Preoperative Considerations
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Pituitary adenomas are benign epithelial neoplasms derived from the pituitary glands. In the absence of a macroscopic capsule, those lesions can infiltrate adjacent structures such as the cavernous and sphenoid sinuses, dura and brain parenchyma. These tumors are considered invasive pituitary adenomas.
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They are characterized by their mass effect on bordering structures such as the optic nerves and chiasm, as well as for their capacity to disturb hormonal homeostasis.
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Visual symptomatology is the most common presentation, bitemporal hemianopsia being the classic symptom. Other symptoms include headaches, diplopia and occasional cerebrospinal fluid rhinorrhea.
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Hormonal disturbances can cause different syndromes; the most common are Cushing’s disease, acromegaly and hyperprolactinemia.
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Treatment options include surgery, medication and radiotherapy. The treatment selection will depend on several factors:
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Size and pneumatization of sphenoid sinus.
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Tortuosity and position of carotid arteries.
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Intracranial tumor extensions.
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Suspicion about the pathology of the tumor.
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Therapeutic history of the patient.
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Indications
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Medical therapy is often required when surgery is delayed, contraindicated, or unsuccessful.
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Drug therapy is especially useful in cases of prolactinomas, where the use of dopamine agonists is highly effective to achieve hormonal control and reduction of the tumoral mass.
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Patients with acromegaly may benefit from the adjuvant use of somatostatin analogues postoperatively in order to achieve control of the disease.
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Radiation therapy may be considered in patients whose disease is not controlled by surgical or medical therapy.
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Indications for surgery:
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Visual deficits secondary to compression of the optic apparatus.
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Cranial nerve deficits secondary to cavernous sinus invasion.
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Apoplexy.
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Macroprolactinomas that do not respond to medical treatment.
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GH- or ACTH-secreting adenomas.
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The goals of the surgery are:
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Resection of the tumor.
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Decompression of the visual apparatus.
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Improvement of the endocrinological symptoms.
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Pathology diagnosis of the lesion.
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Complete resection is challenging to achieve in large invasive pituitary tumors that present marked cavernous sinus invasion, regardless of the approach.
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Transcranial approaches are especially useful in the treatment of invasive pituitary adenomas, which present:
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Significant extension into the anterior, middle or posterior cranial fossa; as well as those tumors that have lateral extension beyond the middle cerebral artery and/or those tumors encasing the anterior cerebral artery.
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Suspicion of an important fibrous component of the tumor.
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Small or underpneumatized sphenoidal sinus.
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Uncertainty about the actual pathology of the tumor (i.e. a possible meningioma).
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If transsphenoidal surgery has already failed.
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Some examples of feasible approaches for invasive pituitary tumors are:
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Transsphenoidal (and transsphenoidal staged).
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Subfrontal , bifrontal or extended-bifrontal, transbasal — used in suprasellar tumors with significant superior, inferior or lateral extension.
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Supraorbital — for adenomas confined to the sellar region with minimal parasellar extension lateralized to one side.
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Pterional — in cases of pre-fixed chiasm with limited extension.
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Orbitozygomatic — in cases with marked suprasellar or parasellar extension or invasion of the cavernous sinus or interpeduncular cistern.
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Extended endoscopic approaches have been recommended as useful alternatives for resection of giant invasive adenomas by experienced surgeons. (For endoscopic management, please refer to Section 5: Endoscopic Approaches to Skull Base Tumors). This may be especially useful in:
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Patients unable to tolerate transcranial surgery.
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Longstanding damage of the chiasm (may not tolerate additional trauma like mobilization required through the transcranial route).
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Palliative debulking.
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Another valid option for extensive cases is a combined approach (staged or not) or endoscope assisted-microneurosurgery.
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Close collaboration between surgeons, endocrinologists and radio-oncologists is essential in the management of invasive pituitary adenomas since combined therapies may be key for tumor control.
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Preoperative evaluation
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Neuroimaging: MRI sequences to describe the size, location and relation with CNS structures of the tumor. CT scans are useful when additional description of the osseous anatomy is required, particularly when the patient has been exposed to previous cranial surgeries.
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Endocrine assessment for hormonal overload and/or deficiency. Special attention is given to thyroxine and cortisol levels.
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Neuro-ophthalmological evaluation.
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Surgical Procedure
The goal of this chapter is to describe the most common alternatives to the transsphenoidal approach for those invasive pituitary adenomas with marked extension to the suprasellar or parasellar regions or after a partial removal via the transsphenoidal approach: pterional, orbitozygomatic and bifrontal approaches. For endoscopic management, please refer to Chapter 23 , Chapter 24 , Chapter 25 .
Patient Positioning
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The patient is placed in a supine position. The head is situated above the heart level with a small degree of head uptilt to facilitate venous drainage.
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Since the nature of the operation can impair the pituitary–adrenal axis, it is customary to administer corticosteroids during the procedure.
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Different techniques might be used to approach these lesions:
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Subfrontal approach: The neck is translated superiorly and placed in a neutral position, using a Mayfield or Sugita headholder.
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Pterional/orbitozygomatic: It is recommended to use a right-sided approach unless there is an important left-sided extension. Place the head in a three-point pinion headholder with 20° rotation to the left and slight extension. The amount of rotation and extension of the head depends on the size and extension of the tumor.
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Skin Incision
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Bifrontal approach skin incisions:
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A bicoronal incision is made coronally from the temporal region to the contralateral temporal region, 2–3 cm behind the hairline.
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The pterional approach requires a curvilinear skin incision. The inferior ending starts 1 cm anterior to the tragus. The superior end varies depending on the patient’s hairline. It can be either at the midline or at the contralateral midpupillary line.
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Orbitozygomatic (OZ) approach
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The incision is similar to the pterional approach.
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The facial nerve needs to be protected with a separate flap with a subfascial or interfacial dissection before exposing the zygoma.
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The temporalis muscle dissection needs to give the surgeon exposure to perform the OZ osteotomies.
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The incision is made 2 cm posterior to the frontozygomatic process carried 1 cm posterior below the superior temporal line. It is angled inferiorly and ends at the root of the zygomatic process. The temporalis flap is reflected inferiorly until the inferior orbital fissure is visualized.
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Craniotomy
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Bifrontal ( Figure 34.1 ):
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Dissection of the dura mater posterior to the crista galli may produce injury to the olfactory nerves.
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In the extended bifrontal craniotomy the orbital bar can also be removed with five osteotomies using a long router bit with footplate or a bone saw. The frontal sinus mucosa needs to be entirely resected to prevent a future mucocele in those cases. The mucosa can be cauterized using a monopolar cautery, followed by the drilling of the inner body surfaces with a diamond burr. A pericranial flap can be harvested in preparation for reconstruction to decrease the chances of postoperative CSF leak.
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Olfactory nerves are covered during dissection to prevent nerve injury. Tumors with an important intrasellar and/or parasellar component need additional removal of bone from the skull base. In these patients olfactory nerve preservation is practically impossible.
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