Abstract
Although the transnasal, transphenoidal route has become the mainstay for treating sellar and suprasellar pathology, certain preexisting factors make the transcranial route more favorable. However, optic apparatus and hypothalamic-pituitary axis injuries are more common with this approach. One must take into consideration the position of the optic chiasm relative to the tuberculum sella, the pituitary gland, infundibulum, and hypothalamus, as well as tumor anatomy and blood supply. Postoperative, vision and pituitary hormone status must be monitored and managed closely to avoid the detrimental effects of diabetes insipidus or hypopituitarism.
Keywords
transcranial approach/craniotomy, pituitary tumor, pituitary hormone dysfunction, diabetes insipidus, optic nerve injury
Highlights
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Depending on certain preexisting factors, the transcranial approach may be more favorable compared with a transnasal approach.
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Optic apparatus and pituitary axis injuries are rare but devastating.
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Visual deterioration and pituitary dysfunction are more common with a transcranial approach.
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The approach should be tailored to the position of the optic chiasm relative to the tuberculum sella and the tumor.
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Careful perioperative monitoring and management of vision and pituitary hormone status are imperative.
Background
The transsphenoidal approach revolutionized pituitary surgery and not only has become the mainstay for treating sellar pathology but also continues to evolve as expanded, endonasal approaches are continuously being developed to access tumors beyond the original limits. However, up to 10% of pituitary tumor cases still present with factors that make a transcranial approach necessary. These include (1) difficult-to-access suprasellar, parasellar, or retrosellar/retroclival extension, (2) involvement or encasement of circle of Willis vasculature or the optic apparatus, (3) “dumbbell-shaped” tumors with significant constriction of the diaphragma sellae, (4) brain invasion or cerebral edema, (5) firm, fibrotic tumor consistency, (6) previous surgery or radiation therapy, (7) coexistent nearby aneurysm, (8) “kissing” carotids, (9) predominantly cavernous sinus involvement, and (10) inaccessible subfrontal extension.
The goal of surgery is maximal resection to decompress neurovascular structures without causing or worsening neurologic and endocrinologic dysfunction. Compared with the endonasal, transsphenoidal route, the transcranial approach is associated with higher rates of anterior pituitary dysfunction, diabetes insipidus (DI), visual deterioration, and hypothalamic injury. Mortality was 2.3% (6 of 259 patients) in one series and 10.6% (7 of 66 patients) in a systematic review of three other studies, compared with 2.0% and 0% for microscopic and endonasal transsphenoidal resection, respectively. Although there may be a selection bias that shifts larger tumors with less favorable prognoses and high complications rates into transcranial series, the risk of visual deterioration, hypopituitarism, and permanent DI is 22.9%, 9.1%, and 9.1%, respectively, for the transcranial approach compared with 0.8%, 9.5%, and 8.7%, respectively, for microscopic transsphenoidal, and 0.0%, 1.1%, and 4.7%, respectively, for endoscopic transsphenoidal.
This chapter will discuss iatrogenic injury to the optic and pituitary apparatuses, which can lead to dreaded vision and hormonal dysfunction.
Anatomic Insights
Common Transcranial Approaches
An ideal transcranial approach offers the shortest distance with the widest corridor to access the tumor. The subfrontal approach is most commonly used to access the supra- and parasellar regions. From medial to lateral, craniotomies include bifrontal interhemispheric, unilateral frontal, frontoorbital, and frontotemporal. The most relevant neurovascular structures encountered during these approaches are the optic apparatus (nerves, chiasm, tracts), the pituitary gland and stalk, and circle of Willis vessels (internal carotid artery [ICA], anterior cerebral artery [ACA], middle cerebral artery [MCA]), including perforators (hypothalamic perforators as well as inferior and superior hypophyseal arteries).
Relevant Operative Anatomy ( Fig. 22.1 )
One of the first structures encountered in open pituitary surgery by the subfrontal approach is the ipsilateral optic nerve, which should serve as a landmark to find neighboring structures. The ICA resides inferolaterally, which can be followed posteriorly until it bifurcates into the ACA and MCA. Whereas the MCA courses laterally out of the surgical field and typically does not pose an obstacle, the ACA curves superomedially over the optic nerve and chiasm and is intimately involved in this approach. Following the optic nerve posteromedially leads to the chiasm and contralateral optic nerve. The lamina terminalis lies posterior to the chiasm and, depending on tumor size, may be thinned and displaced along with the optic apparatus. Deep and medial to the ipsilateral optic nerve and inferior to the optic chiasm resides the pituitary stalk, which extends through the diaphragma sellae into the gland within the sella turcica. Important perforators to be preserved are the superior and inferior hypophyseal arteries, arising from the medial and meningohypophyseal arterial trunks, respectively. These are critical perforators to the pituitary and optic apparatuses.
A key factor in the outcome of a transcranial pituitary surgery involves the relationship of the optic chiasm to the tuberculum sella and, consequently, the pituitary tumor. Bergland et al. described three scenarios: (1) normal position over the diaphragma sellae, (2) prefixed, in which the chiasm overlies the tuberculum sella, and (3) postfixed, in which it overlies the dorsum sella ( Fig. 22.2 ). A normal or postfixed chiasm offers a corridor in which the tumor resides anterior to the chiasm in between the optic nerves and provides access to the tuberculum sella region. A prefixed chiasm necessitates an approach that will provide access to the inferior aspects of the third ventricle through the lamina terminalis and tumor resection within the opticocarotid triangle. Whereas a subfrontal approach is sufficient for the previous scenarios, a bifrontal interhemispheric approach is recommended if there is more significant retrosellar extension.
An additional consideration is that tumors, especially large macroadenomas, frequently distort the normal neurovascular anatomy, obliterate the cisterns, and stretch and displace the optic apparatus against the bony compartments and foramina. Great care must be taken to minimize retraction on these compromised structures.
Mechanisms of Injury
Short of transecting the optic chiasm or nerve, a common mechanism of injury involves a stretch or crush injury, which incites a proinflammatory cascade that not only damages retinal ganglion and support cells but also exacerbates vascular compromise to, and venous congestion within, the nerve or retina. A less obvious injury involves inadvertent devascularization as a result of coagulating or disrupting shared blood supply between the nerve and the tumor. Injury to any part of the optic apparatus can lead to a corresponding visual field deficit.
The pituitary apparatus is likewise vulnerable to these types of injuries, but there is an additional risk of being resected along with tumor tissue, especially in cases of large macroadenomas with substantial perisellar extension, where normal tissue is obscured and significantly distorted. Stalk and posterior pituitary injury leads to DI, whereas anterior pituitary injury manifests as hypopituitarism requiring complete or partial hormone replacement.
Related posts:
Medical Complications in Neurosurgery
Cerebral Vasospasm: Complications and Avoidance
Complications of Surgery for Pineal Region Tumors
Thalamic and Insular Tumors : Minimizing Deficits
Complications of Surgery for Peripheral Nerve Injuries and Tumors
Complications of Surgery for Intrinsic Spinal Cord Tumors
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