Microscope-Guided Endonasal Transsphenoidal Approach
Last Updated: September 27, 2018
The endonasal transsphenoidal approach has evolved during the past century. Harvey Cushing played an instrumental role in popularizing the endonasal route as it was pioneered by surgeons of his time, including Schloffer and Halstead. During the infancy of cranial surgery, transsphenoidal surgery literally placed Cushing on the brink of exploring brain surgery.
As lighting, endoscopy, and instrumentation evolved during the past decades, so have the surgical limits of what is possible through the endonasal corridor. The transsphenoidal approach has become the preferred route of access to parasellar lesions such as pituitary adenomas, meningiomas, and craniopharyngiomas. Endoscopy has obviated the need to resect pituitary tumors through the transcranial corridor. In contrast to the pterional and orbitozygomatic approaches, the transnasal approach eliminates the need for brain retraction.
In the past few years, the endonasal route has been used in conjunction with endoscopy to access the anterior cranial base, parasellar, and paraclival regions. In this chapter, I review the tenets for the microscopic transnasal approach. Since our ear, nose, and throat colleagues often perform the endoscopic transnasal exposure for us, I will not address the corresponding nuances for the endoscopic approach in this Atlas. However, neurosurgeons continue to perform the transnasal exposure using an operating room microscope, and this chapter is dedicated to the description of this technique. The neurosurgical methodologies for resection of skull base tumors through the endoscopic transnasal route are discussed in the Pituitary and Parasellar Tumors chapters of the Brain Tumors Volume.
Indications for the Approach
The microscopic endonasal transsphenoidal approach is ideal for reaching lesions confined to the paramedian sellar and suprasellar territories medial to the carotid arteries and inferior to the subchiasmatic space. Substantial vertical growth of the tumor is not a contraindication as long as the tumor does not have extensive lateral dispersions beyond the limits of the cavernous sinus. Common lesions treated via microscopic transnasal surgery include pituitary adenomas, Rathke’s cleft cysts, and selected parasellar meningiomas, craniopharyngiomas, and clival chordomas. The addition of endoscopy radically expands the reach of the transnasal exposure to access lesions beyond the sella and along almost the entire skull base.
By utilizing an endoscope, the surgeon can gain an expanded view that reaches the anterior skull base for orbital groove, planum sphenoidale, and tuberculum sella meningiomas. Third ventricular craniopharyngiomas, paraclival chordomas, and condrosarcomas are easily uncovered using endoscopic endonasal surgery. For tumors with significant suprasellar and middle fossa extension, I attempt an initial resection through the transnasal route and prepare the patient for a second stage transcranial surgery.
Any parasellar lesion demands that the patient receives a through pituitary axis hormonal evaluation to rule out perioperative inadequate pituitary reserve. The patient’s serum prolactin, cortisol, and thyroxine levels are especially important. Routine use of perioperative glucocorticoids has eliminated intraoperative hypocortisolemia, but pre-existing hypothyroidism may present acutely after surgery. I have been surprised by very unusual asymmetric imaging presentations of prolactinomas centered somewhat away from the midline.
It is critical that the surgeon has a clear understanding of the location of the carotid arteries. The ectatic arteries may be misplaced to the midline and easily injured during the dural opening and later dissection. Invasive tumors can destroy bony landmarks along the sella and significantly disorient the operator.
If the exact site of an arterial injury cannot be immediately discovered and sealed, the operative field should be packed with cotton to control bleeding, surgery aborted, and an arteriogram performed immediately to rule out a pseudoaneurysm.
Neuronavigation (CT guided) or at least lateral fluoroscopy is especially instrumental in successful execution of the microscopic transnasal route. Invasive tumors and scarring from previous operations can distort normal landmarks and easily disorient the operator. An inappropriate trajectory can lead to misguided bone removal at the skull base and place neighboring cerebrovascular structures at great risk.
A prophylactic lumbar drain may be placed intraoperatively if a repeat operation or procedure with a high risk of postoperative CSF leakage is planned. The patient’s abdomen is routinely prepared for fat graft harvest.
I plan a “cross-court” operative working angle. In other words, tumors with a left-sided suprasellar extension are approached from the right nostril. This principle expands the working angle toward the contralateral hidden suprasellar corners.
MICROSCOPIC TRANSNASAL CRANIOTOMY
It is important to emphasize that entry through one nostril deviates the speculum toward the contralateral half of the sella and directly over the corresponding carotid artery. The operator should recognize and resist this natural cross-court deflection and constantly direct the speculum toward the midline.
The most deceiving scenario is when there are two vertical septi located relatively symmetrically from either side of the midline. Removal of one septi gives the false impression that the “midline” septum is removed and the operator is looking at the midsection of the sella. This illusion may lead the surgeon to open the bone over the carotid artery. The usual deflection of the speculum to the contralateral side often complicates this problem further. This erroneous localization of the floor leads to inadequate bone removal and resultant suboptimal tumor resection. Therefore, a clear study of the septal anatomy is necessary before surgery.
Appropriate closure is vital in preventing a postoperative cerebrospinal fluid (CSF) fistula. A fat graft must first be procured from the periumbilical region. I perform this step of the operation at the very beginning of the surgery to avoid subcutaneous contamination of the abdomen by the nasal flora.
I wrap the fat globules with Surgicel. This maneuver allows easy handling of the fat droplets when moving them within the sella and prevents fat from being sucked into the suction apparatus.
The fat is gently packed within the sella to avoid compression of the suprasellar contents. I pack the fat more anteriorly along and above the tuberculum sella because this is the location where occult CSF leaks often occur due to attenuation of the diaphragm sella at the point of its attachment to the anterior sella. The choice of an appropriate method of packing is more important than the amount of packing. Obviously, if there is a visible site of CSF egress within the diaphragma, fat should be packed in the region.
A square segment of a dural replacement material is tucked under the borders of the bony and dural opening along the floor of the sphenoid sinus. If the tear in the diaphragma is large enough to disclose the suprasellar contents, a pedicled septal mucosal flap is rotated onto the defect and a formal skull base reconstruction is completed. A layer of fibrin glue is sprayed over this construct to bolster a watertight seal. The reminder of the bony septum is replaced, the midline mucosae are returned to midline, and both nostrils are packed. I use antibiotic-covered “trumpet drains” to keep the anterior septal mucosae along the midline septum. These drains are removed during the first postoperative day.
A lumbar drain is implanted and allowed to drain for several days if large defects within the diaphragma were noted intraoperatively. I only selectively monitor patients in the intensive care unit; most patients are monitored on the ward. Patients suffering from Cushing’s disease are observed in the intensive care unit to monitor symptoms of hypocortisolism, which can be life threatening if not addressed quickly. Patients with diabetes insipidus may be managed on the ward if they are awake and cooperative. I consider urine output of more than 400cc for two consecutive hours a possible indication for diabetes insipidus, requiring further investigation with laboratory tests, including urine-specific gravity.
Antibiotics are administered to the patient for as long as the nasal packing is present. A routine postoperative magnetic resonance image is performed to assess the extent of resection. This modality is important to the lifetime training process of the surgeon to improve his or her learning curve in achieving desirable resection outcomes. This learning curve is steep and requires years of experience.
Pearls and Pitfalls
- Neuronavigation (CT guided) is especially important for operations on patients with a poorly penumatized sphenoid sinus, repeat operations, or invasive tumors.
- Misplacement of the nasal speculum into the sphenoid sinus and forceful overzealous spread of its valves leads to serious complications such as anterior skull base fractures and optic nerve injury.
- The extent of bony and dural exposure along the floor of the sella often determines the extent of tumor resection.
- Patience and meticulous reconstruction of the sellar floor avoids disappointing postoperative CSF fistulas.
Rhoton AL Jr. The sellar region. Neurosurgery. 2002;51(suppl 1):335-374.
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