Transoral Surgery for Irreducible Atlantoaxial Dislocation Complicated by Concomitant Aberrant Internal Carotid Arteries

Irreducible atlantoaxial dislocation (IAAD) is a rigid C1-C2 deformity that often requires extensive surgical treatment. Because of the narrow surgical field and the complexity of the adjacent neurovascular structures, the surgical treatment of IAAD remains a challenge¹⁾. We describe our experience of transoral anterior release for a patient with IAAD complicated by concomitant bilateral aberrant internal carotid artery (ICAs).

A 69-year-old woman presented with a 10-year history of progressive myelopathy. She also suffered from central sleep apnea and required continuous positive airway pressure (CPAP) therapy. X-ray and computed tomography (CT) showed an upper cervical anomaly of the occipito-atlantal junction and C2-C3 assimilation with basilar invagination (Fig. 1A, B, and C). Magnetic resonance imaging of the cervical spine showed compression of the medulla and spinal cord due to the invagination of the dens (Fig. 1D). Three-dimensional CT angiography was performed for routine preoperative evaluation of the vertebrobasilar arteries, including the circle of Willis, and it coincidentally found marked aberration of the bilateral ICAs, which were tortuous and had abnormal medial courses on the front of the atlantoaxial segment (Fig. 1E).

Figure 1.

Postoperative images.

X-rays and computed tomography (CT) showed the upper cervical anomaly of the occipito-atlantal junction and C2-C3 assimilation with basilar invagination of the odontoid process (A, B, D). Magnetic resonance imaging showed Chiari type 1 malformation and compression of the medulla and upper cervical cord (C). CT angiography showed the aberrant internal carotid arteries through the atlantoaxial segment (E).

Although the presence of aberrant ICAs may pose a substantial risk during the anterior approach to the upper cervical spine, we decided to seek anatomical correction via circumferential atlantoaxial release to maximize the patient's neurological improvement. In the first surgery, we resected the posterior arch of C1 to decompress the cord and further attempted to release the atlantoaxial joint through a posterior approach. However, the posterior surgery alone did not achieve sufficient reduction. Aberrant ICAs have been reported to be associated with degenerative changes and malalignment of the cervical spine²⁾. Therefore, we conducted halo traction after the initial surgery to realign the cervical spine and lengthen tortuous ICAs. Traction was up to 10 kg for 1 week but resulted in no noticeable change. Thus, we proceeded to the second surgery with the transoral anterior release, supported by multimodal medical devices. To monitor the oxygen saturation of the cerebral surface, an in vivo optical spectroscopy (INVOS) monitor was used. Retractors were placed in the oral cavity to ensure the surgical field. The soft palate was fished out and retracted with a nelaton catheter inserted through the nasal cavity, and the reference arc of the navigation system was attached to the halo ring via a metal connector (Fig. 2). Enhanced CT-based navigation was used to guide not only the orientation of atlantoaxial bony structure but also the running courses of the ICAs. Additionally, the retropharyngeal surgical window was assessed via transoral ultrasonography with color Doppler (Fig. 3). While assessing the anatomical structures with navigation and ultrasonography, small incisions were made on the pharyngeal mucosa, and the attachment of the longus colli muscles to the C1 anterior arch was cut. Then, the anterior arch was resected at a width of 15 mm. The ligaments attaching to the odontoid process were also released using an ultrasonic scalpel. Next, the patient was returned to the prone position. Under an image intensifier, manual force was gradually exerted onto the C2-C3 spinous processes, pushing the subaxial spine ventrally. Posterior fixation was finalized with an O-C2 angle of 20° (Fig. 4). At the postoperative 2-year follow-up, the patient was able to walk without any support. Her hand clumsiness also improved, and she could sleep without CPAP. There were no major perioperative complications.

Figure 2.

Setting prior to the second surgery.

The soft palate was fished out using a nelaton catheter inserted through the nasal cavity, and the pharynx was expanded using retractors. The reference arc for intraoperative navigation was attached to the halo crown using a connector (bold arrow).

Figure 3.

Transoral carotid ultrasonography with color Doppler.

Color Doppler ultrasonography showed that the right internal jugular vein and internal carotid artery ran through the C1 anterior arch. The probe was placed at the yellow dotted line. Green areas are safe for incision.

Figure 4.

Postoperative images.

The irreducible atlantoaxial dislocation with basilar invagination was reduced (A), and the spinal cord was decompressed (B) after atlantoaxial anterior release (black dotted line) (C).

Retropharyngeal aberrant ICAs pose a significant risk of inadvertent injury and potentially fatal massive bleeding³⁾. The etiology of aberrant ICA is still controversial. The embryological approach proposed that incomplete straightening and persistence of embryonic angulation may result in the presence of aberrant ICAs in the retropharyngeal space^4,5). In this case, the abnormal medial course of the ICAs remained unchanged, despite halo crown traction after the initial posterior release. Plausible explanations include not only simple cervical shortening but also embryological maldevelopment and/or age-related arteriosclerosis that caused a loss of elasticity in the vessel walls³⁾.

As previously reported, enhanced CT-based navigation is a powerful tool in transoral anterior release surgery for detecting the anatomical location and the abnormal course of ICAs⁶⁾. As an additional surgical guide and for safety, it was beneficial to evaluate the running courses of ICAs and the cerebral blood flow in real-time intraoperatively using pharyngeal ultrasonography⁷⁾, cerebral angiography⁸⁾, and near-infrared spectroscopy (NIRS)^9,10). When making an incision in the pharynx, color Doppler ultrasonography helped identify a safe surgical window. An INVOS monitor (in vivo NIRS device) can noninvasively measure the regional saturation of oxygen (rSO₂) to determine the local blood supply and demand balance in the brain⁹⁾. In this case, the baseline rSO₂ value was approximately 70% and did not decrease below the trigger values during surgery.

Among cases with retropharyngeal aberrant ICAs, spine surgeons should pay meticulous attention to preventing ICA injuries and thrombus formation during surgery when planning an anterior approach to the upper cervical spine.

Conflicts of Interest: The authors declare that there are no relevant conflicts of interest.

Author Contributions: KI designed and executed the experiments and wrote the manuscript. ET was a major contributor to writing the manuscript. FH contributed to introducing the concept of neurosurgery. MT contributed to introducing the concept of intensive care. YM, YI, TM, SI, YK, YT, AH, and SI contributed to introducing the concept of orthopedic surgery and helped write the manuscript. HC is a supervisor and editor of the manuscript. All authors reviewed and approved the final manuscript.

Ethical Approval: The case report has been granted an exemption by our institutional review board (Gunma University).

Informed Consent: Informed consent was obtained from the patient in this study.