The optic apparatus includes the optic nerves, chiasm, and optic tracts. Lesions affecting this region vary widely with respect to histologic type, site, extent, and clinicopathologic behavior. Treatment of these lesions therefore requires a comprehensive multidisciplinary approach with a team consisting of skull base neurosurgeons, otolaryngologists, and ophthalmologists, as well as radiation and medical oncologists. Although observation and radiotherapy may play important roles, surgical treatment represents the mainstay of therapy, with a relatively recent shift from traditional open approaches to less invasive endoscopic endonasal approaches (EEAs).
Some of the common pathologies affecting the optic apparatus are discussed in the following text. Pituitary macroadenomas and meningiomas represent the most common lesions in adults, whereas craniopharyngioma is the most common in children.
Pituitary adenomas represent the most frequent type of sellar mass with a prevalence of 77 to 100 cases per 100,000 population. These tumors can be classified into nonfunctional or functional (hormone-secreting) subtypes.
Among the pituitary tumors that are associated with visual dysfunction, nonfunctioning adenomas are the most common (58%). Superior extension of tumors may compress the optic chiasm and lead to visual field deficits, most commonly bitemporal hemianopia. Other visual symptoms may include loss of visual acuity and diplopia. The progression of visual loss is typically slow (50%); however, rapid (27%) and intermittent progression (12.5%) have also been reported. Importantly, although 75% of patients with pituitary adenomas have visual field defects, fewer than half report these visual changes subjectively. This underscores the importance of obligatory visual field testing in all patients with pituitary adenomas irrespective of whether they report visual impairment.
Treatment of pituitary tumors is individualized and is based on tumor type, size, anatomic location relative to surrounding critical structures (optic apparatus, carotid arteries, third ventricle) and the degree of visual and/or hormonal impairment. Surgical resection is first-line treatment for most tumors except prolactinomas, which typically respond well to medical management with a dopamine agonist. Today at most academic medical centers, the overwhelming majority of surgical resections are performed through an endoscopic transsphenoidal approach.
Meningiomas arising from the optic nerve sheath or in proximity to the optic chiasm in the suprasellar region may affect the optic apparatus. Optic nerve sheath meningiomas, which arise from cap cells of the arachnoid surrounding the optic nerve and spread through subarachnoid spaces, are the most common optic nerve sheath tumors and account for one-third of intrinsic optic nerve tumors. Involvement of the extraorbital portion of the optic nerve without (49%) or with involvement of optic chiasm (40%) is common. These tumors usually present with unilateral symptoms and are frequently seen in middle-aged women. Prognosis depends largely on the size and extent of the tumor and less so on histopathologic features.
Suprasellar meningiomas may arise from the tuberculum sellae (TS), diaphragma sellae (DS), or planum sphenoidale (PS). The majority (50%) arise from the TS 19 and are most commonly seen in women in their fifth decade. Identification of the anatomic subtype is important in selecting the type of surgical approach for resection, which is often challenging. The relative displacement of the optic apparatus, however, can help in distinguishing one subtype from another. For example, TS meningiomas lead to posterior or superoposterior displacement of the optic chiasm, whereas DS meningiomas lead to superior displacement, and PS meningiomas lead to posterior and inferior displacement. The nuances of surgical approach can then be based on these anatomic distinctions. Although TS meningiomas can be resected using a purely supradiaphragmatic approach, a combined supradiaphragmatic and infradiaphragmatic approach is necessary for DS meningiomas.
GTR of tumor (at least Simpson grade I/II) with improvement or stabilization of visual function are the goals of surgical treatment. Surgical treatment is also a viable option for radiation-resistant meningiomas.
Craniopharyngiomas are the most common suprasellar tumor found in children, accounting for 50% of masses in this region. These tumors arise from remnants of the Rathke pouch and demonstrate a bimodal age distribution, most commonly affecting children 5 to 14 years and adults 65 to 74 years.
Although a benign tumor, craniopharyngiomas are associated with significant mortality among all sellar and suprasellar tumors, with a standardized overall mortality rate ranging from 2.88% to 9.28%. In pediatric patients, symptoms of elevated intracranial pressure (ICP) are commonly seen, whereas in adults, visual disturbance and hypopituitarism are also noted. Hypothalamic involvement is frequently seen in pediatric patients. Gross total resection (GTR) of the tumor with maintenance of hypothalamic functionality should be the goal of treatment; however, in patients in whom maintaining hypothalamic functionality is challenging owing to unfavorable tumor localization, subtotal resection followed by adjuvant radiotherapy should be performed. Additionally, adjuvant radiotherapy improves local control and helps prevent permanent endocrine and neurocognitive sequelae, as well as injury to critical neurovascular structures in difficult tumors. If GTR is not possible, our preference is to preserve the pituitary stalk. In addition to visual dysfunction, long-term morbidity associated with craniopharyngiomas includes hypopituitarism, hypothalamic injury, detrimental cardiovascular effects, reduced bone health, neurologic deficits, lower cognitive function, and poorer quality of life.
Rare Pathologies Affecting Optic Apparatus
Other, less common lesions affecting the optic apparatus are listed in Table 36.1 .
|Sub types||Tumors||Non-tumorous lesions|
|Benign||Chiasmatic/Hypothalamic glioma, Germ cell tumors (germinomas, non-germinomatous and teratomas), Hypothalamic hamartoma, Rathke cleft cysts, Yolk sac tumor, Pilocytic astrocytomas, Gangliocytomas||Langerhans cell histiocytosis and other granulomatous disorders, |
Arachnoid cysts, Lymphocytic hypophysistis, Pituitary apolexy,
Aneurysms of circle of Willis,
Colloid cysts, Epidermoid cysts
|Malignant/Potentially malignant||Lymphoma, Paraganglioma |
Ependymal metastases in third ventricle, metastasis to optic chiasm and pituitary gland
Lesions affecting the optic apparatus frequently cause significant visual morbidity and, given their anatomic location, may contribute to hypopituitarism and symptoms of elevated ICP. Specific visual complaints vary depending on the anatomic site, size, and extent of lesion. Gradual and painless visual loss is a common initial symptom, although nonspecific symptoms such as headache and nausea (owing to elevated ICP) and weight disturbance may be present. Other visual findings may include visual field defects, color vision disturbance, optic atrophy, afferent pupillary defect, choroidal folds, presence of opticociliary shunt vessels, and edema of the optic disc and macula. Given the breadth of the possible visual symptoms and signs, a thorough ophthalmologic examination, including measurement of visual acuity, color vision, visual fields, and optic nerve examination, is obligatory in both the preoperative and postoperative setting.
EEAs to the optic apparatus include both transsphenoidal and extended transsphenoidal approaches (ETSAs). Transsphenoidal approaches are commonly used for lesions confined to the sella, whereas ETSAs, such as transplanar or transtubercular, may be necessary to access anatomic areas superior, posterior, or lateral to the sella.
Preoperative assessment using magnetic resonance imaging and computed tomography is recommended. Anatomic ease of access (deviated nasal septum, concha bullosa, sphenoid sinus pneumatization, intrasphenoid and intersphenoid septae, and so on), tumor characteristics (consistency, extension, and so on), and surrounding critical structures (prefixed and postfixed chiasma, bony dehiscence, carotid artery position and encasement, location of the pituitary gland and its stalk, apoplexy, and so on) should be noted. Patient selection is of paramount importance for success of the procedure and hence should be done based on the surgeon’s expertise.
Technological advances in the form of computer-assisted surgical navigation, specialized instrumentation with long handles, suction irrigation, micro drills, and ultrasonic aspirators have improved the surgical precision in the EEA. Rigid 18-cm endoscopes (Karl Storz, Tuttlingen, Germany) of size 4 mm (0-, 30-, and 45-degree) are used in adults, whereas smaller sized endoscopes (2.7 mm) are used in the pediatric age group.
Standard neurosurgical anesthetic practices are followed. Total intravenous anesthesia is recommended to maintain a hypotensive state to reduce bleeding (mean blood pressure maintained approximately 90-100 mm Hg and pulse rate approximately 55-60 beats/min). Paralytic agents should be avoided if intraoperative neurophysiologic monitoring with somatosensory evoked potentials is intended. Intraoperative somatosensory evoked potentials, electromyography for perfusion, electroencephalography, motor evoked potentials, and selective cranial nerve monitoring can be used as required to reduce the risk of injury to adjacent neurovascular structures, including the optic apparatus and carotid artery. Corticosteroids may be administered if preoperative hypocortisolism or visual disturbance is suspected. ICP-lowering agents or lumbar drain are rarely required. Urinary catheterization is performed to monitor fluid balance during the surgery. At all times during the surgery, both suction and cautery are usually maintained at a lower setting to prevent mucosal injury.
The setup of the operating room is similar to other endoscopic neurosurgical procedures. The right-handed surgeon usually stands on the right side of the patient and the monitor screen is set up behind the patient’s head. The patient’s head and body are elevated to 30 to 45 degrees from the horizontal plane to reduce ICP and venous bleeding. The bilateral nasal cavities are filled with cottonoids soaked in vasoconstrictive agents (oxymetazoline or diluted epinephrine) under endoscopic guidance to prevent inadvertent mucosal trauma. The patient is then prepped and draped. The nasal mucosa is injected medially into the posterior nasal septum and middle and inferior turbinates with local anesthesia (lidocaine 1% with epinephrine 1:100,000) using a spinal needle with the tip bent to 20 degrees.
The Endonasal Stage
Creation of an adequately wide endonasal corridor is the first step and is imperative to any endoscopic procedure. Bilateral nasal endoscopy is performed to evaluate both the nasal airways and to identify anatomic entities, such as septal deviation, septal spurs, septal perforations, synechiae (from previous procedures), and turbinate hypertrophy, among others. Surgical procedures, such as septoplasty, turbinate reduction, or lysis of adhesions, may be performed to improve access and postoperative sinonasal function. Bilateral lateralization of the middle and superior turbinates is performed to expose the sphenoid face and natural os sphenoidale in a transnasal fashion. Resection of the middle turbinate is rarely required.
The Sphenoid Stage
To approach the sella, resection of the anterior face of the sphenoid is required, which may result in sacrifice of the pedicle for a nasoseptal flap (NSF). Traditionally, the NSF is raised at the beginning of the operation. However, the majority of cases do not incur an intraoperative cerebrospinal fluid (CSF) leak; even if leak is present, it is usually small and of a low-flow type that does not normally require an NSF for cranial base defect closure. In our center, we do not routinely use an NSF after excision of sellar or suprasellar tumors. Therefore an NSF preservation approach is used. A number of techniques have been described to preserve the NSF pedicle without raising an NSF, which include transseptal approach (tunnel), pushdown or rescue flap, or “1.5 with pushdown” techniques.
We commonly us the 1.5 with pushdown technique that is described briefly. With the sphenoid face well visualized, the natural os on the right side is enlarged superiorly and bony septum is fractured off the rostrum. The mucosa is elevated off the face of the sphenoid on both the right and left sides, preserving the pedicle for a septal flap. The ipsilateral mucosa inferior to the os is then carefully displaced inferiorly to preserve the pedicle to the NSF. Through the contralateral naris, the mucosa is removed superior to the os, preserving the pedicle to the NSF ( Fig 36.1 ). A small posterior septectomy is performed if necessary for exposure.
Alternatively, the ETSA for giant pituitary adenomas, meningiomas, and craniopharyngiomas where a large dural defect (high-flow leak) is expected and an NSF is used as part of the multilayer reconstruction of the cranial base, the NSF is harvested at the beginning of the procedure and is stored in the nasopharynx during the extirpative portion of the case.
The degree of pneumatization of the sphenoid sinus significantly influences the ease of access to the sella and optic apparatus. Presellar and conchal sellar types, such as in pediatric patients, may prevent identification of reliable anatomic landmarks. Ultimately, the distance between the two opticocarotid recesses, the height of dorsum sella, and the size of posterior clinoids determine the dimensions of intrasphenoid corridor. The intersinus septations are removed by either through-cutting hand instruments or a high-speed drill with diamond burr, being careful to identify any attachment to the carotid canals. The paramedian septum often leads to the carotid artery, and therefore it should be excised with particular care. The mucosa is removed from the sellar face to facilitate reconstruction and to avoid postoperative mucocele formation.
Cranial Base Stage
Several key anatomic landmarks can be identified along the cranial base (sellar face), including the sella, PS, TS, lateral opticocarotid recesses, optic canals, and clinoidal carotid protuberance ( Fig 36.2 ). Neuronavigation and micro-Doppler monitoring may be used to confirm anatomic landmarks and to determine the extent of bony removal that is necessary to access the intradural lesion.
For the majority of pituitary tumors, the sellar face is then removed with either a Kerrison punch or high-powered drill from one cavernous carotid to the other, and inferiorly to the level of the sella floor. If a drill is used, copious irrigation is advised to prevent thermal injury. The internal carotid artery should be carefully identified; however, we prefer not to remove the bone over the internal carotid artery for most cases to prevent iatrogenic carotid injury. The caveat is that additional exposure is required for resection of tumor within the cavernous sinus.
For excision of meningiomas and craniopharyngiomas, the optic canal is unroofed using a diamond drill to a thin eggshell of bone that can be removed with a microdissector. The length of canal that needs to be unroofed depends on the extent of the lesion. However, proximal unroofing is performed for most cases to prevent injury to the optic nerve at the entrance to the optic canal during intradural dissection. Bony removal in the rostral direction along TS and PS is usually performed to expose the limbus sphenoidale. Again, the degree of planum drilling depends on the extent of tumor ( Fig. 36.3 ). Removal of the lateral strut of the TS also allows for wider access to the opticocarotid cistern. If additional bone needs to be removed to expose this area, it should be done before opening the dura.
The principles of intradural excision—namely, internal debulking followed by capsule mobilization, extracapsular dissection of neurovascular structures, focal coagulation, and capsule removal—are duly followed. These maneuvers should be performed in a controlled, bimanual fashion. The site and extent of dural incision is tailored to the lesion. Usually a smaller incision is placed first, and upon confirmation of the arachnoid plane, the incision is extended using endoscopic microscissors. The dura may be opened in various methods—a vertical linear incision with crossed extensions, two lateral vertical incisions joined by a transverse one, a set of four incisions to create a rectangular window, or a cruciate incision. We usually prefer the cruciate incision, which is made using a retractable knife ( Fig. 36.4 ). This opening must be precise and generally should not extend beyond the tumor margins initially, especially during the transplanum approach, because excessive exposure can place uninvolved structures at risk and can lead to brain herniation, which in turn limits the visualization. After this opening is made, a blunt nerve hook or microdissector is placed along the circumference of the opening to create a subdural, extraglandular, or extracapsular plane. Bleeding from superior intercavernous sinus is controlled using bipolar electrocautery and/or local hemostatic agents. The superior hypophyseal branches to the optic apparatus and infundibulum are carefully dissected and mobilized superolaterally to prevent inadvertent injury.
Tumor Excision Stage
The tumor can be removed en bloc after internal debulking or in piecemeal fashion. Extracapsular dissection and complete resection is the goal whenever possible. Although these general principles are applicable for excision of most lesions, several more nuanced techniques are applicable in specific circumstances.
For instance, if the optic canal is found to be invaded, then the canal is decompressed in a retrograde manner from the lamina papyracea back to the orbital apex (270 degrees around the canal). The part of the canal adjacent to the carotid artery is completely unroofed, and the bone overlying the more superior and medial portions of the canal is also excised bilaterally for at least 1 to 2 cm distal to the orbital apex. Resection of a TS meningioma may be particularly difficult if it has a firm and rubbery consistency; therefore, sharp dissection rather than simple suctioning may be necessary. In close proximity to critical structures, internal decompression may be performed with a sharp and blunt manual dissection or with ultrasonic tumor aspiration. For craniopharyngiomas the cyst capsule may need to be coagulated before internal decompression to shrink the tumor and contain cyst content spillage. After adequate internal debulking, extracapsular dissection can then be performed ( Figs. 36.5 and 36.6 ). Preservation of the infundibulum should be attempted whenever possible. However, if the stalk is invaded, it needs to be excised to prevent increased need for postoperative radiation and risk of tumor recurrence. At no point during the intradural excision should the lesion be blindly or indiscriminately pulled or retracted.