The most common tumors of the lower clival region are chordomas and chondrosarcomas, followed by other rare tumor types such as plasmacytomas and pituitary adenomas, which have been described either with extra- or intra-dural localization (1). The surgical management of pathologies involving the clivus and craniocervical junction has always been considered a complex procedure because of the deeply located surgical site and the surrounding complex neural and vascular anatomical structures. Many approaches have been proposed in the literature with variable advantages and disadvantages (2). The routes of these approaches utilize endonasal, transoral, and transcervical corridors while using a surgical endoscope or microscope (3).

In 1957, Southwick and Robinson described the anterior retropharyngeal pre-vascular approach to the upper cervical spine (4). In 1986, Lesoin et al. (5), and in 1987 McAfee et al., modified and popularized this approach and provided extended access to the clivus and the upper three cervical vertebrae (6). Stevenson et al. reported the first study describing a high anterolateral retropharyngeal pre-vascular approach for the management of a clival chordoma (7). In 2011, Russo et al. performed 20 mm × 20 mm minimal anterior clivectomy with the high anterior cervical approach. It also showed the anatomical structures exposed by minimal anterior clivectomy and demonstrated that the cranial nerves (CN) VI, IX, X, and XI; the anterior inferior cerebellar artery (AICA); and the basilar artery (BA) can be seen in any cadaver with minimal anterior clivectomy (8). With the widespread use of endoscopes in neurosurgery, it has been shown that the endoscope can also be used in the anterior cervical approach (9, 10).

In this cadaveric study, we described a modified endoscope-assisted high cervical anterolateral retropharyngeal approach (HCALR) in which each stage of the dissection was demonstrated step by step with endoscopic camera views. Comparisons of previously described approaches that emphasize advantages and disadvantages have also been discussed.

Six adult cadaveric specimens with colored silicone-injected vasculature were used. While performing the high cervical anterolateral retropharyngeal approach, each step of the dissection was documented using a rigid endoscope (Karl Storz®, STORZ, Germany) 4 mm in diameter, 18 cm in length, with 0° lenses, and a Canon EOS 5D Mark II (Canon Incorporated, Tokyo, Japan) camera. This approach was performed bilaterally in one cadaver and unilaterally in the remaining cadavers. C-arm fluoroscopy was used to confirm localization. The dissections were performed at the Center of Advanced Simulation and Education Neuroanatomy Laboratory, Acıbadem MAA University in İstanbul, Turkey.

The High Cervical Anterolateral Retropharyngeal Approach is primarily suitable for ventral and midline lesions of the lower clivus, foramen magnum, and C1 and C2 vertebrae. The most common tumors in this region are chordomas, chondromas, meningiomas, and schwannomas (1). However, in tumors with lateral and posterior extensions, suboccipital or transcondylar approaches may be a more appropriate choice. In addition, this approach is also suitable for cases of atlantoaxial dislocation to allow for anterior decompression at the upper cervical level.

Conventionally, the first popularized approach to the lower clivus and upper cervical spine is the transoral approach. The main drawbacks of the transoral route compared with the retropharyngeal approach are the infection rates and oropalatopharyngeal complications (11, 12). In addition to the sterile surgical environment, the post-operative recovery time is significantly shorter in the anterolateral retropharyngeal transclival approach due to tracheostomy and the nasogastric tube not being needed (12). The anterior retropharyngeal approach provides an adequate exposure of the anterior superior cervical spine and lower clivus, while maintaining a wider lateral exposure. One of the disadvantages of this approach is the apparent focused exposure to the lower clivus. In the study of Agrawal et al., where the transoral and extraoral approaches were compared, the working area of the transcervical approach was wider than the transoral approach, but the working length on the C1 anterior tubercle was longer than the transoral approach. In addition, the transoral approach has an anterior-to-posterior trajectory, while the transcervical approach has a lateral-to-medial trajectory. This means that the transoral approach provides a straight view, while the transcervical has an oblique view to the C1 anterior tubercle (11).

The endoscopic endonasal approach has become a popular mode of approach to the lower clivus in recent years. Its main advantage is the wide viewing angle and access to the upper and middle clivus. Since using a natural anatomical corridor, the lack of the need for retraction makes this approach superior in certain regards (13). However, a major disadvantage is the relatively common cerebrospinal fluid (CSF) leakage. In the review study conducted by Morales-Valero et al. where 20 studies describing the endoscopic endonasal approach in craniovertebral junction pathologies were examined, 18% of the patients developed intraoperative and 4.2% post-operative CSF leaks (14). The risk of CSF leakage is also possible in the transcervical approach, sans the risk of meningitis due to the aforementioned sterile environment. As the target of the surgery shifts caudally to the lower clivus and upper cervical spine, the retropharyngeal approach takes the advantage in exposure, which allows instrumentation if needed (10). The endoscopic endonasal approach also has an anterior-to-posterior trajectory, similar to the transoral approach.

Modified transcervical approach is only indicated for lesions involving the lower and middle clivus. In this approach, clivus access can be achieved by partial resection of the ipsilateral C1 anterior arch. This allows for preservation of the atlantoaxial ligaments and consequently reduces the risk of post-operative craniocervical instability. In addition, if the target lesion extends into the upper cervical region and requires a more extensive resection, craniocervical instability is inevitable.

On the other hand, the limited space in which important neurovascular and visceral structures are located can cause difficulties for surgeons using the retropharyngeal approach. Extensive retraction may lead to injury of pharyngeal musculature, whereas limited retraction of the midline musculature may lead to limited visualization of the midline and contralateral structures.

The purposed approach of this study differs from the previously described transcervical approaches with regard to patient positioning, initial incision, used corridor, and dissection. Many of the previously described incisions include a horizontal incision starting from the tip of the mastoid coursing until the symphysis menti. Complementary vertical (T-shaped) (6, 7) or oblique (L-shaped) (5) oriented incisions have been described in the literature (3, 8). The V-shaped (modified Schobinger) incision of this study starts with a horizontal incision starting at a point just inferoposterior to the angle of the mandible. The aim here is to expose the anterior part of the parotid gland to visualize branches of the facial nerve arising at the posterior pole of the parotid gland. The incision continues parallel to the body of the mandible just 3 cm inferior to it to decrease the possibility of injuring the marginal mandibular branch of the facial nerve (8). The oblique part of the V starts from the apex, and courses parallel to the SCM muscle until it reaches the posterior belly of the digastric muscle. The oblique part of the V, parallel to SCM, is also shorter than previously described incisions (Figure 3) (11). The oblique part of the incision can be lengthened inferiorly and the corridor below the digastric muscle can be used to reach pathologies below the C2 vertebra. This corridor coincides with the angle of the mandible, which can be used as a landmark to maintain good orientation of this corridor in deeper dissection. In addition, retraction of the submandibular gland is not necessary, which decreases the risk of injuring the marginal mandibular branch of the facial nerve, lingual nerve, HN, facial artery, and vein (15–17). However, a large submandibular gland can still cause obstruction to the surgical view and exposure. Another cause for limited visualization can be a small jaw. In these cases, the submandibular gland can be dissected from the surrounding tissues and mobilized or excised. Freeing the digastric muscle from the hyoid bone by removing the fibrous sling and separating the posterior belly of the digastric muscle from the stylohyoid muscle was used previously to allow an open corridor for the surgery, but a similar exposure can be gained with gentle retraction of the posterior belly of the digastric muscle inferiorly. Limiting this step minimizes the risk of injuring the ansa cervicalis, HN, glossopharyngeal nerve, superior laryngeal nerve, superior thyroidal artery, carotid sheath, and the structures in them (18–20).

One of the most serious difficulties of the transcervical approach is the need for retraction. More severe complications can also occur including paresis of the superior laryngeal nerve and HN due to excessive retraction (21, 22). However, since the approach is submandibular rather than subdigastric, the injury risk of the superior laryngeal nerve compared with the conventional submandibular approach is lower. The superior laryngeal nerve is out of the operation field, and this minimizes the possibility of superior laryngeal nerve palsy in the post-operative period (20). It can also be argued that performing this approach with endoscope will reduce the need for retraction and therefore reduce the complication rate.

Marginal mandibular nerve injury is another complication of the transcervical approach. While dissecting the superficial layer of the deep cervical fascia, one should be extremely cautious with the marginal mandibular branch of the facial nerve. This nerve is one of the structures that has a higher injury potential. To decrease this probability, the incision is made 3–4 cm below the body of the mandible and after identifying the marginal mandibular branch of the facial nerve, it can be dissected and mobilized superiorly to avoid injury in the following steps of the dissection (23). At the same time, it is thought that using an exoscope in these macroscopic stages of the procedure will make it easier to recognize the anatomical structures and reduce the possibility of injury to these structures. In case of injury, an asymmetrical smile, and elevation of the lower lip on the ipsilateral side can be seen post-operatively (17, 24).

Maintaining midline orientation is vital for the safety of the deep dissection for the submandibular retropharyngeal approach. To provide adequate exposure to the surgical target, head rotation is required, which makes the midline orientation harder. This positioning may vary depending on the surgeon, but an angle of 15° extension and 30° rotation to the contralateral side is adequate (6–8, 25, 26). The angle of the mandible should be positioned to avoid obscuring the surgeon's field of view (Figure 5). Another important point is the position of the clivus. Clival drilling should not be started without proper orientation. Without proper orientation, the vertebral arteries at C1 level remain at risk while looping over the posterior arch of the C1 vertebra. At this stage, the pharyngeal tubercle or C1 anterior tubercle can be used as a landmark. Another stage where midline orientation is vital is dissection of the retropharyngeal space. Retromandibular fossa is located superolateral to the retropharyngeal space and it includes the temporal artery, the spinal accessory nerve, and the hypoglossal nerve, which necessitates care while working on the superior and lateral borders (27).

The use of an endoscope in the transcervical approach makes this approach more reliable as previously discussed in the literature (2). Since rigid retraction is not needed as much, the risk of retraction-related injuries decrease. At the same time, angled endoscopes provide a wider field of view compared with microscopes. In this way, the major shortcomings associated with the long working corridor can be overcome.

There are various but a limited number of anatomic studies in the literature describing the high cervical retropharyngeal approach in clivus and craniocervical junction pathologies (3, 8, 11, 28, 29). The study of Russo et al. is an endoscope-assisted anatomical study that uses the corridor under the digastric muscle and includes minimal anterior clivectomy (8). Baird et al. compared endoscopic endonasal, transoral, and transcervical methods in terms of working angle, distance to clivus, and working areas (3). Agrawal et al. compared transoral and transcervical approaches in terms of distance from the dura and tip of the anterior tubercle of C1 (11). Salle et al. described the endoscopic anatomical study using the submandibular corridor and measured the distances between eight predetermined points in the craniocervical region (28). Li et al. used the corridor under the digastric muscle to reach the craniocervical junction and discussed the lateral boundaries of the clivectomy. In addition, the effects of the atlas and odontoid resections on the extent of exposure have been shown (29). Although it had been shown in previous literature, our experience shows that it is impossible to view the clival region straight on similar to the endonasal approaches in the transcervical approaches because the pharyngeal muscles and the midline structures cannot be lateralized beyond the midline (8, 29). In addition, in lesions that are limited to the clivus that do not extend into the upper cervical vertebral area, using the corridor in the submandibular trigone over the digastric muscle will both reduce the working distance to the clivus and reduce the risk of neurovascular injury due to retraction.

This study has several limitations that should be considered when interpreting the results and translating them to clinical practice. Our study focused solely on the anatomical feasibility of the modified HCALR approach using cadaveric dissections. While this provides valuable insights into the approach's potential, it does not translate directly to clinical outcomes in live patients. The study did not assess the potential complications and risks associated with the modified HCALR approach in a clinical setting. Real-world factors like bleeding, tissue manipulation, and surgical technique can significantly impact complication rates. It is recommended to use the exoscope during superficial macroscopic stages to better simulate real surgical procedures.

By addressing these limitations and conducting further research, we can gain a more comprehensive understanding of the modified HCALR approach’s potential role in clinical practice.

In this study, a modified high cervical anterolateral retropharyngeal transclival approach is described and compared with the methods described previously. This study is highly demonstrative of the regional anatomy, all important neurovascular structures, and their relations to each other with stepwise dissections. It could serve as a guide for other surgeons to lower complications.