Key points

Introduction

The goal of the endoscopic endonasal odontoidectomy for craniovertebral junction degenerative disease is cervicomedullary decompression when irreducible ventral bony, ligamentous, or other disease compresses the brainstem. The standard approach for odontoidectomy is represented by transoral or transcervical approaches with microsurgical support. In 2005, Kassam and colleagues proposed the endoscopic endonasal route as a less invasive approach to avoid splitting the soft or hard palate, retraction of the tongue, or the need for glossotomy or mandibulotomy with the related sequelae and possible complications for the patients. This approach provided a more direct, panoramic, and straightforward route to the odontoid process. Since this pioneering report, many series have documented the safety and feasibility of this approach and demonstrated high patient tolerance. It also provides quick recovery time due to immediate airway reestablishment without the need for tracheostomy or prolonged orotracheal intubation. There is also rapid resumption of oral feeding, avoiding the risk of postoperative dysphagia or swallowing impairment. However, some questions still remain open. In particular, the identification of proper and accurate surgical indications to maximize the advantages and minimize the limits of this approach. Furthermore, some technical points, such as whether to open the anterior arch of cervical vertebrae (C) 1 and management of the rhinopharyngeal mucosa (midline incision, direct skeletonization, or flap), are still debated. This article reviews the authors’ surgical experience and the current literature with the aim to identify in which conditions this approach is indicated. A variant of the surgical technique for the cases of craniovertebral malformation with platybasia and high position of the dens, which can provide a more direct access to the odontoid for such patients, is discussed.

Endoscopic endonasal anatomy of craniovertebral junction

The craniovertebral junction comprises the occipital bone and C1 and C2. It envelops and protects the brainstem, the upper spinal cord, the last cranial and the first spinal nerves, and the vertebral arteries. From the endoscopic endonasal perspective, this region represents the more caudal midline extension of this approach. Indeed, the inferior limit of the endoscopic endonasal approach is represented by a line passing through the anterior nasal spine and the posterior border of hard palate. Thus, normally, the endoscopic endonasal approach allows the surgeon to reach the body of C2, giving excellent exposure of the craniovertebral junction ( Fig. 1 A). The superior part of the craniovertebral junction is formed by the occipital bone, which constitutes the foramen magnum, where the medulla passes to the spinal canal and the atlas articulates with the skull base. Thus, from the endoscopic endonasal view, the dissection of the inferior third of the clivus permits reaching and exposing the foramen magnum at the midline (see Fig. 1 B). Laterally, this exposure is hampered by the presence of the 2 occipital condyles, where the first cervical vertebra connects with the skull and the hypoglossal canals are contained. To approach the dens, a strictly midline approach is sufficient because occipital condyles do not limit the view (see Fig. 1 B). Furthermore, their preservation prevents injuries to cranial nerve XII, passes the hypoglossal canal, and reduces postoperative instability. The posterior surface of anterior arch of the C1 arch joins with the odontoid process of C2 (see Fig. 1 B). Indeed, the axis has a typical shape, with this upward process, deriving from its body, which is completely visible, from an endoscopic endonasal point of view after the removing of the anterior arch of the atlas (see Fig. 1 C, D). Occipital bones, C1 and C2, are also connected by multiple ligaments. The anterior longitudinal ligament must be opened to access to the craniovertebral junction from a ventral route. It covers the atlantooccipital membrane, a broad and dense fibrous tissue that extends from the inferior border of the foramen magnum to the superior edge of C1. Its incision allows access to the bony structures of the region. When exposed, the dens is surrounded by a complex ligamentous system, formed by the alar, the apical, and the cruciform ligaments. The alar ligaments are thick fibrous structures that connect the dens to the occipital condyles. The apical is a midline structure that rises from the tip of odontoid to the foramen magnum. All of these ligaments should be sectioned to free the odontoid process and allow its removal.

Cadaveric dissection, 0° scope. ( A ) After posterior septostomy, the rhinopharyngeal region is exposed. The craniovertebral junction is in the midline by the inferior third of the clivus and the first cervical spine segments. This area is covered by the rhinopharyngeal mucosa and muscles. ( B ) After removal of the rhinopharyngeal mucosa and muscles, the occipital bone and the anterior arch of C1 are completely exposed. In normal conditions, the odontoid is covered for the most of its extension by this arch. ( C , D ) After opening of the anterior arch of C1, the dens can be exposed and resected, showing the dura behind the odontoid process. *, eustachian tubes; ˆ, Rosenmüller fossae; §, occipital condyles; °, odontoid process; #, cervical spine dura.

Introduction

The goal of the endoscopic endonasal odontoidectomy for craniovertebral junction degenerative disease is cervicomedullary decompression when irreducible ventral bony, ligamentous, or other disease compresses the brainstem. The standard approach for odontoidectomy is represented by transoral or transcervical approaches with microsurgical support. In 2005, Kassam and colleagues proposed the endoscopic endonasal route as a less invasive approach to avoid splitting the soft or hard palate, retraction of the tongue, or the need for glossotomy or mandibulotomy with the related sequelae and possible complications for the patients. This approach provided a more direct, panoramic, and straightforward route to the odontoid process. Since this pioneering report, many series have documented the safety and feasibility of this approach and demonstrated high patient tolerance. It also provides quick recovery time due to immediate airway reestablishment without the need for tracheostomy or prolonged orotracheal intubation. There is also rapid resumption of oral feeding, avoiding the risk of postoperative dysphagia or swallowing impairment. However, some questions still remain open. In particular, the identification of proper and accurate surgical indications to maximize the advantages and minimize the limits of this approach. Furthermore, some technical points, such as whether to open the anterior arch of cervical vertebrae (C) 1 and management of the rhinopharyngeal mucosa (midline incision, direct skeletonization, or flap), are still debated. This article reviews the authors’ surgical experience and the current literature with the aim to identify in which conditions this approach is indicated. A variant of the surgical technique for the cases of craniovertebral malformation with platybasia and high position of the dens, which can provide a more direct access to the odontoid for such patients, is discussed.

Endoscopic endonasal anatomy of craniovertebral junction

The craniovertebral junction comprises the occipital bone and C1 and C2. It envelops and protects the brainstem, the upper spinal cord, the last cranial and the first spinal nerves, and the vertebral arteries. From the endoscopic endonasal perspective, this region represents the more caudal midline extension of this approach. Indeed, the inferior limit of the endoscopic endonasal approach is represented by a line passing through the anterior nasal spine and the posterior border of hard palate. Thus, normally, the endoscopic endonasal approach allows the surgeon to reach the body of C2, giving excellent exposure of the craniovertebral junction ( Fig. 1 A). The superior part of the craniovertebral junction is formed by the occipital bone, which constitutes the foramen magnum, where the medulla passes to the spinal canal and the atlas articulates with the skull base. Thus, from the endoscopic endonasal view, the dissection of the inferior third of the clivus permits reaching and exposing the foramen magnum at the midline (see Fig. 1 B). Laterally, this exposure is hampered by the presence of the 2 occipital condyles, where the first cervical vertebra connects with the skull and the hypoglossal canals are contained. To approach the dens, a strictly midline approach is sufficient because occipital condyles do not limit the view (see Fig. 1 B). Furthermore, their preservation prevents injuries to cranial nerve XII, passes the hypoglossal canal, and reduces postoperative instability. The posterior surface of anterior arch of the C1 arch joins with the odontoid process of C2 (see Fig. 1 B). Indeed, the axis has a typical shape, with this upward process, deriving from its body, which is completely visible, from an endoscopic endonasal point of view after the removing of the anterior arch of the atlas (see Fig. 1 C, D). Occipital bones, C1 and C2, are also connected by multiple ligaments. The anterior longitudinal ligament must be opened to access to the craniovertebral junction from a ventral route. It covers the atlantooccipital membrane, a broad and dense fibrous tissue that extends from the inferior border of the foramen magnum to the superior edge of C1. Its incision allows access to the bony structures of the region. When exposed, the dens is surrounded by a complex ligamentous system, formed by the alar, the apical, and the cruciform ligaments. The alar ligaments are thick fibrous structures that connect the dens to the occipital condyles. The apical is a midline structure that rises from the tip of odontoid to the foramen magnum. All of these ligaments should be sectioned to free the odontoid process and allow its removal.

Cadaveric dissection, 0° scope. ( A ) After posterior septostomy, the rhinopharyngeal region is exposed. The craniovertebral junction is in the midline by the inferior third of the clivus and the first cervical spine segments. This area is covered by the rhinopharyngeal mucosa and muscles. ( B ) After removal of the rhinopharyngeal mucosa and muscles, the occipital bone and the anterior arch of C1 are completely exposed. In normal conditions, the odontoid is covered for the most of its extension by this arch. ( C , D ) After opening of the anterior arch of C1, the dens can be exposed and resected, showing the dura behind the odontoid process. *, eustachian tubes; ˆ, Rosenmüller fossae; §, occipital condyles; °, odontoid process; #, cervical spine dura.

Indications and preoperative management

Odontoidectomy is indicated in craniovertebral junction abnormalities of any origin (congenital, inflammatory, developmental, or traumatic) when there is a pure ventral irreducible cervicomedullary compression. When the abnormality is reducible, such as in some atlantoaxial subluxations, its realignment, followed by immobilization and/or posterior arthrodesis, is the treatment of choice. When the abnormality is irreducible, decompression is necessary and the surgery should be tailored based on the origin of the brainstem compression. For instance, in a case of dorsal compression, such as Arnold-Chiari malformation, a posterior decompression is indicated. Conversely, in a case of ventral compression, such as odontoid invagination, a ventral decompression (ie, odontoidectomy) is indicated. As a first step, the stability of the craniovertebral junction should be assessed. Radiographic studies in a flexion-extension position are the gold standard for this evaluation. If an unstable craniovertebral junction condition is observed, a posterior fusion (or a revision, if already performed) should be the first line of treatment. This is because, in multiple degenerative diseases of this region, such as rheumatoid arthritis, instability plays a key role in the development of the an inflammatory pannus around the odontoid, which exerts compression on the brainstem. For these cases, a simple posterior arthrodesis, resolving the first element of the pathogenesis of the disease, can be curative and favors the regression of the ventral reactive pannus through the immobilization of the junction. Thus, only for cases of ventral irreducible compression, the odontoidectomy is properly indicated. These cases are extremely rare, which justifies the low number of series and cases reported in literature. The indication for an endoscopic endonasal approach should be evaluated case by case. Two main elements should be considered: the radiological features of the brainstem compression and patient-related peculiarities, such as the presence of relative or absolute contraindications to the transoral approach. Crockard observed that, in cases of oral sepsis, reduced oral opening, or fixed flexion deformity of the head on the neck that does not allow sufficient opening of the mouth, the transoral approach should be discouraged. Macroglossia is another complication for the surgeon because it reduces the surgical field and increases the risk of postoperative ventilatory impairment. For these cases, the authors advise the endoscopic endonasal approach as an alternative to the transoral because the reduced oral surgical field does not impair it. CT scan and MRI give different information that should be considered in the preoperative surgical planning. In particular, the presence of cranial-vertebral junction malformations, especially platybasia and high position of the dens that causes brainstem compression, are relevant features for the surgical planning. Indeed, in these cases, the transoral approach is complicated by the vertical direction of work and the long route, whereas the endoscopic endonasal approach allows a direct and short path to the odontoid, thus it can be advantageously chosen.