A mouse model of repeated traumatic brain injury-induced hearing impairment: Early cochlear neurodegeneration in the absence of hair cell loss

PURPOSE

Traumatic Brain Injury (TBI) is a major cause of death and disability worldwide. Mounting evidence suggests that even mild TBI injuries, which comprise >75% of all TBIs, can cause chronic post-concussive neurological symptoms, especially when experienced repetitively (rTBI). The most common post-concussive symptoms include auditory dysfunction in the form of hearing loss, tinnitus, or impaired auditory processing, which can occur even in the absence of direct damage to the auditory system at the time of injury. The mechanism by which indirect damage causes loss of auditory function is poorly understood, and treatment is currently limited to symptom management rather than preventative care. We reasoned that secondary injury mechanisms, such as inflammation, may lead to damage of the inner ear and parts of the brain used for hearing after rTBI. Herein, we established a model of indirect damage to the auditory system induced by rTBI and characterized the pathology of hearing loss.

METHODS

We established a mouse model of rTBI in order to determine a timeline of auditory pathology following multiple mild injuries. Mice were subject to controlled cortical impact at the skull midline once every 48 h, for a total of 5 hits. Auditory function was assessed via the auditory brainstem response (ABR) at various timepoints post injury. Brain and cochleae were collected to establish a timeline of cellular pathology.

RESULTS

We observed increased ABR thresholds and decreased (ABR) P1 amplitudes in rTBI vs sham animals at 14 days post-impact (dpi). This effect persisted for up to 60 days (dpi). Auditory temporal processing was impaired beginning at 30 dpi. Spiral ganglion degeneration was evident at 14 dpi. No loss of hair cells was detected at this time, suggesting that neuronal loss is one of the earliest notable events in hearing loss caused by this type of rTBI.

CONCLUSIONS

We conclude that rTBI results in chronic auditory dysfunction via damage to the spiral ganglion which occurs in the absence of any reduction in hair cell number. This suggests early neuronal damage that may be caused by systemic mechanisms similar to those leading to the spread of neuronal death in the brain following TBI. This TBI-hearing loss model provides an important first step towards identifying therapeutic targets to attenuate damage to the auditory system following head injury.

Post-concussive Dizziness: A Review and Clinical Approach to the Patient

Dizziness is a frequent complaint after head trauma. Among patients who suffer a concussion (mild traumatic brain injury or mTBI), dizziness is second only to headache in symptom frequency. The differential diagnosis of post-concussive dizziness (PCD) can be divided into non-vestibular, central vestibular and peripheral vestibular causes with growing recognition that patients frequently exhibit both central and peripheral findings on vestibular testing. Symptoms that traditionally have been ascribed to central vestibular dysfunction may be due to peripheral dysfunction. Further, our ability to test peripheral vestibular function has improved and has allowed us to identify peripheral disorders that in the past would have remained unnoticed. The importance of the identification of the peripheral component in PCD lies in our ability to remedy the peripheral vestibular component to a much greater extent than the central component. Unfortunately, many patients are not adequately evaluated for vestibular disorders until long after the onset of their symptoms. Among the diagnoses seen as causes for PCD are (1) Central vestibular disorders, (2) Benign Paroxysmal Positional Vertigo (BPPV), (3) Labyrinthine dehiscence/perilymph fistula syndrome, (4) labyrinthine concussion, (5) secondary endolymphatic hydrops, (6) Temporal bone fracture, and (7) Malingering (particularly when litigation is pending). These diagnoses are not mutually exclusive and PCD patients frequently exhibit a combination of these disorders. A review of the literature and a general approach to the patient with post-concussive dizziness will be detailed as well as a review of the above-mentioned diagnostic categories.

Cell Transplantation to Restore Lost Auditory Nerve Function is a Realistic Clinical Opportunity

Hearing is one of our most important means of communication. Disabling hearing loss (DHL) is a long-standing, unmet problem in medicine, and in many elderly people, it leads to social isolation, depression, and even dementia. Traditionally, major efforts to cure DHL have focused on hair cells (HCs). However, the auditory nerve is also important because it transmits electrical signals generated by HCs to the brainstem. Its function is critical for the success of cochlear implants as well as for future therapies for HC regeneration. Over the past two decades, cell transplantation has emerged as a promising therapeutic option for restoring lost auditory nerve function, and two independent studies on animal models show that cell transplantation can lead to functional recovery. In this article, we consider the approaches most likely to achieve success in the clinic. We conclude that the structure and biochemical integrity of the auditory nerve is critical and that it is important to preserve the remaining neural scaffold, and in particular the glial scar, for the functional integration of donor cells. To exploit the natural, autologous cell scaffold and to minimize the deleterious effects of surgery, donor cells can be placed relatively easily on the surface of the nerve endoscopically. In this context, the selection of donor cells is a critical issue. Nevertheless, there is now a very realistic possibility for clinical application of cell transplantation for several different types of hearing loss.

Cochlear implantation after traumatic brain injury without otic capsule fracture: A case report and literature review

OBJECTIVE

The aim of this study was to report a case of cochlear implantation (CI) for a patient with an otic capsule-sparing traumatic brain injury (TBI) and to review the relevant literature.

METHODS

A patient with history of TBI received a CI for bilateral profound hearing loss. A systematic review of the literature was performed to identify and compare similar cases.

RESULTS

A 36-year-old male with a history of hearing loss from right acute labyrinthitis was referred for bilateral profound sensorineural hearing loss (SNHL) after a fall with associated injury to the central auditory nervous system (CANS) including the brainstem. On the right, behavioral acoustic threshold measurements were in the profound range with absent OAEs. On the left, testing revealed no measurable behavioral acoustic thresholds and variable physiologic measures. A right unilateral cochlear implant was performed with most recent follow-up demonstrating speech awareness thresholds of 25 dB HL with excellent detection of all 6 Ling sounds. However, the patient also continues to suffer from other neurologic sequelae related to his TBI, which challenge his ability to demonstrate objective and subjective benefit. A systematic review of the literature demonstrates variable outcomes for patients with TBI and SNHL.

CONCLUSIONS

Patients with profound SNHL and TBI present a distinct rehabilitative challenge for clinicians. CI may provide meaningful benefit in this population, though care should be taken in patient selection and counseling.

Long-term cochlear implantation outcomes in patients following head injury

OBJECTIVE

In cases of a severe to profound sensorineural hearing loss following head injury, the cochlear implant (CI) is the primary option for auditory rehabilitation. Few studies, however, have investigated long-term CI outcomes in patients following head trauma, including those without temporal bone fracture (TBF). Herein, the aim of this study is to examine CI outcomes following cases of head injury with and without TBF.

METHODS

Audiometric outcomes of patients who received a CI due to a head injury resulting in severe to profound hearing loss at two tertiary care hospitals were analyzed. Patients were divided into those who received a CI in a fractured temporal bone (group A, n = 11 patients corresponding to 15 ears) and those who received a CI in a non-fractured temporal bone (group B, n = 8 patients corresponding to nine ears). Primary outcomes included duration of deafness prior to CI and postoperative consonant-nucleus-constant whole word (CNC) scores.

RESULTS

Nineteen patients (84% male), corresponding to 24 CIs, were identified. Fifteen CI were performed on ears with TBF (group A), and nine CI were performed on ears without TBF (group B). No patients had an enlarged vestibular aqueduct (EVA). The mean duration of deafness was 5.7 and 11.3 years in group A and group B, respectively. The mean duration of CI follow-up (CI experience) was 6.5 years in group A and 2.1 years in group B. The overall mean postoperative CNC score for all subjects was 68.6% (±21.2%, n = 19 with CNC testing). There was no difference in CNC score between group A and group B (69.8% and 66% respectively, P = .639).

CONCLUSION

The study is among the largest series examining long-term outcomes of CI after head injury. CI is an effective method for auditory rehabilitation in patients after head injury.

LEVEL OF EVIDENCE

IV.

Abnormal Tectorial Membranes in Sensorineural Hearing Loss: A Human Temporal Bone Study

HYPOTHESIS

This study evaluates the morphological changes of the tectorial membrane (TM) in conjunction with degeneration of hair cells, interdental cells, and presence of endolymphatic hydrops (EH) in sensorineural hearing loss (HL) in the human using histopathology techniques.

BACKGROUND

The TM plays an important role in mechanical transduction of acoustic energy, and pathology of the TM may result in HL.

METHODS

All temporal bone (TB) specimens from the Massachusetts Eye and Ear Otopathology Laboratory from patients with various causes of sensorineural HL and morphological abnormalities of the TM were evaluated. Cases with a history of cochlear trauma (other than acoustic trauma) and/or severe postmortem artifacts were excluded. The TBs were processed histologically, and the status of hair cells, supporting cells, interdental cells, presence of EH, and postmortem time were tabulated.

RESULTS

Two thousand two hundred ninety TBs from 1340 individuals were evaluated, and 164 of 748 TBs from the otological disorders in which the TM were abnormal, met the inclusion criteria. The most common disorders were idiopathic sudden deafness (57.1%), genetic etiology (53.7%), and ototoxicity (40.0%), as compared with cases with presbycusis (2.9%). EH was found in 33.3% of all cases with an identified abnormality of the TM.Abnormalities of the TM were 1) deformed, 2) shrunken, 3) detached from the limbus, 4) encapsulated, or 5) missing. Encapsulated, shrunken and missing patterns (36, 35, 31%, respectively) were the most common.

CONCLUSION

A relative high prevalence of EH among disorders with TM abnormalities suggests a possible common pathophysiology in both. In addition, anatomic abnormalities of the TM may play a role in the pathophysiology of HL in these disorders.

Peripheral Vestibular System Histopathologic Changes following Head Injury without Temporal Bone Fracture

Objective

Vestibular symptoms such as dizziness and vertigo are common after head injury and may be due to trauma to the peripheral vestibular system. The pathophysiology of peripheral vestibular symptoms following head injury without temporal bone (TB) fracture, however, is not well understood. Herein, we investigate the histopathology of the peripheral vestibular system of patients who sustained head injury without a TB fracture.

Study Design

Otopathology study.

Setting

Otopathology laboratory.

Subjects and Methods

TB of subjects with a history of head injury without TB fractures were included and evaluated by light microscopy. Specimens were assessed for qualitative and quantitative characteristics, such as number of Scarpa’s ganglion cells in the superior and inferior vestibular nerves, vestibular hair cell and/or dendrite degeneration in vestibular end organs, presence of vestibular hydrops, and obstruction of the endolymphatic duct.

Results

Five cases (n = 5 TBs) had evidence of vestibular pathology. There was a decrease of 48.6% (range, 40%-59%) in the mean count of Scarpa’s ganglion cells as compared with that of normative historical age-matched controls. Moderate to severe degeneration of the vestibular membranous labyrinth was identified in the posterior, superior, and lateral canals in several cases (50%, n = 4 TBs). The maculae utriculi and sacculi showed mild to severe degeneration in 2 cases. Additional findings include vestibular hydrops (25%, n = 2 TBs) and blockage of the endolymphatic duct (n = 1 TB).

Conclusions

Otopathologic analysis of patients with a history of head injury without TB fracture demonstrated peripheral vestibular otopathology. Future studies are necessary to determine if otopathology findings are directly attributable to head injury.