July 2020 The Hearing Journal

Traumatic sensorineural hearing loss (SNHL) may occur because of head trauma. Cases of SNHL following a head injury with associated temporal bone fracture (TBF) are well described and thought to be caused by direct disruption of the otic capsule with damage to the membranous labyrinth and/or cochlear nerve. However, a subset of patients who sustain a head injury develop SNHL even in the absence of TBF.

scansEstimates of SNHL following head injury without associated TBF can be as high as 58 percent. Although the exact pathophysiology of auditory dysfunction following head injury without TBF remains unknown, possible etiologies for this phenomenon include trauma to the peripheral and central auditory pathways. Peripherally, mechanisms of injury include stretching or tearing of the cochlear nerve, excessive fluid in the inner ear, development of endolymphatic hydrops, and trauma to the membranous labyrinth. In contrast, central deafness may occur from direct injury to the inferior colliculi or at the upper pontine level, potentially as a result of diffuse axonal injury. However, due to limited pathologic data, identifying a clear etiology remains challenging.

Cochlear implants (CIs) have been suggested to be effective in the auditory rehabilitation of patients with severe to profound SNHL following a head trauma and an intact cochlear nerve. Postoperative audiometric evaluations have also shown similar outcomes in post-lingually deaf patients due to other etiologies.  However, most of these studies are limited by small sample sizes and mainly involve patients with SNHL following a head injury and an otic capsule fracture, and existing published literature provides little information on CI performance when the otic capsule is spared.

AUDITORY REHABILITATION IN PROFOUND BILATERAL SNHL AFTER HEAD INJURY

Lindquist, et al., investigated the clinical findings of a 36-year-old man who fell from a multistory building, resulting in bilateral profound SNHL. His medical history was positive for a remote history of right acute labyrinthitis with sudden SNHL approximately 10 years prior. Computed tomography (CT) imaging workup revealed a minimally displaced fracture in the squamous portion of the right temporal bone without any involvement of the otic capsule, in addition to signal changes in the midbrain, posterior pons, right temporal lobe and normal cochlear fluid signals bilaterally on magnetic resonance imaging (MRI). The patient underwent a right CI procedure for auditory rehabilitation approximately one year after his injury. Postoperative follow-up after 28 months revealed speech awareness thresholds of 25 dB HL with no open-set word recognition. The patient continued to suffer from other neurologic sequelae related to his brain injury, such as verbal aphasia, that was thought to challenge his ability to demonstrate the objective and subjective benefit of his CI.

CI OUTCOMES IN PROFOUND BILATERAL SNHL AFTER HEAD INJURY

Although many studies have reported successful auditory rehabilitation with a CI after bilateral traumatic deafness, most of them are limited by small sample sizes, short follow-up periods, and heterogenous audiometric outcome metrics. Additionally, most of these studies did not do a distinct analysis of patients without an otic capsule-violating TBF. In Khwaja, et al.  series of patients with SNHL following a head injury, seven CI procedures were performed on ears without a TBF. Even though the specific CI performance in this subgroup was not provided, the authors reported successful auditory rehabilitation.

While the case report by Lindquist, et al.,presented poor post-operative CI performance, prior studies that examined the audiometric performance post-cochlear implantation of patients with SNHL after a head injury without an otic capsule-violating TBF have mostly reported favourable outcomes, with average post-implantation speech recognition scores of 80 percent.  In one of the largest series examining CI performance after a head injury, Lubner, et al., retrospectively reviewed 24 CIs in patients following a head injury, in which nine were completed on ears without a TBF. Their auditory outcomes were similar to those in the TBF population, as well as to outcomes previously reported in the literature.

CONSIDERATIONS FOR AUDITORY REHABILITATION

Brain injury extent and location may affect CI outcomes across many domains. Previous studies examining patients with major brain injury and SNHL who received a CI often presented with behavioural, emotional, social, cognitive, and physical disabilities, which made their rehabilitation process challenging. Additionally, due to the injury severity, barriers to CI usage may include pragmatic issues such as transportation to appointments, post-traumatic brain injury depression, and cognitive deficits that are likely to result in the nonuse of a patient's CI. While Lindquist, et al., suggested that the neurocognitive sequela in their case may have resulted in poorer hearing testing performance and personal benefit from a CI, other authors have reported positive auditory rehabilitation outcomes despite major brain injury.  In a case report by Coligado, et al., a CI was found to ultimately improve the patient's ability to participate in therapy and performance status despite paraplegia and bitemporal lobe damage with antisocial behaviour. In another small case series by Siegal, et al., the authors highlighted the importance of resources (e.g., family support) to undergo maintenance (e.g., appointments) and training to use a CI in hopes of achieving successful auditory rehabilitation. Thus, the extent of brain injury should not necessarily preclude CI candidacy.

Duration of deafness may be another critical factor for predicting post-CI auditory performance in this population. Only a few studies have commented on how the duration of deafness may correlate with CI outcomes for traumatic SNHL without otic capsule involvement, and a negative correlation has been suggested between these variables. A case series by Lubner et al., demonstrated a similar trend in this population, although it was not significant. Despite the patient in the study by Lindquist, et al., having a much shorter duration of deafness prior to implantation than those reported in the relevant literature (one year v. the review's mean of nine years), the patient was unable to achieve any open-set recognition. The authors reported that his post-injury neurological symptoms, such as verbal aphasia, may be the reasons that likely limited him from getting an extensive hearing evaluation and an improved quality of life with his CI. Their report highlights the importance of considering clinical outcomes and managing expectations of providers and patients when determining CI candidacy in this population.

Anatomically, there are several other predictors of successful CI outcomes. Cochlear patency during surgery is ideal to ensure complete electrode insertion, but may be compromised due to ossification following trauma. To better visualise cochlear patency, MRI is the preferred imaging modality given that CT may miss up to 22 percent of cochlear luminal obstruction cases subsequently found at surgery.  Additionally, given that spiral ganglion neurons (SGN) are the target cells for electrical stimulation in cochlear implants, their survival may also influence CI auditory rehabilitation. In a post-mortem study of the cochlea following head injury without a TBF, Ishai, et al., found a range of 25-79 percent loss of SGN compared with controls without head injury. Although the loss of SGN reached up to three-quarters of the normal range in this study, other authors report sustained auditory sensation after cochlear implantation with even relatively poor ganglion cell survival.

While CIs have proven benefits in auditory rehabilitation, the cochlear nerve must be intact to ensure proper auditory perception.  Even without otic capsule involvement, the cochlear nerve may be damaged and should be assessed via MRI. If the cochlear nerve is compromised, an auditory brainstem implant (ABI) may provide an alternative for auditory rehabilitation. Traditionally indicated for neurofibromatosis type 2 (NF2), ABI have also been used for non-NF2 cases. In a study by Medina and colleagues, the authors compared CI and ABI auditory performance in patients with bilateral deafness after head trauma. While some patients regained their hearing with an ABI, the degree of hearing restoration was much more variable than in those who received a CI. These findings were similar to those in a systematic review of ABI for non-NF2 cases, which found more favourable CI results regardless of the cause of deafness. These studies suggest that ABIs should not be recommended unless bilateral cochlear nerve damage occurs, which is very unlikely.

FUTURE DIRECTIONS

Overall, the current literature depicts different cases of traumatic SNHL with a spared otic capsule with varying efficacy of auditory rehabilitation post-cochlear implantation. While CI outcomes for SNHL following a head injury with a TBF are well documented, these studies have limited generalisability due to heterogeneity in auditory testing methods and small sample sizes. Additionally, only recently have studies examined direct comparisons in patients with and without a TBF. Future longitudinal studies with standardised audiometric measures are needed to better characterise CI performance in patients with SNHL following a head injury and eventually compare outcomes across various CIs.

Furthermore, given how the extent of injuries to other body systems (e.g., cognitive, behavioural, emotional, social, and physical domains) may influence CI candidacy, a multidisciplinary and individualised approach should be taken during patient selection and counselling. While stratification by the Glascow Outcome Scale as a neurosurgical tool to assess outcome after major brain injury has not shown effectiveness in predicting CI outcomes in this population, the development of new metrics to assess brain damage severity and functional outcomes may help to anticipate disabilities that could potentially impact CI candidacy and challenges regarding post-implant management.

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