Neurosurgery of the Visual Pathway

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The visual pathways take an extraordinary and extensive intraorbital and intracranial course from the globe to the visual cortex within the occipital lobes. Hence, a large num- ber of orbital and intracranial pathologies interfere with the optic pathways. The diagnosis and treatment of these pathologies demands an interdisciplinary team with ophthal- mologists, neuroradiologists, neurosurgeons, and radiation therapists. The management algorithm takes into consideration presenting signs and symptoms, as well as ophthalmologic and imaging findings. It requires a multimodal treatment protocol depending on the biological nature and location of the pathology.

Modern neurosurgery applies a vast range of operative approaches and microsurgical techniques to remove intra- and extra-axial lesions of the optic pathways. The neurosurgical management is enhanced by recent advances of intraoperative image guidance and electrophysiological monitoring to face the challenge of preserving function.

Neurosurgery of the Visual Pathway

A. Gharabaghi, J. Honegger, and M. Tatagiba

Tumors Compromising the Visual Pathway When visual loss is associated with a mass lesion, the physi- cian in charge has to consider a large variety of aspects such as the biological nature and the location of the lesion. Is the tumor extra-axial, i.e., originating in the vicinity of the optic system and compressing it, or is it intra-axial, i.e., aris- ing from the visual pathway itself? Is the lesion slow or fast growing? Does it threaten the patient’s life? What is the probable time course of further deficits to be expected?

Can the lesion be removed completely, or does the surgery have to be limited to decompression of the visual pathways?

Furthermore, the surgical accessibility and appropriate operative approach has to be considered in terms of adequate tumor exposure and minimal invasiveness with satisfying cosmetic results at the same time. Which additional imaging and electrophysiological information is available to increase the safety of the intervention?

The intention of the following chapter is to address these questions for the most frequent tumors of the orbita, anterior skull base, sellar/parasellar region, and intraparen- chymal hemispheres that compromise visual function, and to introduce the required neurosurgical techniques including intra- and extracranial approaches to treat these lesions.

Intraorbital Lesions

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^Definition

Intraorbital tumors include optic nerve sheath meningeomas, optic nerve gliomas, cavernous hemangiomas, peripheral nerve tumors, dermoid and epidermoid cysts, osteomas, fibrous dysplasia, hemangiopericyto- ma, metastatic lesions, and less frequent pathologies.

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Orbital tumors that are located lateral to the nerve may be resected via a lateral orbitotomy. Lesions superior to the optic nerve are removed using an orbitofrontal craniotomy. When the tumor is located below the nerve or medially, a transethmoidal or transmaxillary approach or a transconjunctival approach can be chosen.

Optic nerve sheath meningeomas (^■ Fig. 22.1) may be treated with a variety of modalities, depending on the current visual status of the patient. If vision is lost completely, the tumor can be removed totally. When useful visual function is present, fractionated stereotactic radiotherapy is the method of choice (see Chap. 23). From the neurosurgical point of view a histological confirmation should precede this therapeutic option.

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^Pearl

Optic nerve gliomas are often associated with neurofi- bromatosis type 1 and occur typically in the first decade of life (^■ Fig. 22.2). Surgery has to be considered when loss of vision or radiological progression occur. In these cases, the nerve is resected with the tumor via a frontolateral approach to prevent chiasmatic involvement (^■ Fig. 22.3).

Cavernous hemangiomas are the most common benign primary orbital tumor of the adult and can be removed completely. The surgical approach depends on the location of the lesion in relation to the optic nerve. Most often fron- to-orbital craniotomies are performed.

In special cases of endocrine orbitopathy with deteriorating visual function, orbital decompression may be helpful (^■ Fig. 22.4).

After trauma, decompression of the optic nerve is indicated in cases of deteriorating visual function or in associa- tion with the treatment of cerebral spinal fluid fistula and/

or intraorbital hematoma (^■ Fig. 22.5).

Fig. 22.1. Bilateral optic nerve sheath meningiomas. The CT images show an extensive calcification of the lesions

Fig. 22.2. a Microscopic view of an intra-axial optic nerve glioma (pilocytic astrocytoma) on the right side. b Intraoperative view after removal of the pilocytic astrocytoma and decompression of

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Fig. 22.4. Pre- and postoperative images (a, b) of a patient treated for endocrine orbitopathy Fig. 22.3. a Intra-axial glioma of the chiasm and the optic tract

(preoperative contrast-enhanced T₁-weighted axial MR image).

b Preoperative T2-weighted axial MR image of the same intra-axial glioma on the left side leading to almost complete ipsilateral visual loss. c Postoperative CT image after tumor removal. The ipsilateral optic tract had to be sacrificed to prevent infiltration of the contralateral side

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Anterior Skull Base Tumors

Meningiomas of the sphenoidal wing, the anterior clinoid, and the cavernous sinus may involve the optic canal and the superior orbital fissure (^■ Fig. 22.6) as well as the intracranial optic nerves and optic chiasm.

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Frontolateral or pterional craniotomies allow exposure and removal of these lesions with good functional outcome.

Fig. 22.5 CT images of a trauma patient presenting with a fracture of the orbital roof (a) with secondary visual deficit in the same side and an associated intracranial hematoma above the orbit (b). This constellation allows decompressing the orbit and evacuating the hematoma during one surgery

Fig. 22.6. Meningioma of the left clinoid process in axial (a) and coronal (b) planes of contrast-enhanced T1-weighted MR images.

Patient positioning and skin incision behind the hairline for a frontolateral approach to remove this lesion (c)

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^Pearl

If the cavernous sinus is massively infiltrated, partial tumor removal is followed by stereotactic fractionated radiotherapy or radiosurgery. Radical removal of meningiomas from within the cavernous sinus has been used in the past, but has since been abandoned because of high morbidity and mortality and in particular, dete- rioration of oculomotor function.

When intraosseous meningiomas or fibrous dysplasia lead to hyperostosis and compression of the optic nerves, bony decompression is performed via a frontolateral or pterional approach using a diamond high-speed drill (^■ Fig. 22.7).

Sellar and Parasellar Tumors

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^Definition

Pituitary adenomas are the predominating lesions of the sellar region. Hormone-secreting pituitary adenomas (i.e., prolactinomas, growth hormone [GH]-secreting adenomas, adrenocorticotrophic hormone [ACTH]-secreting adenomas) are often diagnosed by symptoms due to hormonal hypersecretion, and visual impairment is less frequent. In contrast, nonfunctioning pituitary adenomas only become symp- tomatic when a large space-occupying lesion develops (^■ Fig. 22.8). Visual failure is the prevailing symptom of nonfunctioning pituitary adenomas. It is caused by suprasellar tumor extension.

Neuro-ophthalmological evaluation typically detects chiasmal syndrome. Early diagnosis and appropriate surgical therapy are crucial, as progression of chiasmal syndrome would ultimately result in blindness.

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The majority of pituitary adenomas can be removed by a transnasal, trans-sphenoidal approach (^■ Fig. 22.9).

Total removal is feasible unless massive invasion of adjacent structures such as the cavernous sinus is found. Visual outcome of trans-sphenoidal surgery is favorable. Improve- ment of chiasmal syndrome or even total restoration of visual function is accomplished in up to 90% of the cases.

Residual tumor within the cavernous sinus can be treated by fractionated radiotherapy or radiosurgery see Chap. 23.

Only in those cases with massive intracranial extension and perforation of the diaphragma sellae does a transcranial approach have to be performed using a frontolateral or pterional craniotomy.

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^Definition

Craniopharyngiomas are also benign lesions of the pituitary and hypothalamic region and make up 1% of intracranial tumors. They occur both in childhood and in adult life. Craniopharyngiomas often show the typical triad with solid tumor, cysts, and calcifications (^■ Fig. 22.10).

Chiasmal syndrome is frequently encountered. Surgical therapy of craniopharyngiomas is challenging. Total removal is only accomplished in 50% of cases, and craniopharyngiomas tend to recur. Removal is incomplete in the presence of severe hypothalamic involvement and extensive tumor size.

Fig. 22.7. Intraosseous sphenoid wing meningioma compressing and displacing the left optic nerve [preoperative contrast- enhanced T1-weighted axial MR image (a) and bone-window CT image (b)]

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Only 30% of craniopharyngiomas can be removed by a trans-sphenoidal operation, while 70% of craniopharyngiomas have to be operated on by craniotomy. A frontolateral or pterional approach is most frequently used.

Some neurosurgeons prefer a bifrontal approach in the presence of major retrosellar extension. Decompression of the optic chiasm is accomplished in the majority of cases.

Hence, most patients experience postoperative visual im- provement. However, a rate of 10 to 15% of visual deterio- ration is encountered following transcranial tumor resec- tions.

Fig. 22.8. Intra- and suprasellar pituitary macroadenoma with typical lobulated figure-of-eight or “snow man” appearance in sagittal (a) and coronal (b) contrast-enhanced T1-weighted MR images

Fig. 22.9. Microscopic transnasal, trans-sphenoidal view of a pi- tuitary adenoma before (a) and after resection (b), exposing the diaphragma sellae

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Hemispheric Tumors

Gliomas, metastases, and ventricular tumors like ependy- momas may compromise the dorsal part of the visual system by displacing or infiltrating the optic radiation (^■ Fig. 22.11). These lesions are often approached via a craniotomy of the skull in the direct vicinity of the lesion.

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^Pearl

Image-guidance techniques help to localize the less invasive trans-sulcal or interhemispheric approach (^■ Fig. 22.12). On the other hand, the operative approach has to avoid critical areas, e.g., speech area or optic radiation (^■ Fig. 22.13). Recent technological innovations allow imaging of the optic radiation fibers using special MRI sequences (diffusion tensor imaging [DTI]), thereby facilitating the preservation of these structures (see also “Technology” section [^■ Fig.

22.21]).

Intraoperatively, direct electrophysiological stimulation of these fiber tracts may help to confirm these imaging findings, thereby increasing the safety of the procedure (see also “Technology” section [^■ Fig. 22.20]).

Fig. 22.10. Pre- and postoperative sagittal MR images (a, b) of a cystic craniopharyngioma that was completely removed via a transnasal approach

Fig. 22.11. Intracranial space-occupying lesion (ependymoma of the lateral ventricle) displacing the optic radiation laterally

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Fig. 22.12. Image-guided planning of a craniotomy in the vicinity of a hemispheric occipital lesion. The surgeon uses a handheld pointer with infrared light-emitting diodes (a) that are detected by the camera of the navigation system. The location of the pointer tip is indicated on the monitor of the navigation system in relation to the patient-specific MR images of a cystic metastasis (b) in triaxial planes (contrast-enhanced T1 and flair- weighted T2 images)

Fig. 22.13. Image-guided planning of the operative trajectory to avoid critical areas, e.g., optic radiation. Online visualization of preoperative MR (a) and intraoperative ultrasound images (b) allows compensation for brain shift during resection of the cystic metastasis

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Preoperative Workup

The neurosurgeon has to base the decisions concerning the treatment strategy on the sum of information accessible. The ophthalmologist will support this process by contributing the following facts: When was the onset of ophthalmologic symptoms, and what was their course since then? What are the current neuro-ophthalmologic findings like visual acuity and visual field deficits? Is oculomotor function impaired, and which motor nerves are involved? Does the optic disc show any signs of atrophy or papilledema? Which topographic conclusions in terms of the probable side of compression of the visual pathway can be drawn from these findings? Are there any additional neurological findings, e.g., indicating the involvement of other cranial nerves? Furthermore, all available imaging findings, as detected by modern magnetic resonance and computer tomography imaging, are included in decision- making.

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Today, the nature of the pathology can be determined preoperatively with a high degree of accuracy.

Is a recent internal medicine evaluation available indicating the patient’s suitability for surgery and concomitant risk factors?

Operative Technique

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The treatment strategy depends on the biological nature of the pathology. In benign tumors, the surgeon intends to remove the tumor completely while seeking optimal preservation of function. In these cases, the resection often follows a transtumoral approach, reduc- ing the size of the lesion in a piecemeal fashion.

En bloc resection is a surgical principle in the treatment of malignant tumors. Malignant tumors demand an en bloc resection that includes healthy tissue around the lesion. In these cases, the indication for operation depends on the possibility to resect the lesion radically, including the necessity to sacrifice vision on one side.

However, en bloc resection is mostly not feasible in intracranial, intra-axial lesions. In glioma surgery, curative surgery is not possible because of the infiltrative nature of the tumors. The aim of surgery is the removal of visible tumor while preserving function. Postoperatively, adjuvant

radiotherapy and/or chemotherapy are necessary. Radical resection is not performed when sacrifice of vision does not reflect cure or significant increase of life expectancy, and when the tumor is invading the cavernous sinus. If the tumor is infiltrating the surrounding structures but shows a slow growing pattern, a partial removal is performed with the aim of decompressing the visual pathway in order to maintain the quality of life.

Extracranial Approach

Tumors of the paranasal sinuses, several intraorbital lesions, and sellar tumors can be accessed via extracranial approaches.

Transorbital Approach

Both malignant and benign tumors of the anterior skull base and paranasal sinuses that compress structures of the visual system but do not show any intradural extension can be exposed via different orbitotomies, depending on their localization in relation to the optic nerve (^■ Fig. 22.14).

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Extended bone infiltration necessitates a fronto-orbital approach with resection of the osseous skull base (^■ Fig. 22.15). The intraoperative orientation is provided by anatomical landmarks, e.g., superior orbital fissure, optic foramen, foramina ovale, and rotundum.

When the underlying dura mater is infiltrated, it will be resected and replaced as well.

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^Definition

The extent of surgical tumor removal in meningiomas can be classified according to the Simpson grad- ing system, from macroscopically complete removal with excision of dural attachment and abnormal bone (grade I) to simple decompression (grade V).

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The transconjunctival approach is a useful way with good intraoperative visibility, especially for lesions located in the inferior medial and basal compartment of the orbit. Deep intraconal lesions at the orbital apex and extraconal superior lesions are less accessible by this approach. However, selected intraorbital lesions may be resected without muscle dissection, leading to excellent cosmetic and functional results.

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Fig. 22.14. Skin incision and bone removal for a lateral orbitotomy, exposing the orbital contents

Fig. 22.15. Bicoronal skin incision and location of burr holes for a fronto-orbital approach. The straight line shows the location and the size of the bone removal

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Trans-Sphenoidal Approach

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More that 95% of pituitary adenomas and most adenomas with suprasellar extension can be removed via a transnasal, trans-sphenoidal procedure.

Classically, a trans-septal, submucosal approach was performed. Today, most pituitary surgeons prefer the direct pernasal approach to the sella. With this technique, the nasal septum is disconnected from the sphenoid and dis- placed laterally with the speculum. The direct pernasal approach is minimally invasive. It avoids major dissection of the nasal septum, and it is well tolerated by the patients, with minimal postoperative discomfort and minimal nasal swelling (^■ Fig. 22.16).

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Today, extended trans-sphenoidal approaches are available and provide access not only to the pituitary fossa, but also to the clivus, posterior ethmoid, cavernous sinus, and suprasellar area. Decompression of the optic nerves and optic chiasm as well as decompression of the oculomotor nerves within the cavernous sinus is

accomplished. The extended trans-sphenoidal approach allows not only removal of extensive adenomas, but also removal of other pathologies such as perisellar metastasis or chordomas.

For example, skull base chordomas frequently produce abducens nerve paresis that recovers after trans-sphenoidal tumor removal. Trans-sphenoidal surgery is performed with microsurgical techniques. Today, selective adenomec- tomy is performed, with preservation of pituitary function.

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Modern technical tools enhance tumor removal and minimize the complication rate. Parasellar and suprasellar tumors are directly visualized and removed with the use of endoscopes. Neuronavigation systems are used in extensive lesions to avoid injury to the carotid artery and provide intraoperative orientation.

The complication rate of trans-sphenoidal surgery is low. In experienced centers, the frequency of meningitis and cere- brospinal fluid rhinorrhea, which are typical complications of trans-sphenoidal procedures, is below 1%.

Fig. 22.16. Transnasal, trans-sphenoidal access to the sella, using a trans-septal, submucosal approach. The nasal septum is disconnected

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Intracranial Approach

If tumors of the anterior skull base are located primarily extradural, they can be visualized by elevating the dura without the necessity to open it. Intradural tumors, e.g., meningiomas, have to be exposed by opening the dura via either a bifrontal or a unilateral frontal craniotomy.

Extradural Approach

Extradural frontal approaches can be performed using a bilateral or a unilateral craniotomy, depending on the size of the lesion to be removed. The optic canal and the superior aspect of the orbital contents can be visualized by this approach, while the optic chiasm and the intradural optic nerve cannot.

Intradural Approach

In addition to the intradural frontal approach, both uni- and bilateral craniotomies are possible.

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In recent years, the unilateral frontolateral craniotomy is preferred because of less invasiveness and sufficient exposure of the anterior cranial base and the suprasellar region. The skin incision is hidden behind the hairline. The small flap is located close to the floor of the cranial base, allowing exposure of both optic nerves and the chiasm.

This approach is preferred for lesions of the anterior visual system (^■ Fig. 22.17). It can be performed for sphenoidal wing meningioma, meningioma of the anterior clinoid, and suprasellar tumors.

For the exposure of the parasellar area, the frontotemporal craniotomy often becomes necessary with retraction of the temporal muscle (^■ Fig. 22.18).

Technology

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^Definition

Several technical innovations like image-guidance systems, computer simulations, three-dimensional image renderings, and intraoperative monitoring allow the neurosurgeon to precisely localize lesions and evaluate their extent in relation to adjacent neural structures, and determine their functional integrity by electrophysiological means. This combined approach im- proves patient safety.

Neuronavigation

Preoperative CT and MR images can be used for surgical planning and intraoperative orientation when using neuro- navigational systems. These devices are based on the principle of frameless stereotaxy, and they allow for image guidance during surgery. The target localization accuracy of these tools reaches a precision of up to a few millimeters.

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A major limitation of navigational systems that work with preoperative images is that of intraoperative shifts in brain position. This problem is minimal during skull base surgery. Therefore, this technique is most appropriate to define target trajectories and to plan the surgical approach with custom-tailored craniotomies when removing lesions of the skull base near the visual system.

During frontolateral craniotomies, for example, image guidance helps to avoid an unintended opening of the frontal sinuses by visualizing these structures prior to skin incision and craniotomy. In hemispheric lesions, neuronavigation is most helpful to determine the exact location and extent of the lesion prior to skin incision and craniotomy (^■ Fig. 22.19).

Intraoperative Monitoring

Continuous monitoring of nerve function is possible during surgery. The immediate feedback of the electrophysiological response allows the neurosurgeon to adjust his microsurgical technique to the current functional status of the manipulated structure. The integrity of the visual pathway can be monitored measuring visually evoked potentials at the occiput. Changes of latency and amplitude of these electrophysiological signals help to detect slightest distur- bances during surgery. This technique is very susceptible to artifacts, when other electrical systems and machines are in use.

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More recently, intraoperative stimulation of white matter tracts can be applied by using subcortical electrical stimulation to induce evoked potentials. When the patient is awake, even phosphenes can be induced using this technique. Whenever these responses are evoked during iterative stimulation, tumor resection has to be interrupted in order to preserve the functional integrity of the optic radiation ( Fig. 22.20).

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Fig. 22.17. Head positioning and skin incision for the frontolateral approach. The straight line shows the location and the size of the craniotomy

Fig. 22.18. Head positioning and skin incision for the frontotemporal approach. The straight line shows the location and the size of the craniotomy

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Imaging

Modern imaging techniques allow acquiring a huge amount of additional information to plan the strategy and extent of tumor removal.

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With surgical MRI units or with navigated ultrasound systems, even an intraoperative update of the imaging information is possible.

This allows for compensating the brain shift during the procedure. For many years the anterior parts of the visual system up to the lateral geniculate body could be visualized with classical imaging sequences but the optic radiation and the visual cortex could not be delineated as precisely as necessary for surgical planning.

Fig. 22.19. Image-guided localization of an intracranial lesion of the occipital lobe. The lower (a), upper (b), medial (c), lateral (d) edge of the tumor can easily be projected on the skin surface to plan a custom-tailored approach

Fig. 22.20. Intraoperative direct electrical stimulation of the subcortical white matter tracts of the visual pathway will limit the ex-

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Functional magnetic resonance (fMRI) and DTI are recent and innovative modalities that allow visualization of both the cortical and the subcortical structures of the posterior part of the visual system in detail. In particular, the optic radiation can now be localized, when planning the surgical approach to hemispheric, periventricularly located lesions (^■ Fig. 22.21).

Conclusion

Optimal management of intra- and extra-axial lesions of the visual pathway can be provided when an interdisciplinary team cooperates closely during the diagnostic and treatment process. The ophthalmologist, who is confronted first with these patients plays an important role initiating the proper course of action based on an intimate under- standing of the treatment philosophy and management algorithms of the multi-specialist team.

Further Reading

Kaye AH, Black McL P (eds) (2000) Operative Neurosurgery, Volume 1, Churchill Livingstone, Edinburgh, London, New York

Winn HR (ed) (2004) Youmans Neurological Surgery, 5th edition, Else- vier Saunders, Edinburgh, London, New York

Rengachary SS, Ellenbogen RG (2005) Principles of Neurosurgery, 2nd ed, Elsevier Mosby, Edinburgh, London, New York

Fig. 22.21. Visualizing the optic radiation by MR diffusion tensor imaging (DTI)-based tractography