Neurofibromatosis 2: Primary Modality of Hearing Rehabilitation with Cochlear Implant

Abstract

Auditory Brainstem Implants have been recommended as the gold standard in hearing rehabilitation of Neurofibromatosis Type 2 patients who lose hearing completely in both the ears and the cochlear nerves are not stimulable. Some patients have undergone cochlear implantation, in whom the cochlear nerve was spared during surgery or have undergone stereotactic radiotherapy preserving the function of the cochlear nerve. Here we report a case in whom we chose cochlear implantation prior to any definitive treatment for the tumour itself during the ‘wait and watch’ period. The reasons in favour of this approach have been discussed in this article. Post switch-on the implant is benefitting the patient satisfactorily and she is on regular follow up for monitoring the bilateral tumours.

Introduction

Neurofibromatosis Type 2 (NF2) is a dominantly inherited syndrome that predisposes individuals to multiple tumours of the nervous system. The most common of these are bilateral vestibular schwannomas (90–95%). Intracranial and spinal meningiomas and other spine tumours, including intrinsic ependymomas, are also prominent components of this condition. Mutations in the NF2 tumour suppressor gene on chromosome 22q12, which produces merlin, are responsible for this syndrome [1].

Vestibular schwannomas in NF2 are typically bilateral and cause tinnitus, hearing loss and balance dysfunction. The hearing loss is usually gradually progressive and becomes profound, although sudden hearing loss may also occur. NF2-related vestibular schwannomas are usually multifocal and occur equally on the superior and inferior vestibular nerves [1].

The hearing rehabilitation of the people who have small volume tumour, yet with profound hearing loss is challenging. Auditory Brainstem Implant (ABI) has been recommended as the mode of rehabilitation in these candidates, as the surgery in most instances would compromise the cochlear nerve integrity on both sides. There are several reports of cochlear implants being used as the rehabilitation mode after careful preservation of cochlear nerve anatomical integrity or after achieving no growth of tumour with stereotactic surgery.

The rate of growth of the tumour dictates the need for radiosurgery. Hence this would be thought after years of follow up with periodic re-scans. Rehabilitation of these small tumour volume patients who have profound deafness takes the priority while waiting for tumour growth pattern. Traditionally hearing aids would not help much. Hence cochlear implant, with carefully informed consent, is an acceptable alternative until a definitive decision is made about the tumour management.

Case Report

A 32-year-old lady presented with decreased hearing in both ears and decreased speech understanding over the previous 2 years which was insidious in onset, and gradually progressive in nature. She was using behind the ear digital hearing aids for about 6 months without much benefit. Pure tone audiograms showed bilateral profound Sensori-Neural Hearing Loss (SNHL) with the aided thresholds outside the speech spectrum on both sides. She was evaluated further by Auditory Brainstem Responses (ABR), which confirmed severe to profound hearing loss on both sides. Radiological evaluation including Magnetic Resonance Imaging (MRI) and High Resolution Computerized Tomographic Scanning (HRCT) revealed bilateral vestibular schwannomas. Gadolinium enhanced MRI showed the tumour dimensions to be 33 × 15 × 17 mm on the left side and 33 × 11 × 15 mm on the right side (volumetric) (Figs. 1, 2, 3).

Fig. 1.

Axial section MRI with Gadolinium contrast showing the contrast enhancing lesions in both internal auditory canals with the approximate dimensions as labelled

Fig. 2.

Coronal dimension of the tumour on right side (MRI, T2 weighted)

Fig. 3.

Coronal dimension of the tumour on left side (MRI T2 weighted)

She was offered surgical resection of the tumours with due risk of developing facial nerve weakness and the need for ABI following it or, alternatively, serial imaging for growth of tumour and Gamma Knife (GK) Surgery if significant growth was demonstrated.

The patient chose not to go for tumour removal and ABI option, but preferred the “wait and rescan” plan. She chose to get GK irradiation should the tumour show growth. She most definitely needed hearing for her daily activities. Hence the option of cochlear implantation for hearing rehabilitation and optional radiosurgery subsequently (provided growth of the tumour is demonstrated), was considered. She was also counselled and consented about the risk of implant being not functional if tumour growth occurred or if the tumour was irradiated later. She underwent Transtympanic Electrically evoked ABR (TT-EABR) to know the integrity of the cochlear nerves (Fig. 4).

Fig. 4.

Transtympanic EABR responses were obtained on both sides, with better morphology on the left side

There were Wave V responses on both sides but the morphology and latencies were better on the left side compared to the right side, Kumar–Dutt Classification Type B on the left side and Type C on the right side [2]. With a special consent, cochlear implantation with Nucleus Freedom CI24RE(ST), was performed in the left ear with complete insertion (Fig. 5).

Fig. 5.

Post-operative X ray of the left ear shows complete electrode insertion

The implant is 1.5 Tesla MRI compatible and rescan with MRI for volumetric analysis of the tumour is possible with magnet removal. Intraoperative Neural Response Telemetry (NRT) was almost equivalent to those that would be with non-tumorous conditions. The implant was switched on after 3 weeks. She could listen and respond to the sounds and speech with a Category of Auditory Performance (CAP) score of 7 and a Speech Intelligibility Rating (SIR) score of 5 after 6 months of implant use [3, 4].

Discussion

Bilateral vestibular schwannomas in a NF2 patient can invade and grow within the cochlear nerve, while unilateral sporadic vestibular schwannoma (VS) only compresses it. [5].

The mechanism of hearing loss from vestibular schwannomas in NF2 is not well understood, and there is poor correlation between tumor size and/or growth rate and degree of hearing loss [6]. One proposed mechanism is the accumulation of intralabyrinthine protein as a result of cochlear aperture obstruction, which can be visualized by high-resolution magnetic resonance imaging (MRI) [7].

The rate of hearing loss often differs between the ears in affected individuals. These tumours have highly variable growth rates that decrease inversely with age. Typically, they are also more adherent to adjacent cranial nerves and incorporate more native nerve fascicles. Surgery can be more complex than with sporadic tumours, since NF2 tumours are often multifocal [8, 9]. These tumours may extend to involve fibers of the facial nerve and there is a significant risk of damage to the facial nerve during surgery. Identification of the surgical plane between the tumour and the cochlear nerve is more demanding on an NF2 patient than on sporadic VS [10, 11].This is particularly problematic, since the facial nerve controls the blink reflex and innervates the lacrimal gland. Loss of the blink reflex or loss of tearing may complicate other eye problems [6].

Management of vestibular schwannomas in Neurofibromatosis 2 comprises of two parts, one to address the tumour and the other is to rehabilitate the hearing loss in the patient.

Complete surgical resection is curative but the timing of treatment remains controversial and the risks of surgery should be balanced against the tumour’s natural history.

Early surgical management of small vestibular schwannomas (less than 3 cm in diameter) can preserve serviceable hearing and normal function of the facial nerves in 30–65% and 75–92% of patients, respectively. Even when hearing is lost, early removal might allow for maintenance of cochlear nerve integrity, enabling cochlear implantation for auditory restoration. Cochlear nerve function is not always preserved in spite of anatomic preservation of the cochlear nerve. An intact nerve does not necessarily mean a normal nerve histologically [12]. In spite of preservation of the cochlear nerve during surgery, several patients lose hearing in the postoperative period. Theories that explain this finding are direct neuronal disruption of the cochlear nerve and vascular injury to the cochlea [13].

Neff et al., also reported cases with postoperative hearing loss with hearing preservation techniques in spite of anatomically preserved cochlear nerve. They suggested cochlear implantation in these patients even if blood supply is interrupted as long as electrical promontory stimulation (EPS) is positive [14].

The only study found to compare ABI versus CI after VS resection from the same institution was performed by Vincenti et al. [15]. The authors concluded that when possible, CI should be the preferred hearing rehabilitation device for patients with VS, not only because of the better hearing results, but also because of the reduction of the surgical risks and lesser extent of the operation [15].

Overall, surgical results for preservation of hearing and facial nerves in neurofibromatosis type 2—even for small tumours—has generally been much poorer than is reported. Thus, conventional management strategies are typically conservative and recommend surgical resection of tumours 2–3 cm in size only if serviceable hearing is lost or rapid growth identified.

To avoid the risks of surgery without delaying treatment, researchers in several centres have examined the potential effectiveness of stereotactic radiosurgery for vestibular schwannomas in neurofibromatosis type 2 [16]. Local control (represented by stable size) has been reported in 74–100% of vestibular schwannomas in neurofibromatosis type 2 (mean follow-up 54 months). Measurable hearing and normal function of the facial nerve were maintained in 33–57% and 92–100% of patients, respectively [17, 18]. The current indications for radiosurgery are (1) a growing tumor less than 30 mm in mean diameter, (2) the ipsilateral ear has no serviceable hearing, and (3) there is risk of brain stem compression or brain stem dysfunction. Treatment is generally indicated when there is a risk of brainstem compression, deterioration of hearing, and/or facial nerve dysfunction. Reports using radiation in patients with NF2 have shown variable outcomes, and long-term follow-up is lacking. Furthermore, surgical resection may be more difficult following stereotactic radiosurgery. There are also concerns that such patients may have an increased risk of secondary malignancies [19].

Wackym et al. [20] examined 59 patients and found a hearing loss pattern consistent with stria vascularis devascularization after gamma knife surgery, which is, hearing loss across all frequencies and relative preservation of speech discrimination ability. No neural hearing loss was registered. Based on this data, direct cochlear damage could be the sole cause of post radiotherapy hearing loss [20].

Patients who are deaf with an anatomically and physiologically intact cochlear nerve are candidates for cochlear implantation that can provide sustained auditory improvement. Early surgical intervention for vestibular schwannomas in NF2 patients when the cochlear nerve can be spared is an important consideration to allow for possible cochlear implantation. A 6- to 8-week recovery period for the anatomically intact cochlear nerve may be necessary to obtain a positive promontory stimulation response following tumour resection and should be performed prior to cochlear implantation. When the physiological integrity of the cochlear nerve is compromised, an auditory brainstem implant might be an option for hearing rehabilitation.

CIs offer many advantages over ABIs, and the most important benefit is that CIs can provide better speech understanding as intracochlear electrode placement permits reliable tonotopic stimulation, better auditory performance is generally expected [11].

The goal of treatment for vestibular schwannomas in individuals with NF2 is preservation of function and maintenance of quality of life. As such, the identification of a tumour per se is not an indication for treatment and the potential benefits must be balanced against the risks of active intervention.

In this case report, the patient had profound hearing loss in both ears with small volume tumours at a young age. The available options for tumour management were, periodic imaging and monitoring, surgical resection or Stereotactic Radiosurgery alone. The available options for hearing habilitation were, Cochlear implantation (following radiosurgery/cochlear nerves sparing surgery) or ABI (Following Complete tumour resection). Follow up for the growth pattern can be done by contrast enhanced MRI or Contrast CT if cochlear implants were considered for rehabilitation. Radiosurgery would be considered if tumour growth is observed in future.

Ex vivo and in vivo studies indicate the safe use of 1.5 Tesla MRI scan in patients with cochlear implant [21, 22].

A decision to ‘wait and watch’ for tumour growth and cochlear implantation for left ear for hearing rehabilitation was taken. The Transtympanic Electrically Evoked Auditory Brainstem Response (TT-EABR) by round window stimulation (TT-EABR) was performed to ascertain the integrity of cochlear nerve stimulation in the postoperative setting. TT-EABR was present in both ears with more robust responses in the left ear. The following are the reasons in favour of the decision for choosing Cochlear implant in this patient as the primary modality of the rehabilitation even before the definitive tumour management.

1. Young lady with economically weaker background, needed hearing for her daily needs.

2. The need for stereotactic radiation or surgical resection of VS arises after establishing significant tumour growth, which may take a couple of years. During this waiting period patient will be deprived of any auditory stimulation for her daily needs.

3. Patient and the family after detailed discussion chose to defer definitive surgery for the tumours considering the difficulty in preserving cochlear nerve integrity during surgery and the associated risk of facial nerve weakness.

4. Confirmed sturdiness of the cochlear implant electrodes to stereotactic radiation, and positive responses on TT-EABR were important considerations for better hearing outcomes for the patient [23–25].

Compliance with Ethical Standards

Conflict of interest

The authors certify that they have NO affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.

Informed Consent

The relevant informed consent has been obtained from the patient for presenting this study.