17 Implantation in Skull Base and Temporal Bone Lesions In this chapter additional indications for auditory implantation using a cochlear implant or an auditory brainstem implant are discussed; this group is miscellaneous and contains various skull base lesions and some more exceptional neurotologic pathology not discussed in the previous chapters. In general this group of extended indications is small and concerns a limited subset of patients who are severely bilaterally hearing deprived because of miscellaneous disorders. The group includes patients with a skull base lesion in one ear and a deaf contralateral ear not available for implantation (which otherwise would have been the first option), as well as rare bilateral pathology such as temporal bone fractures and Meniere disease. While the same pathology in just a single ear would not lead to cochlear implantation, in this group of patients an individualized, well considered, and sometimes experimental decision is made in the quest for hearing restoration. The outcomes cannot always be predicted in advance; only afterward does the result become clear. The pathology in this group can be divided into 1. Skull base lesions: tumor or expansive lesions of the skull base and petrous bone 2. Nontumor lesions violating the integrity of the otic capsule (petrous bone fractures or labyrinthectomy) Neurofibromatosis type 2 (NF2) would fall in the first category and presents the largest group of patients in whom experience with both types of auditory implants (cochlear implant [CI] and auditory brainstem implant [ABI]) has been gained. NF2 cannot be considered as an “additional” indication for auditory implantation; the ABI was invented for this type of pathology, but in management and decision making basically the same principles apply. Auditory implantation in NF2 is dealt with separately in Chapter 16. There is growing evidence that better results in speech perception are achieved with a CI (whenever feasible) than with an ABI.1–5 This appears to be related to easier access, better stabilization of the implant, and use of the tonotopy in the cochlea as opposed to that of the cochlear nucleus. Cochlear implantation should therefore always be considered as a first choice in this category of patients, when possible. Additionally the surgical risks in cochlear implantation are smaller than those in auditory brainstem implantation (see Chapters 5 and 8). The feasibility of cochlear implantation depends crucially on two important anatomical conditions in CI decision making: 1. Preservation and integrity of the cochlear nerve 2. Patency and integrity of the cochlea These are especially important for the pathology dealt with in this chapter, because the cochlear nerve or the cochlea itself could be deranged by the pathology, making deployment of a CI less practical or even impossible, and resulting in an ABI as the only possible option. In tumor or expansive lesions of the skull base and petrous bone, both factors could be distorted, depending on location and type of pathology. In skull base lesions it is possible that the decision for CI or ABI can only be made intraoperatively, after the anatomical situation has become clear. Preoperative audiologic measurements can give useful but not prognostic information (pure tone audiogram, auditory brainstem response [ABR] electric promontory stimulation). The loss of vascular supply to the labyrinth can lead to labyrinthitis ossificans affecting the patency of the cochlea and leading to loss of spiral neurons in the long term.6,7 This makes it preferable, if not mandatory, to perform cochlear implantation in the same surgery before the occurrence of the ossification due to the vascular damage. In the nontumor lesions especially, the patency and integrity of the cochlea is an important factor. In fractures due to trauma or due to active labyrinthitis ossificans the cochlear patency might be lost. The risk of the cochlear nerve being torn off the modiolus as a result of the trauma is in our opinion infinitesimal: the forces necessary to create this kind of lesion are not compatible with survival1; moreover, not one case with bilateral cochlear nerve rupture has been published. Therefore, the first factor is of minor concern in fractures of the otic capsule.5 After labyrinthectomy the presence of labyrinthitis ossificans is usually limited to only the round window area7 because the vascularization of the internal auditory canal (IAC) is left intact during the labyrinthectomy, which is different from the case in surgery for removal of an IAC tumor. Also, the cochlear nerve is preserved. In cases with preoperative or intraoperative violation of the otic capsule there is also a risk of cochlear ossification due to labyrinthitis ossificans. Because of this risk, it is best to perform cochlear implantation during the same procedure (even though delayed implantation up to a maximum of 18 months is mentioned in the literature8,9). More information on cochlear ossification and labyrinthitis ossificans can be found in Chapter 11. Electrical stapedial reflex (ESR) measurements, electrically evoked compound action potential (ECAP) measurements using neural response testing (NRT or NRI), and electrically evoked auditory brainstem response (EABR) testing can only be performed after insertion of the electrode array. Thus, in some cases, the cochlear implant functions as a diagnostic tool in addition to the surgeon’s intraoperative findings. Although the cochlear nerve and nucleus may be anatomically intact, their functionality could be hampered by compression due to the skull base lesion. At present no reliable test is available to gain information on this issue, although electrical promontory stimulation (EPS) seems to be useful to assess the survival of neural elements preoperatively.10–12 However, when EPS results are negative it does not necessarily mean that the cochlear implant will not function (it could be a false negative). Its value is therefore limited.13,14 Tysome et al suggested that a temporary intracochlear electrode would be the ideal tool for gaining this information.13 A brainstem electrode array and cochlear electrode array combined in one implant, allowing one to use whichever delivers the best result, would also be a valuable tool. Such a tool or implant has only been used in a research project and is not commercially available. Sporadic reports have appeared in the literature regarding these particular cases; they include cochlear implantation in isolated vestibular schwannoma (VS) in the only hearing ear (and in NF2),4,7,13,15–19 in petrous bone cholesteatoma,20 and in a paraganglioma case.21 Cochlear implantation in pediatric medulloblastoma22 and in Langerhans cell histiocytosis (LCH)23,24 has also been described. The combination of surgical excision of the local pathology, chemotherapy, and radiotherapy as well as cochlear implantation makes these indications challenging. The outcome can be affected by the finding that the cochlear nerve or nucleus has been compressed by the tumor, or can be influenced by loss of vascularization or spasm of the internal auditory artery.8 These factors can lead to a temporary or long-term disappointing outcome of the cochlear implantation, which cannot be predicted preoperatively. Previous radiotherapy can also negatively affect the CI/ABI outcome.25–28 Recurrence of pathology or postsurgical scarring around the cochlear nerve can also negatively influence the outcome in the long term.2,8,29 Neff et al in their long-term study on cochlear implantation in VS patients reported that neither of these effects was found with a minimum follow-up of 5 years.2 Roehm et al reported a patient with a recurrence of VS only after 22 years, showing that cochlear implantation is sometimes a valuable and long-lasting interim step (as in this case lasting 22 years) in treatment to achieve hearing restoration.29 When the cochlea or the cochlear nerve cannot be preserved, ABI remains the only available option. Unfortunately, anatomical preservation of the cochlea and cochlear nerve does not guarantee their functionality and presently no intraoperative test is available to evaluate this functionality. Such an evaluation is extremely important because, in the case of a nonfunctioning CI, revision surgery for ABI insertion in a cerebellopontine angle already operated on and filled with fat is particularly demanding if not actually impossible due to fibrosis in the presence of delicate structures. In these skull base cases surgery is often performed through translabyrinthine approach. In this approach the hearing is sacrificed but the cochlea is preserved; this keeps both the CI and the ABI option available during the surgery. During the translabyrinthine approach the vascularization of the IAC is usually sacrificed: this can lead to labyrinthitis ossificans.7 When a CI is planned the approach is usually combined with a subtotal petrosectomy to reduce the risk of a postoperative CSF rhinorrhea by plugging the eustachian tube and obliterating the residual cavity (Chapter 10). In this situation the possibility of a subcutaneous CSF collection is actually increased somewhat, because after transection of the EAC the musculoperiosteal layer is incomplete. Radiologic follow-up remains necessary to identify recurrence of the primary pathology but also to identify possible inclusion cholesteatoma in the occluded cavity. CT follow-up is performed at 1, 3, 5, and 10 years after surgery. The presence of the CI impedes the routine use of MRI because the magnet and implant create artifacts (see also Chapter 3). When the sensorineural hearing loss is caused by radiotherapy treatment, the results of cochlear implantation show reasonable to good responses, as reported in several studies.25,28,30–32 Radiotherapy has been proven to affect the stria vascularis and the organ of Corti with its hair cells, while the higher auditory pathways (brain-stem, cochlear nerve) generally remain functionally intact, even in the longer term as shown by Low et al.27 Counseling on the likelihood of a modest outcome after radiotherapy seems wise. Other side effects of radiotherapy can hamper implantation: pathologic changes leading to middle ear effusion, eustachian tube fibrosis, and chronic suppurative otitis media. Osteoradionecrosis has been reported as a lasting source of infection33 and softening of the temporal bone, making facial nerve damage more likely. Radiotherapy can also lead to intracochlear fibrosis and ossification,26 fibrosis and scarring of tissues in the internal auditory canal, and obliterative endarteritis that has an influence on vessels throughout the radiation field.32 As also the skin can be affected (radiation dermatitis), thought should be given to a minimal incision and skin flap design.32 Long-term hearing outcomes of cochlear implantation in irradiated patients may deteriorate because of recurrence of pathology or the causes mentioned above. This should be discussed with the patient, whose expectations should not run too high. Non–Skull Base Indications for Auditory Implantation • CI combined with labyrinthectomy in Meniere disease • CI in fracture of the otic capsule Performing a labyrinthectomy in Meniere disease can be considered one of the last steps in gaining control of vestibular function. This surgical step should only be considered when several pharmaceutical, dietary, and lifestyle options, as well as intratympanic gentamicin have been attempted.34–37 Also endolymphatic sac decompression and selective vestibular nerve sectioning have been suggested as surgical methods. The anatomical destruction of the vestibular system via transmastoid three-canal labyrinthectomy (drilling of all three semi-circular canals with removal of all neuroepithelial tissues in vestibulum and ampullae) allows resolution of the vertigo spells in 93 to 100% of cases.36 The nonresponsive cases usually suffer bilateral Meniere disease. However, the direct consequence of a labyrinthectomy is the loss of residual hearing. Therefore usually only severely hearing deprived patients are elected for this approach. This is not a large group as only 10% of Meniere cases drop below 61 dB on average pure tone audiometry, and fewer than 5% drop below 81 dB.38 Bilateral Meniere disease develops in 10 to 15% to 15 to 40% of cases and usually presents within 5 years after the unilateral onset. In these cases the sensorineural hearing loss in the first affected ear is usually worse.17 In presence of contralateral severe hearing loss, simultaneous labyrinthectomy and cochlear implantation may be considered.35 In addition, patients affected by bilateral Meniere disease fitted with common hearing aids usually do not attain good performance due to the hearing fluctuation and the minimal compliance between detection and complaint threshold. Simple transmastoid labyrinthectomy, because of the preservation of the vascularization of the cochlea, often does not produce extended ossification/fibrosis.7 Therefore, theoretically, implantation may also be delayed to a second step. Combining the two surgeries (transmastoid labyrinthectomy and cochlear implantation through a standard facial recess approach), however, is relatively easy and the procedure is safe to carry out in the same stage. Although loss of any residual hearing after labyrinthectomy is a fact, quality of life measurements made after labyrinthectomy in Meniere disease show 98% of patients to be positive.34 Late cochlear implantations, years after the labyrinthectomy, show improvement in auditory gain, but the results in general are worse than with simultaneous implantation.39 Another consideration in Meniere disease regarding cochlear implantation is the fact that the spiral ganglion cell count and function of the cochlear nerve might be lower because of the primary disease. Histologic study of postlabyrinthectomy bones demonstrated survival of around 10,000 spiral ganglion cells: only 10% of cases drop to around 3,000. Both these amounts have been proven to be enough for proper CI functioning.6,36,39 Based on the explanations above, hearing results of cochlear implantation combined with labyrinthectomy are usually good. Temporal bone trauma can lead to fracture lines that damage the facial nerve (palsy), labyrinth (vertigo), skull base (CSF leakage), ossicles (conductive hearing loss), and cochlea (sensorineural hearing loss) in various combinations.40–42 Damage to the otic capsule can be found especially in transverse fractures (mostly due to occipital trauma). There are several causes for this sensori-neural hearing loss: disruption of the membranous labyrinth with mixture of perilymphatic and endolymphatic fluids, loss of the integrity of the membranous cochlea with loss of the cochlear transducer function, and vascular compromise or hemorrhage. The presence of a persistent perilymph fistula or secondary endolymphatic hydrops can also be of influence.43 The spiral ganglion cells and modiolus usually remain intact and also the cochlear nerve itself usually remains intact and in connection with the cochlea.1 Cochlear implantation in patients with otic capsule–damaging fractures usually leads to good results in hearing rehabilitation,5 although some patients do not perceive this outcome as successful because of the sudden loss of their normal hearing after the trauma.41 In spite of this psychological drawback, the risk of labyrinthine ossification triggered by the trauma (not by loss of vascularization in the IAC) makes urgent implantation necessary, and might lead to a less than perfect outcome due to the development of fibrous intracochlear tissue surrounding the electrode array.44–46 When implantation is performed late (after months) the risk of ossification or loss of spiral ganglion cells might also compromise the outcome45; transverse fractures may lead to loss of spiral ganglion cells over time47 and can lead to progressive decrease of CI results. Another reported drawback is that cochlear implantation may be challenging because of a displaced fracture line that can impede electrode insertion.48 Several histopathologic studies show that in fractures the enchondral part of the cochlear capsule heals with fibrous tissue instead of with new bone, thus presenting a lasting risk for meningitis.46,49 Another complication described is a higher incidence of facial nerve stimulation (especially at the level of the geniculate ganglion) caused by the low resistance of the fracture line.44 The inner endosteal layer and the outer periosteal layer do heal, but only with very thin layers and often incompletely.49 This necessitates isolation from the external environment by means of subtotal petrosectomy with closure of the eustachian tube and external ear canal and obliteration with fat, to minimize the risk of otitis media leading to meningitis and also in cases without cochlear implantation.50 In general, the risks related to these procedures are a combination of the risks of both surgeries: the risks of implantation and the risks during surgery for the primary pathology. The presence of the implant may increase risks of infection, CSF leak, and meningitis, but the limited number of reported cases hinders statistical confirmation. In these exceptional cases, CI and ABI placement are good treatment options and we should take into consideration that the CI or ABI is sometimes the only option available for these patients if they are not to remain completely deaf. There seem to be no absolute contraindications, but the higher risks involved in some of the skull base procedures must be considered carefully in the counseling. The risk–benefit ratio needs to be discussed with the patient before the definitive decision is made. Fig. 17.1.1 Preoperative MRI showing a intracanalicular vestibular schwannoma on the left side (arrow).
17.1 CI versus ABI
17.2 Preoperative Conditions
17.3 Intraoperative Audiometric Testing
17.4 Indications
17.4.1 Skull Base Lesions
17.4.2 Previous Radiotherapy
17.4.3 Non–Skull Base Lesions
Labyrinthectomy in Meniere Disease
Fracture of the Otic Capsule
17.5 Contraindications and Risks of Implantation in Skull Base Surgery/Neurotology
Case 17.1 Cochlear Implantation Combined with Removal of Small Vestibular Schwannoma Left Ear (Fig. 17.1.1–Fig. 17.1.13)
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
