The External Carotid Artery and the Styloid Process

Abstract

Background. The external carotid artery (ECA) is typically regarded as coursing between the styloid muscles to continue into the parotid space. The anatomical possibility of an ECA with an ascending parapharyngeal trajectory continuing posteriorly to an elongated styloid process (ESP), thus retrostyloid, to the parotid space is overlooked. It was, therefore, aimed to document the prevalence of this retrostyloid variant of the ECA's course. Methods. We investigated a retrospective random cohort of 160 archived CT angiograms of 97 males and 63 females aged between 47 and 76. The presence of an ESP and the retrostyloid course of the ECA were bilaterally documented. Results. An ESP was identified in 99/320 sides (30.94%), regardless of the ECA course. In the overall group, we obtained 35% null cases for the two variables on the right and 34.06% for the left. ESPs were identified in 8.75% on the right side and 10.31% on the left. The ECAs had retrostyloid courses in 6.25% on the right side and 5.63% on the left. Thus, of the 320 ECAs documented on both sides, 221 (69.06%) had no retrostyloid courses, and we did not identify any ESP in those cases. ESPs were detected in 19.06% of the sides but without retrostyloid ECAs, and retrostyloid courses of the ECAs were detected in 11.88%. Conclusions. The possibility of a retrostyloid course of the ECA should not be ignored. An ESP may misinform the surgeon about the main carotid artery located immediately deep to it.

Introduction

The styloid process of the temporal bone (SP) is derived from the second branchial arch. The morphology and geometry of the PS are variable [1]. The external carotid artery (ECA) leaves the carotid bifurcation and ascends to the parotid space. The anatomical literature considers that the ECA has a course between the styloid muscles and continues into the parotid space [2, 3], thus lateral to SP [4]. The anatomical possibility of an ECA with an ascending parapharyngeal trajectory continued posteriorly to the SP, thus retrostyloid, to the parotid space is overlooked.

During previous studies of the carotid system of arteries (Dumitru et al., 2024; Manta et al., 2023), the ECA was sometimes noticed to cross posteriorly the SP of the temporal bone. We, therefore, aimed to document the prevalence of this retrostyloid variant of ECA's course on a relevant batch of computed tomography (CT) angiograms.

Materials and Methods

We investigated a retrospective random cohort of 160 archived CT angiograms of 97 males and 63 females aged between 47 and 76.

The anatomic variants were documented using the Horos 3.3.6 software (Horos Project, Annapolis, MD, USA) on planar slices and three-dimensional volume renderings. The research was conducted ethically following the World Medical Association's Code of Ethics (Declaration of Helsinki). The responsible authorities (affiliation 2) approved the study (approval no.10540/16.02.2022).

The variables were documented as follows: (1) ESP without the distorted course of the ECA; (2) the retrostyloid course of the ECA, posterior to an ESP. Recording of results was performed bilaterally.

Results

In the male group (NM=97), we did not identify the tracked variables on the right side in 34.54% of cases and on the left side in 33.51%. We found the ESP in 10.31% of this sublot on the right side and, on the left (Figure 1), in 10.82% of cases. The ECA had a retrostyloid course on the right side in 5.15% and on the left in 5.67%.

Figure 1.

Three-dimensional volume rendering. Anterior view. Unilateral elongated styloid process (left side). External carotid arteries have no retrostyloid course. 1. right common carotid artery; 2. right internal carotid artery; 3. right external carotid artery; 4. right styloid process; 5. left common carotid artery; 6. left internal carotid artery; 7. left external carotid artery; 8. left elongated styloid process.

In the female group (NF=63), we did not identify the tracked variables on the right side in 35.71% of cases and on the left side in 34.92%. We found ESP on the right side in 6.35% of this subgroup (Figure 2) and on the left in 9.52% of cases. The ECA had a retrostyloid course on the right side in 7.94% and on the left in 5.56%.

Figure 2.

Three-dimensional volume rendering. Anterior view. Unilateral (right side) retrostyloid course of the external carotid artery. 1. right internal jugular vein; 2. right common carotid artery; 3. right internal carotid artery; 4. right external carotid artery; 5. right styloid process; 6. left internal jugular vein; 7. left common carotid artery; 8. left internal carotid artery; 9. left external carotid artery; 10. left styloid process.

Thus, in the overall group (N=160), we obtained 35% null cases for the two variables on the right and 34.06% for the left. ESPs were identified in 8.75% on the right side and 10.31% on the left. The ECAs had retrostyloid courses in 6.25% on the right side and 5.63% on the left.

Thus, of the 320 ECAs documented on both sides of the median plane, 221 (69.06%) had no retrostyloid courses, and we did not identify any ESP in those cases. ESPs were detected in 19.06% of the sides but without retrostyloid ECAs, and retrostyloid courses of the ECAs were detected in 11.88%. Referring only to the evidence of ESP, it was identified in 99/320 sides (30.94%), regardless of the ECA course.

In the male subgroup (NM=97), we found bilateral null combinations of the two variables in 65.98%. The unilateral presence of an ESP was detected in 3.09% and unilateral retrostyloid ECA in only one case, 1.03%. A bilateral combination of ESP but without retrostyloid ECA was present in 17.53%, a combination of ESP with contralateral retrostyloid ECA (Figure 4) in 4.12%, and a bilateral combination of retrostyloid ECAs (Figure 5) in 8.25%.

Figure 4.

Three-dimensional volume rendering. Anterior view. The styloid process is elongated bilaterally, but the external carotid artery has a unilateral retrostyloid course (left side). 1. right common carotid artery; 2. right carotid bifurcation; 3. right internal carotid artery; 4. right external carotid artery; 5. elongated right styloid process; 6. left common carotid artery; 7. left carotid bifurcation; 8. left internal carotid artery; 9. left external carotid artery; 10. elongated left styloid process.

Figure 5.

Three-dimensional volume rendering. Anterior view. Bilateral elongated styloid process and external carotid artery with retrostyloid course are identified. 1. right carotid bifurcation; 2. right external carotid artery; 3. right styloid process; 4. right internal carotid artery; 5. left carotid bifurcation; 6. left external carotid artery; 7. left styloid process; 8. left internal carotid artery.

In the female subgroup (NF=63), we found null bilateral combinations of variables in 66.67%. Unilateral presence of ESP was detected in 3.17%, and unilateral retrostyloid ECA in 4.76%. A bilateral combination of ESP but without retrostyloid ECA course was found in 7.94% (Figure 3), a combination of ESP with contralateral retrostyloid ECA in 12.7% and a bilateral combination of retrostyloid ECA in 4.76%.

Figure 3.

Three-dimensional volume rendering. Anterior view. The styloid process is elongated bilaterally, but the external carotid artery has no retrostyloid course. 1. right carotid bifurcation; 2. right internal carotid artery; 3. right external carotid artery; 4. right elongated styloid process; 5. left carotid bifurcation; 6. left internal carotid artery; 7. left external carotid artery; 8. left elongated styloid process

The bilateral distribution of variables in the overall group (N=160) was determined. Bilateral absence of the two investigated variables was detected in 106/160 (66.25%) cases, unilateral ESP in 5/160 (3.13%) cases, unilateral retrostyloid ECA in 4/160 (2.5%) cases, bilateral ESP in 22/160 (13.75%) cases, bilateral combination ESP-retrostyloid ECA in 12/160 (7.5%) cases, and bilateral retrostyloid ECAs in 11/160 (6.88%) of cases.

Discussion

The styloid process, posterior belly of the digastric muscle and styloid muscles make up the styloid diaphragm. Posteromedial to it are located the internal carotid artery (ICA), internal jugular vein (IJV), cranial nerves 9-12 and the cervical sympathetic chain [5, 6]. Thus, the SP is located in the external wall of the parapharyngeal carotid space [7]. The IJV, ICA and glossopharyngeal, vagus and accessory nerves lie medial to the SP, while the occipital artery and hypoglossal nerve run along the lateral aspect of the SP [8]. Various studies of the SP have determined the distances between it and internally located elements such as the palatine tonsil and ICA [9] but have not observed the possible course of the ECA on its internal side [7, 10].

A dissection study of six cadavers followed arterial variations in the parapharyngeal space [11]. Variational possibilities of the ascending palatine and ascending pharyngeal arteries were observed [11]. In 11/12 cases (92%), the ICA and ECA were separated by the styloglossal diaphragm and pharyngeal venous plexus. Still, in 1/12 cases (8%), the ECA was embedded in the parapharyngeal fat between the styloglossal and stylopharyngeal muscles [11]. As the batch was small, the authors did not observe or note the morphology and geometry of PS.

Surgical approaches to the parapharyngeal space are challenging [12]. The transoral route is rarely used and only for small tumours [12]. The transoral window extends between the medial pterygoid and superior pharyngeal constrictor muscles [12]. The stylopharyngeal and styloglossus muscles are critical landmarks in this approach, and it is usually considered that posterior to them, the ICA, IJV and last four cranial nerves are exclusively found [12]. The present study demonstrates that the ECA also passes occasionally here, whose iatrogenic lesion should thus be avoided. A clinically evident ESP by palpation should raise the suspicion of an ECA with a retrostyloid course. Recently, a meta-analysis calculated an overall ESP prevalence of 30.2%, with a trend towards bilaterality, but not for gender, age or geographical location of the study population [13]. In the present study, we detected ESP with a prevalence of 30.94%.

A retrostyloid ECA enters the parotid space. The access is made behind the SP and not between styloid muscles. Thus, in various surgical procedures on the pathological parotid gland [14], the SP can be a critical bony landmark for identifying the ECA or, on the contrary, a landmark that confuses the surgeon and increases the risk of iatrogenic injury to the ECA. There are surgical techniques in which parotidectomy is facilitated by the section of the digastric and styloid muscles and by the fracture or removal of the SP [14]. A surgical fracture of the SP with retrostyloid ECA puts this artery at risk.

The ECA is typically located between the styloglossus and stylohyoid muscles, and the glossopharyngeal nerve runs deep to this artery [5]. A retrostyloid course of the ECA will cause it to continue between the SP and the stylohyoid muscle to enter the parotid space. In these situations, the typical landmark represented by the styloglossus muscle is replaced by the SP.

The SP, the stylohyoid ligament and the lesser horn of the hyoid bone together form the stylohyoid chain [9]. Differences in the degree of ossification and how the segments of the stylohyoid chain connect on either side lead to variations in the SP's length, orientation, thickness and straightness [9]. The SP is difficult to observe by magnetic resonance imaging (MRI), and computed tomography (CT) remains the primary diagnostic method [9].

A recent study demonstrated lateral loops of the ECA in the parotid space, which alters the usual topography of this space [15]. The present study also shows the possibility of a retrostyloid course of the ECA in the parapharyngeal space, proximal to the parotid space. However, open surgery makes a progressive surgical dissection possible by observing anatomical variants that may alter the exposure and maneuverability of the ECA and ICA [16].

Eagle (1948) described the elongated SP (ESP) syndrome, which would later receive its eponym, and classified cases into two groups: those with typical oropharyngeal syndrome and those with pain occurring in the neck, in the territory of distribution of a carotid artery, either ICA, through the medial angulation of the ESP, or ECA, through the lateral angulation of the PSA [17, 18]. Arterial damage by ESP is also accompanied by irritation of the pericarotid nerves in the arterial sheath [19]. Eagle objectified the ECA syndrome in one surgical case [17]. If the ECA is involved, facial pain occurs up to the eye [18]. The ICA involvement results in head pain from the ophthalmic to the occipital region [18]. If the ECA has a retrostyloid course, both the ECA and the ICA could be affected, and therefore, the typical clinical picture would be blurred.

In conclusion, surgeons should not assume a predefined trajectory of the ECA but document its possible anatomical variations on a case-by-case basis.

Funding

This research received no specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data Availability

The datasets used and analyzed during the current study are available from the corresponding author upon reasonable request

Conflict of interests

The authors have no conflict of interest to declare.