FIGURE 37.1 CT angiogram midsagittal reconstruction with a line drawn from the bony nasal bridge to the posterior hard palate and extended to the superior aspect of the spine. This nasopalatine line approximates the caudal extent of endonasal access.

The endonasal transodontoid approach avoids the morbidity of transoral/transpalatal approaches to the inferior clivus and superior cervical spine and is associated with a faster recovery (Table 37.1). Surgical advantages include improved visualization, greater access superiorly, and decreased bacterial contamination of the surgical field. Disadvantages include limited access to the inferior cervical spine and potentially greater risk of cerebrospinal fluid (CSF) leak following reconstruction of the dura. In our opinion, an endonasal approach is the preferred approach for lesions in the region of the foramen magnum that are bounded by the neural and vascular structures and for the resection of the odontoid process in the setting of basilar invagination.

HISTORY

Presenting symptoms will depend on the diagnosis, location, extent, and age of the patient. Patients may present with a serous middle ear effusion and conductive hearing loss due to eustachian tube dysfunction or obstruction. Involvement of cranial nerves can result in a wide variety of symptoms, including hypernasal speech and nasal reflux due to palatal dysfunction (CN IX/X); weak voice, dysphagia, and aspiration (CN X); and dysarthria (CN XII). Lower cranial nerve dysfunction can result from intradural compression or nerve involvement at their respective foramina. Coughing associated with meals or drinking should be questioned to check for active aspiration. A cervical mass (metastatic lymphadenopathy) is often the first symptom of nasopharyngeal cancer.

Bone disease with degenerative pannus or basilar invagination presents with myelopathy with or without dysphagia. Progressive loss of ambulation is a common complaint, and diagnosis is often delayed because of its insidious onset in elderly patients. Pain in the neck or occipital neuralgia can be indicative of active instability of the craniocervical junction.

PHYSICAL EXAMINATION

Physical examination includes a full examination of the head and neck including otoscopy, endoscopic visualization of the nasopharynx and hypopharynx, palpation of the neck, and assessment of cranial nerve function. Middle ear effusion may be the result of obstruction of the eustachian tubes. The type of hearing loss (conductive vs. sensorineural) can be confirmed with tuning fork testing (Weber and Rinne tests). The upper aerodigestive tract can be examined with a rigid nasal endoscope or flexible fiberoptic scope. The extent of mucosal lesions or masses should be noted. In particular, the fossa of Rosenmuller posterior to the eustachian tube should be carefully examined, as this is a common site for nasopharyngeal cancer. The neck should be palpated for metastatic cervical adenopathy. Assessment of cranial nerve function focuses on the lower cranial nerves: palatal dysfunction, vocal cord palsy, pooling of secretions in the hypopharynx with aspiration, and paresis of the tongue (deviation of tongue to the paralyzed side with protrusion). Range of mobility of the cervical spine can be grossly assessed with flexion, extension, and rotation. Signs of brainstem compression include weakness of the extremities and hyperreflexia. Gait assessment is important to determine if the gait is myelopathic or the result of another condition.

INDICATIONS

The primary principle of endonasal skull base surgery applies to the transodontoid approach: Avoid transgression or manipulation of major neural and vascular structures. If the lesion extends lateral to major vascular structures (carotid and vertebral arteries) or cranial nerves, an alternative approach or combination of approaches should be considered. For example, a large meningioma may require a combined approach (endonasal and retrosigmoid or far lateral) to access different areas of the tumor depending on its relationship to the neural and vascular structures.

Clinical indications for a transodontoid approach include neoplasms, inflammatory and degenerative disease with craniocervical compression, and trauma. The most common neoplasms include intracranial tumors (meningioma), neoplasms arising from bone (chordomas, chondrosarcoma), and malignancies of the soft tissues (nasopharyngeal carcinoma). Inflammatory or osteoarthritic degeneration or congenital abnormalities of the atlantoaxial joint and associated ligaments can result in chronic instability of the spine and resultant inflammatory pannus or progressive invagination with brainstem compression. Generally, only ventral bony compression needs to be addressed with an anterior approach, whereas soft tissue/pannus usually will resolve over time with posterior decompression and arthrodesis. In cases of severe pannus compression, we still perform an anterior approach to facilitate rapid decompression and avoid a posterior decompression, leaving the surface of the posterior C1 ring intact for fusion. Rarely, traumatic fractures or dislocations of C1 and C2 can be decompressed endonasally. This is more common in the delayed setting of an os odontoideum. The transodontoid approach is applicable to both adult and pediatric populations.

Neoplasms arising from the bone of the inferior clivus (chordoma, chondrosarcoma) are ideally suited for an endonasal approach. These tumors tend to have a midline or paramedian origin and displace neurovascular structures laterally. As a result, resecting the tumor through a midline approach minimizes or obviates any manipulation of these structures. Intracranial tumors such as meningiomas of the foramen can be approached endonasally if they are mainly midline and without significant involvement of the vertebral artery. Tumor relationship to the lower cranial nerves is key, with tumors that displace the nerves laterally being well suited for a midline, endonasal approach. CONTRAINDICATIONS

Active sinus infection is a contraindication for intracranial transnasal surgery but can usually be cleared rapidly with antibiotics with or without surgical drainage. Lesions that do not directly involve the occipitocervical joints in patients without preexisting instability should be approached with caution, taking care to avoid destabilizing the craniovertebral junction. Craniocervical meningiomas, which would require resection of the odontoid, should be considered instead for a posterolateral approach if it is more likely to maintain occipitocervical stability.

Rarely, especially in elderly patients, the parapharyngeal ICAs can become ectatic and loop medially to approach the midline behind the nasopharynx. This should be considered a relative contraindication to an anterior approach. Depending on the lesion and its location, though, dissection can still be performed superior and deep to the ICAs, mobilizing them in a cuff of soft tissue. PREOPERATIVE PLANNING

Any patient with signs of aspiration or subjective dysphagia should undergo a formal swallowing evaluation, including laryngeal endoscopy and an esophagram (barium swallow). If there is active aspiration preoperatively, the need for tracheostomy in the postoperative period should be discussed. Tracheostomy is not necessary for endonasal access to the craniocervical junction or superior spine, but it may be safest to prevent aspiration in a weakened postoperative state.

Magnetic resonance imaging (MRI) and computed tomography angiography (CTA) are complementary for both planning and intraoperative navigation. MRI demonstrates soft tissue involvement and neural compression as well as tumor characteristics that can help with the differential diagnosis. Craniocervical degenerative pannus can have a heterogeneous and atypical appearance but will appear more chronic on CT. CT can also show the degree of joint involvement, anomaly, or degeneration. CTA will demonstrate the degree of vertebral artery involvement and evaluate for an abnormal course of the parapharyngeal ICA, which can affect access. MRI should include both the skull base and cervical spine in the setting of pannus to determine if there is associated subaxial disease. Both modalities in addition to the clinical history and physical examination should be examined for evidence of acute or chronic sinus infection.

A midline sagittal reconstruction of the CTA should be evaluated for caudal extent of endonasal access. The simplest way to do this is to draw a line from the tip of the bony nasal bridge to the posterior hard palate and extend it to the spinal column (Fig. 37.1). This NPL is a rough approximation of the lowest point of access. The usual degree of access illustrated by postoperative imaging was 12.7 mm above that predicted by the NPL, likely due to the failure of the line to account for soft tissue limitations and the lack of need to always reach the most inferior point possible.

Patients with signs or symptoms of instability should undergo flexion/extension lateral cervical spine radiographs to check for evidence of atlantoaxial subluxation or subaxial mobility as this can be made worse by anterior decompression or impact the inferior extent of subsequent spinal fixation.

SURGICAL TECHNIQUE

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