The common carotid artery typically bifurcates at the level of the superior margin of the thyroid cartilage into the internal (ICA) and external (ECA) carotid arteries. Different collateral branches are further given off from the ECA: the superior thyroid (STA), lingual (LA), facial (FA), ascending pharyngeal (APA), occipital (OA), and posterior auricular arteries. Successive collateral branches of the ECA may combine into common trunks, such as the thyrolingual, linguofacial (LFT), thyrolinguofacial (TLFT) or occipitoauricular trunks [1, 2]. A recent study found LFTs in 34/147 cases, and the evidence demonstrated that they could not be assigned a certain morphological and topographical pattern [3].

The present study reports on a unique instance of an infrahyoid-originated LFT with remarkable length and diameter. The anatomical implications of this uncommon variant and the analytical description of its features were assessed.

During a retrospective study on computed tomography angiograms (CTAs), a unique anatomical variant was identified in an adult male patient. The CTAs were conducted using a 32-slice scanner (Siemens Multislice Perspective Scanner), featuring a 0.6 mm collimation and reconstructed with a 0.75 mm thickness, with a 50% overlap for multiplanar maximum intensity projection and three-dimensional (3D) volume rendering technique. The case was documented using Horos for iOS (Horos Project). Findings were verified on two-dimensional planar reconstructions and recorded with 3D volume renderings. This research adhered to the ethical standards outlined by the World Medical Association (Declaration of Helsinki). The Bagdasar-Arseni Emergency Clinical Hospital approved the study (approval no. 2093/1 March, 2022). Informed consent was obtained from all participants.

On both sides, the tips of the styloid processes (SP) reached deep to the submandibular glands on the sides of the oropharyngeal isthmus (Fig. 1). On the right side, the tip of the SP was at 22.8 mm infero-medial to the mandibular foramen. On the left side, the distance between the tip of the SP and the mandibular foramen was 18.7 mm.

On each side, the FA traversed through the submandibular gland, postero-inferiorly to the tip of the respective SP (Fig. 1). The distance between the tip of the SP and the FA was 1.24 mm on the right side and 1.08 mm on the left side.

The lengths of the SPs were measured on thick, oblique sagittal slices. The right SP was 64.3 mm long, and the left one was 59.9 mm long. The right SP was angled at 64^o, referring to the plane of the zygomatic arch and the left at 70^o.

On the right side, the tip of the SP reached 13.5 mm postero-superiorly to the lesser horn of the hyoid bone (HB). On the left side, the distance lesser horn-to-SP was 20.4 mm.

On the right side (Fig. 2), the carotid bifurcation was at the level of the upper margin of the thyroid cartilage. The ECA was anterior to the ICA. The APA left from the medial side of ECA’s origin as the first collateral branch of it. The STA left as the second collateral at 3.2 mm distally to the APA, from the anterior side of the ECA. Before the right ECA reached the superior horn of the thyroid cartilage laterally, it gave off a LFT. The ECA and the LFT they continued applied on the lateral side of the greater hyoid horn (Fig. 3). Posterior to these two, the ICA and APA crossed laterally the hyoid tubercle at the tip of that greater horn. The LFT divided into the FA and LA at 9.5 mm above the greater hyoid horn. The bifurcation of the LFT was at 7.8 mm inferior to the right elongated SP. The right OA origin was at 6.2 mm distally to that of the LFT, also inferiorly to the greater horn. The OA further crossed the ICA laterally at the level of the greater horn. The length of the right LFT was 20.9 mm. The maximum outer diameters of the ECA and LFT were 3.4 mm and, respectively, 3.7 mm.

The left carotid bifurcation was also at the level of the superior margin of the thyroid cartilage. The STA and APA originated from the ECA inferiorly to the greater hyoid horn, while the LA and FA originated above the greater horn.

On both sides, the ECAs crossed posteriorly the SPs at the level of the mandibular foramen and 9.3 mm anterior to the inferior margin of the transverse process of the atlas.

Although the LFT’s prevalence, length, and diameter have been previously assessed [3-7], there is no documented evidence of an LFT originating from the infrahyoid region with a diameter comparable to that of the ECA, as reported here. Fazan et al. [4] reported that the LFT tends to have a higher origin, with no significant differences observed between sides and groups. The average length of the LFT was found to be 9.4±1.7 mm on the right and 7.6±1.3 mm on the left, and the average diameter was 2.1±0.2 mm on the right and 2.4±0.2 mm on the left, with no significant differences between sides and groups [4]. Dnyanesh et al. [5] documented a LFT length of 12 mm but did not provide any data on the calibre or its position relative to the HB. Simsar et al. [6] measured the LFT length as 9.19 mm and noted that it varies between 4.36 mm and 11.46 mm. Ozgur et al. [7] reported LFT lengths of 11.46 mm, 6.58 mm, and 4.36 mm in three distinct cases but did not mention the calibre nor the origin in relation to the HB. Another study measured the LFT diameter as 3.1 mm on the right and 3.3 mm on the left, originating from the carotid bifurcation at 7.6 mm and 6.8 mm, respectively. The lengths before dividing into the LA and FA were 3.1 mm and 3.4 mm on the right and left, respectively [8]. A cadaveric study documented a LFT length of 22 mm on the right, originating below the greater horn of the hyoid and 30 mm on the left, originating anteromedially from the ECA at the upper border of the thyroid cartilage; however, no report of the calibre was provided [9]. Notably, the LFT’s diameter does not significantly differ from that of the LA and FA, which are around 2.5–3 mm [4], despite expectations that it is larger due to the combined flow of the LA and FA [8]. Gonzalez et al. [10] reported a length of 8.84 mm from its origin in the ECA to its bifurcation into the FA and LA, with a diameter larger than 2.17 mm. Narayanan and Murugan [11] described two LFTs with infrahyoid origin. The left one originated from a TLFT, with a length of 20 mm, while the right one originated from the ECA and presented a length of 18 mm [11]. A bilateral TLFT-originating LFT, each measuring about 18 mm, was also reported without mentioning the origins relative to the hyoid level or the diameters [12]. Another LFT originating from a TLFT was mentioned, with a length of 20 mm and an origin at the level of the hyoid, but no calibre was specified [13]. Additionally, a study reported two LFTs with suprahyoid origins that measured 50 mm on the right and 60 mm on the left from the ECA. Still, no information on their diameter was given. The reported results are questionable due to the poor quality of the presented figures, which fail to demonstrate such considerable lengths [14] convincingly. A CTA study reported a mean length of the LFT of 9.23 mm and an internal diameter of 2.97 mm [15]. Unlike the previously stated studies, our case features a right LFT with an infrahyoid origin of 20.9 mm in length and 3.3 mm in outer diameter, in contrast to the 3.5 mm diameter of the ECA on the same side (Table 1).

Even though the LFT’s origin and anatomical course have been previously described, only a limited number of studies [4, 8, 10, 15] report the diameter of this vessel, with recorded values ranging from 2.1 to 3.3 mm (Table 1). Notably, the diameter we have presented, along with that reported by Troupis et al. [8], represents the upper limit of this range. Despite these findings, none of the studies provide data on the diameter of the ECA, which would allow for a comparison between the two vessels. Consequently, we recommend that future research on the LFT also includes measurements of the ECA diameter. This would offer a more comprehensive understanding of the regional anatomy and its clinical implications.

Large LFTs require careful consideration during surgical procedures, as they may be misidentified as the ECA. Damaging an LFT of comparable caliber to the ECA can result in significant intraoperative hemorrhage. Therefore, it is crucial to perform thorough imaging evaluations to distinctly identify and differentiate these vessels prior to surgery.