequivocal otic artery, about which we continue to teach our trainees? Searching the small literature on this elusive vessel, we see that most cases have actually been low-lying trigeminal arteries, while others have described what appear to be stapedial artery remnants (2). The poor quality of reproduction of images in some publications and the frequent presence of only a single angiographic projection make it difficult to be sure of the origin, course, and termination of the vessel, and therefore of its true nature. Does the otic artery really exist? Does this case provide the missing link?
The trigeminal, hypoglossal, and proatlantal arteries are surely segmental arteries related to the metameric embryonic structure of the diencephalon, rhombencephalon, and spinal cord and their related nerves. These three embryonic arteries follow a generally anteroposterior, slightly oblique, course, supplying blood to the developing basilar system. The otic structures clearly are not segmental and develop mainly from the otic placode. Thus, there seems no reason to expect a segmental communication at this level. Further, as Lasjaunias has pointed out (3), unlike the other three embryonic vessels, there is no evidence for the existence of an otic artery in lower animals. If there were an otic artery, it would necessarily have to follow a lateral course into the internal auditory meatus (Fig 1), a very different orientation from its fellow vessels.
Of course, anastomoses may occur between the internal auditory artery (branch of the anterior inferior cerebellar artery [AICA] and thus basilar artery) and the internal carotid artery, via trigeminal and stapedial remnants (3) and the “otic” artery shown in Newton and Potts’ classic textbook (4) would fall into this category. Similarly, dangerous anastomoses are well recognized for example between external and internal carotid arteries (eg, via ophthalmic artery) and reflect overlapping vascular territories, rather than representing a single embryonic vessel in the sense of the trigeminal or hypoglossal artery.
Padgett (5) illustrates the otic artery arising below the level of the hyoid artery. Her reconstructions were based on sections of embryos, traced onto paper and then overlaid to give a three-dimensional effect. We are in awe of the ground-breaking nature of her classic work, and yet the sectioning of the embryos or the tracing process, could introduce artifacts and lead to misidentification of a vessel, especially one that she was expecting to see. Kelemen (6) stated that the hyostapedial (caroticotympanic) artery origin lies between the medial and apical turns of the petrous internal carotid artery and agrees with Padgett that the otic should arise proximal to that. Thus, the otic artery would arise in the adult from the lateral and proximal part of the petrous carotid (Fig 2). Reference to Figure 1 shows that it would therefore be in close proximity to the IAM, through which it must travel.
As Patel et al note, Lie (7) quotes three logical criteria for the putative otic artery. First, it should arise in the lateral portion of the petrous canal, close to the medial turn; in Patel et al, conversely, it arises from the medial portion of the petrous carotid, as the ICA turns up toward the cavernous sinus—ie, close to the apical turn (a well-recognized site of low origin of a trigeminal artery [Lie, p. 58]). Second, it should run through the IAM; this would be confirmed by MR imaging, but, although the authors state that an MR imaging was performed, unfortunately they do not show this. From their angiograms, it seems unlikely that the vessel traverses the IAM. Third, it should join the basilar artery at a caudal point. In the authors’ case, conversely, it joins the midbasilar, clearly above the level of the AICA, a typical location for a trigeminal artery. Unfortunately, adding to the confusion, the model Lie used to illustrate the predicted course of the otic artery shows the vessel arising from the midportion of the petrous ICA; according to the adjoining text description it should arise more laterally, proximal to the caroticotympanic artery (hyostapedial remnant), and thus close to the medial turn.
For all these reasons, we believe that this case is actually, simply a low-lying trigeminal artery. The only other “convincing” case the authors refer to, by Reynolds et al (8), shows the anomalous vessel clearly, only in the anteroposterior plane. As in the current case, it arises from the medial part of the petrous portion of the ICA and does not appear to traverse the IAM. We believe this also to be a low trigeminal artery. Thus, we are still not convinced of the existence of the otic artery as an independent embryonic vessel. In view of the size of the anomalous artery in the current case, it must be clearly visible at MR imaging that the authors refer to in the report; we are intrigued as to whether it was visible entering the IAM, which would certainly support the authors’ argument.
Axial T2-weighted MR imaging at level of petrous canal (A, white arrows) and at level of IAM (B). In both, dotted lines show posterolaterally the predicted course of an otic artery through the IAM to join the lateral portion of the petrous ICA. In A, the anteromedial dotted line shows the course of a low-lying trigeminal artery and, in B, the typical course of a trigeminal artery. C, Actual recent case of a trigeminal artery joining the cavernous ICA. The anomalous vessel is readily visible on standard cross-sectional images, and a true otic artery of the size shown by Patel et al would be equally visible on any standard MR imaging sequence.
Anteroposterior and lateral drawings of the internal carotid artery. The rings denote the entry and exit from the petrous canal. Note the typical foreshortening of the petrous portion on the lateral view. Caroticotympanic artery (adult successor to the hyostapedial artery) arises close to the medial turn. Asterisk denotes predicted origin of a stapedial artery.
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