81.1 Introduction

The hallmark of neurofibromatosis type 2 (NF2) is the development of bilateral vestibular schwannomas (VSs). NF2 is caused by mutation of the NF2 gene on the 22nd chromosome, which is inherited in an autosomal dominant fashion. About half of all NF2 patients are originators, presenting with a new mutation. The incidence of NF2 is approximately 1/33,000, with no ethnic or racial predilections.s. Literatur

In addition to VSs, patients with NF2 develop other tumors, including schwannomas, meningiomas, and ependymomas, in other locations in the neuroaxis. The phenotypic expression of NF2 is highly variable. Patients with more severe disease may present in adolescence with numerous large, rapidly growing tumors, while more mildly affected individuals present in middle age with only small VSs. Traditionally, NF2 has been subcategorized into either the more severe Wishart type or the more mild Gardner type.s. Literatur More recently, mosaic forms of NF2 have also been recognized.s. Literatur It is probably best, however, to view NF2 as occurring along a continuum of severity.

Since NF2 is not uniform in its presentation, it is impossible to make firm recommendations as to the best surgical treatment in all cases. Indeed, it is critical to understand that treatment must be individualized. Over-riding pronouncements about “the best” or “the worst” treatment must be taken with due skepticism.

81.2 General Considerations

In considering microsurgical resection of NF2-associated VSs, there are three issues that differentiate these tumors from those seen in sporadic cases.

First and most obvious is the fact that patients with bilateral VSs are at risk for total deafness. Indeed the natural history of NF2 is one that eventually leads to profound hearing loss in most cases. Thus, preservation and rehabilitation of hearing becomes much more critical in NF2 patients than in those with sporadic tumors, who are generally at risk only for unilateral hearing loss. The goal of hearing preservation may lead in a limited number of cases to a more proactive approach to small tumors; however, in most other cases, it may lead to a less aggressive strategy of continued observation.

In addition, the risk of bilateral profound hearing loss in NF2 mandates consideration of other methods of auditory rehabilitation. Auditory brainstem implants (ABIs) have been used to restore auditory perception to patients with NF2.s. Literatur These are generally placed at the same time as tumor resection because, in general, it is usually not feasible to preserve the cochlear nerve either anatomically or functionally. More recently, cochlear implantation has been utilized in patients with NF2 in whom cochlear nerve integrity can be maintained.s. Literatur It is critical to understand that no current method of auditory rehabilitation can be expected to restore normal hearing. A detailed discussion regarding auditory rehabilitation for NF2 can be found in Chapter 84.

The second consideration is that NF2-associated VSs are frequently seen in combination with other tumors, either meningiomas or schwannomas of other nerves. These must be considered in the plan of resection. Most notably, tumors of the facial nerve are often encountered, making differentiation and preservation of this structure more problematic. In addition, VSs are frequently seen in collision with schwannomas of either the trigeminal nerve or of the jugular foramen. Resection of either of these types of tumor together with a VS may lead to additional morbidity. In the case of trigeminal involvement, any combination of facial and trigeminal dysfunction is likely to result in significant corneal pathology and possible vision loss. In the case of lower cranial nerve involvement, swallowing difficulties are likely to ensue.

The third consideration is the often polyclonal nature of NF2-associated VSs.s. Literatur Although in sporadic tumors the cochlear nerve can reliably be found on the surface of the tumor mass, in NF2 all eight nerve components may run between tumor lobes. That is, it is often more useful to think of an NF2-associated tumor as a “bunch of grapes” rather than as an “orange or apple.” This fact makes consideration of hearing preservation much more daunting. Indeed, the results of hearing preservation surgery in NF2, at least for larger tumors, are generally considered to be poor. Similarly, results of hearing preservation with radiosurgery are considered to be poorer than for sporadic tumors as well.s. Literatur

81.3 Decision Making: When to Proceed with Surgery

In the setting of NF2, the decision to proceed with microsurgery, or indeed with any treatment, may be considerably more complicated than with sporadic tumors. Of course, this is not always the case. For instance, one patient may present with a large and growing tumor on a side with poor hearing, while having a small tumor and good hearing on the contralateral side. In this case, the decision to proceed with resection of the larger tumor is straightforward. Another patient may present with a large and growing tumor in his or her only hearing ear—a much more difficult problem (see Chapter 75; Fig. 81‑1 ).

The goal of hearing preservation can be expected to hinder the achievement of other objectives in many instances. Not infrequently, a patient will present with a giant tumor on the side of an only hearing ear at the point that the goal of facial nerve preservation has already been made difficult and the goal of safe tumor resection has even been called into question. Furthermore, patients and their caregivers often deliberately watch and wait until this point is reached. It has been postulated that large NF2 tumors are apt to maintain good hearing due to the fact that cochlear fibers may traverse them rather than being stretched around them as with sporadic tumors.s. Literatur Thus with NF2 patients, surgeons may be placed in a position of advocating tumor resection in cases in which bilateral profound hearing loss will be the known result. Of course, this should be done only after other options, such as the use of bevacizumab, and other potential medical therapies have been exhausted (see Chapter 83).

81.4 Preoperative Evaluation

In addition to the usual considerations with VSs, potential impairment of other functions in NF2 makes further evaluation critical. Magnetic resonance imaging (MRI) of the entire brain and spine should be carried out prior to surgery. Other, larger tumors may take precedence over VSs. This depends on both tumor appearance and symptomatology. Furthermore, pathology in the cervical spine may limit operative positioning, and in cases with spinal disease somatosensory evoked potential monitoring is mandatory.

Thorough preoperative vision assessment is also necessary prior to proceeding with VS surgery in NF2 patients. Unilateral vision loss in NF2 patients is frequent and may be unrecognized.s. Literatur Vision loss on the side contralateral to the side of planned VS surgery is of particular concern as even transient facial nerve weakness may result in significant disability in patients with good vision only ipsilaterally.

Even more critical is evaluation of vocal cord and swallowing function. Again, the particular concern is possibility of preexisting contralateral lower cranial neuropathy. Rather than being directly related to VSs, the most common cause of mortality in NF2 is lower cranial nerve dysfunction.s. Literatur These nerves may be put at risk with VS tumor resection, and the creation of bilateral difficulties could be expected to lead to severe morbidity. Especially when resection of larger tumors is being considered, in-office flexible laryngoscopy should be part of the routine preoperative evaluation.

81.5 Tumor Resection: General Considerations

Except in cases of surgical misadventure, VSs are rarely the cause of death in patients with NF2. Medium- to giant-sized VSs may be approached via either the translabyrinthine or the retrosigmoid route, the most important consideration being surgeons’ comfort.

As with sporadic tumors, there is considerable variability in tumor consistency, adherence, and vascularity. NF2-associated VSs have been shown to express more vascularity-associated growth factors than sporadic tumors, a finding which served as the basis for the initial trials with bevacizumab.s. Literatur While the vascularity of these tumors may provide significant challenges in resection, it is the authors’ experience that excessive bleeding occurs primarily from discrete vascular channels between areas of tumors of likely distinct clonal origins.

NF2-associated VSs may also be found in collision with other tumors, including meningiomas, trigeminal schwannomas, and jugular foramen schwannomas (Fig. 81‑2 ). Meningioma tissue may be found interspersed with schwannoma.s. Literatur Resection of trigeminal schwannomas should only be considered for large trigeminal tumors with significant brainstem compression. Indeed, the trigeminal nerve should be handled with great care. Surgery resulting in both motor and sensory deficits involving the eye is likely to lead to considerable disability.

Failure of airway protection is the most common cause of mortality in patients with NF2.s. Literatur Because of this, preservation of lower cranial nerve function must be of utmost importance. It is our practice to never resect tumors arising from the lower cranial nerves unless absolutely necessary, which typically grow slowly. Indeed, even patients with preexisting ipsilateral dysfunction are likely to suffer further decline after disruption of these nerves in the posterior fossa. In the absence of ipsilateral vocal cord movement, for example, the vagal nerve may still provide laryngopharyngeal tone and sensation that may worsen considerably after lower cranial nerve schwannoma resection.