Ординатура / Офтальмология / Английские материалы / Clinical Neuro-ophthalmology A Practical Guide_Schiefer, Wilhelm , Hart_2007

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

• 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, deterioration 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

:Definition

Pituitary adenomas are the predominating lesions of the sellar region. Hormone-secreting pituitary adenomas (i.e., prolactinomas, growth hormone [GH]-se- creting 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 symptomatic 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.

!Note

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. Improvement 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.

: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. 22.8. Intraand 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 pituitary adenoma before (a) and after resection (b), exposing the diaphragma sellae

!Note

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 improvement. However, a rate of 10 to 15% of visual deterioration is encountered following transcranial tumor resections.

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

Hemispheric Tumors

Gliomas, metastases, and ventricular tumors like ependymomas 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.

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

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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]).

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 decisionmaking.

!Note

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, reducing 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).

!Note

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.

:Definition

The extent of surgical tumor removal in meningiomas can be classified according to the Simpson grading 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.

Trans-Sphenoidal Approach

!Note

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 displaced 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).

• Pearl

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 adenomectomy is performed, with preservation of pituitary function.

!Note

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 cerebrospinal fluid rhinorrhea, which are typical complications of trans-sphenoidal procedures, is below 1%.

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 uniand bilateral craniotomies are possible.

• Pearl

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

: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 improves patient safety.

Preoperative CT and MR images can be used for surgical planning and intraoperative orientation when using neuronavigational 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.

• Pearl

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 disturbances during surgery. This technique is very susceptible to artifacts, when other electrical systems and machines are in use.

• Pearl

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).

Fig. 22.20. Intraoperative direct electrical stimulation of the subcortical white matter tracts of the visual pathway will limit the extent of tumor resection whenever eliciting phosphenes/visually evoked potentials

Imaging

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

!Note

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.