The Role of Transsphenoidal Surgery in the Management of Complex Lesions Involving the Skull Base

Introduction

Since the repopularization of the transsphenoidal approach by Guiot and Hardy, most surgeons have adopted this technique as the primary method for the removal of pituitary adenomas. Following the development of modern microinstrumentation and the use of the operating microscope, the transnasal transsphenoidal approach has become the preferred method for approaching lesions of the sellar and parasellar region. The transsphenoidal approach and its various modifications (the extended and parasellar transsphenoidal approaches) are also versatile approaches in the armamentarium of the cranial base surgeon to remove complex lesions of the skull base. Lesions of the skull base, particularly the anterior skull base, the clivus, and the cavernous sinus and parasphenoid region, can be readily accessed when the exposure of the standard transsphenoidal approach is extended with additional bone removal or combined with other transfacial approaches. Transsphenoidal approaches can be used alone or in combination with other skull base approaches depending on the extent and location of the lesion. The procedure can be performed microsurgically, endoscopically, or both. Knowledge of the anatomical limitations and the indications for using variations on the transsphenoidal approach is important when choosing the appropriate approach. The transsphenoidal approach provides a minimally invasive technique that avoids a craniotomy and prolonged brain retraction. In this chapter, we discuss the applications of the transsphenoidal approach and its variations in the management of complex skull base lesions.

Anatomical Limitations of Variations on the Transsphenoidal Approach

The transsphenoidal approach and its modifications are suitable for midline lesions that occupy locations from the cribriform plate down to the craniocervical junction ( Figure 14-1 ). Excellent visualization of the anterior skull base, suprasellar cistern, clivus, and cavernous sinuses can be achieved. The anterior communicating artery complex and the optic apparatus can also be visualized superiorly. It is critical to recognize the anatomical limits of the extended transsphenoidal approach when selecting the appropriate skull base approach for a given lesion. The anatomical limits are primarily defined anterosuperiorly by the posterior cribriform region, superiorly by the suprasellar cistern and optic apparatus, laterally by the cavernous sinus and carotid arteries, and inferiorly by the inferior clivus and foramen magnum. The inferior limit can vary with each individual patient and can be determined radiographically on a sagittal MRI by drawing an imaginary line from the hard palate toward the clivus (see Figure 14-1 ). This relationship of the hard palate to the clivus determines the extent of inferior visualization, which is based on the maximal inferior mobilization of the nasal retractor. In some patients, the craniocervical junction can be visualized. The extended transsphenoidal approach is best suited for accessing midline structures but is limited for removing tumors that extend laterally into the maxillary sinuses or superolaterally in the intracranial cavity. If the lateral extent of the lesion exceeds that of the transsphenoidal approach, the transmaxillary route can be used to supplement lateral exposure. Visualization beyond the limits of standard microsurgical visualization can be enhanced with adjunctive use of the endoscope, especially in the suprasellar direction or laterally into the cavernous sinus.

Surgical Indications for Variations of the Transsphenoidal Approach

The transsphenoidal approach and its variations can be used for a variety of skull base lesions. The location and extent of the lesion will determine the most appropriate skull base approach. The standard transsphenoidal approach is favorable for midline lesions arising from the pituitary fossa that may extend into the suprasellar region, into the clivus, or into the sphenoid sinus and nasal cavity. The exposure of the transsphenoidal approach can be extended by removing additional bone at the anterior skull base or laterally and inferiorly to access lesions of the parasellar and clival region, respectively.

Lesions of the anterior skull base can be accessed by removing bone from the planum sphenoidale and tuberculum sellae. We have used this approach for removing small midline tuberculum sellae/planum sphenoidale meningiomas that range from 2 to 3 cm in size ( Figure 14-2 ). This approach offers the advantage of early devascularization of the tumor at its base and removal of the tumor beneath the chiasm and optic nerves with no manipulation of the neural structures.

Craniopharyngiomas that occupy both sellar and suprasellar regions are favorable for transsphenoidal resection, especially if the sella is enlarged ( Figure 14-3 ) . However, if the tumor is purely suprasellar with a normal-sized sella, extended variations such as the transsellar/transdiaphragmatic approach or the transsphenoidal/transtuberculum approach can sometimes be used. In these cases, the additional bony exposure provides improved visualization of the suprasellar portions of the tumor and reduces the amount of blind curettage in this region. Cystic tumors are particularly amenable to drainage and removal by this approach. Excellent results of craniopharyngiomas resected via the transsphenoidal route or an extended variation have been reported by Laws et al, Maira et al, and Couldwell et al. The transsphenoidal approach may not be suitable for cases that have significant lateral extension. In these cases, a frontotemporal or a combined approach may be necessary. We prefer to use a transbasal or frontotemporal pterional approach if the tumor is located entirely suprasellar. We often use an orbitozygomatic approach for craniopharyngiomas that extend superiorly into the third ventricle.

Tumors of the clivus, such as chordomas or chondrosarcomas, are readily accessible with extended transsphenoidal approaches because these lesions are primarily extradural and are located in the midline ( Figure 14-4 ). Any extensions into the sphenoid sinus and nasal cavity are easily removed with the transsphenoidal approach. As mentioned previously, the lateral limitations of the approach are the cavernous carotid arteries; larger lesions with significant lateral extension may require combined approaches. Clivus chordomas with a significant lateral component may require a lateral skull base approach (petrosal, transtemporal, far lateral) for their removal, and those with extension below the inferior clivus level are approached via a transoral route.

Lateral exposure of the cavernous sinus to facilitate additional tumor removal may be achieved via the transsphenoidal exposure ( Figure 14-5 ). Additional bone over the carotid grooves can be unroofed, exposing the C3 portion of the internal carotid artery. The cavernous sinus is entered by opening the dura just medial to the carotid artery. The development of this particular technique has enabled more complete resection of adenomas, thereby reducing the need for blind curettage of the tumor, which is performed by reaching into the cavernous sinus via the sella. More recently, the indications for cavernous dissection in the setting of a pituitary tumor have been further reduced with the increased application of stereotactic radiosurgery. In cases where significant residual tumor is left in the cavernous sinus, we use this extended transsphenoidal variation to transpose the pituitary away from the tumor and interpose a fat graft between the cavernous sinus and the gland (hypophysopexy) in preparation for planned adjuvant radiosurgery for the residual tumor in the cavernous sinus. This technique reduces radiation exposure to the pituitary gland to minimize the risk of hypopituitarism. Tumors that extend laterally into the cavernous sinus may be visualized by placing an angled endoscope into the sella.

The spectrum of tumors accessed in a large series of patients in whom variations on the transsphenoidal approach was used is presented in Table 14-1 .

Table 14-1

Location and Histological Characteristics of Lesions Resected Via Variations of the Extended Transsphenoidal Approach

Location	Histological Characteristics	Cases
Anterior cranial base	Tuberculum/diaphragma sellae meningioma	11
Sellar/parasellar region	Pituitary adenoma with cavernous extension	30
	Sellar/suprasellar craniopharyngioma	27
	Sphenoid mucocele	10
	Sphenoid sinus carcinoma	4
	Breast adenocarcinoma	2
Clivus	Chordoma	18
	Monostotic fibrous dysplasia	3

(Reprinted with permission from Couldwell WT, Weiss MH, Rabb C, et al: Variations on the standard transsphenoidal approach to the sellar region, with emphasis on the extended approaches and parasellar approaches: surgical experience in 105 cases. Neurosurgery 2004; 55:539-547.)

Operative Techniques

Patient Positioning and Exposure of the Sella

The patient is placed supine on the operating table with the head elevated approximately 15 degrees on a horseshoe headrest. This positioning is preferred to rigid pin fixation, which limits intraoperative manipulation of the patient’s head to improve visualization. Further flexion allows visualization of lesions that extend inferiorly along the clivus. Extension of the head allows visualization of lesions with suprasellar extension ( Figure 14-6 ). In the standard transsphenoidal transsellar approach, the face is placed parallel to the ceiling. The sella can be exposed with either a sublabial or an endonasal approach as described elsewhere. A bivalve speculum is placed, the sphenoid sinus is exposed in the usual fashion, and all mucosa is exenterated. Specialized microinstruments, such as curved and angled alligator microscissors, a long monopolar coagulator with a malleable tip, and malleable-tip microring curettes, facilitate resection at the lateral extremes of the exposure. In addition, long, narrow bipolar forceps with up- or down-angled fine tips are used.