Correction: The auditory outcomes in non-blast related traumatic brain injury and the role of severity, aetiology and gender: a scoping review

1IntroductionTraumatic brain injury (TBI), specified as a traumatic structural injury and/or physiological deterioration of brain functions caused by an external force (1), can result in many physical, cognitive, behavioural and emotional impairments (2–4). TBI is estimated to affect 64-74 million people worldwide each year (5). There are different ways of classifying the severity of TBI; most commonly, the Glasgow Coma Scale (GCS) at the time of injury and duration of post-traumatic amnesia are used to classify TBI as mild, moderate or severe (6). The most common type of TBI is "mild" (GCS 13-15; post-traumatic amnesia duration <24 hours; loss of consciousness <30 minutes) with males aged 18-65 years being at highest risk of experiencing TBI (7). There is a range of causes associated with TBI, including falls, traffic accidents, assaults, sports injuries (non-blast related) and explosions (blast-related). While there will be some similarities in the way the brain is affected by each aetiology, blast-related TBI has consistently been recognised to have some particular mechanisms- e.g., typically involves the transmission of high-pressure waves through air and/or fluid-filled spaces, which can cause widespread damage to the brain and inner ear by disrupting vascular structures, neuronal tissue, and the blood-brain barrier (8,9). In contrast, non-blast related TBI generally results from mechanical forces such as direct impact or acceleration-deceleration forces, and may lead to more focal injuries including contusions, diffuse axonal injury and blood-brain barrier disruption (10). While both mechanisms can affect similar structures, the pattern and distribution of the resulting injuries may differ. Blast-related TBI is also more likely to be seen in a military population, which may differ from the civilian population in a range of characteristics. Given the various injury patterns and population characteristics, this review focuses specifically on non-blast-related TBI, as it more accurately reflects the injuries encountered in civilian life and may offer a clearer framework for understanding auditory outcomes.Auditory conditions (such as hearing loss, tinnitus (ringing in the ear), hyperacusis (sound sensitivity)) can be observed in patients with TBI due to impairments or damage to the central and peripheral auditory systems (11,12). Characteristically, auditory conditions occur directly in fractures or damages in the temporal bone region. For instance, sensorineural hearing loss (SNHL) is in general associated with transverse fractures, whilst conductive hearing loss (CHL) is associated with longitudinal fractures (13). In a nationwide population-based study in Taiwan, individuals with TBI were found to have a 2.125 times higher risk of developing hearing loss (14). Moreover, in a study investigating trauma-related tinnitus, 1.7% of 1604 patients reporting experiencing tinnitus due to head trauma (12).Although there are studies assessing auditory functions related to TBI, there is currently no comprehensive review synthesising common auditory findings related to non-blast related TBI, in particular aetiology and severity of TBI related to auditory conditions. Addressing this gap in the knowledge will provide evidence to clarify the diagnosis and treatment methods, to help establish appropriate management strategies for auditory conditions in this patient group, and in turn reduce the negative effects of these comorbidities caused by TBI.Specifically, the objectives here are to identify:•What are the common auditory impairments of non-blast related TBI,•Whether auditory outcomes vary according to severity of non-blast related TBI,•Whether auditory outcomes vary according to aetiology of non-blast related TBI,•Whether auditory outcomes vary by gender following non-blast related TBI.For this purpose, a scoping review was determined to be the most appropriate method, as it is specifically designed to explore broad and diverse research questions, map the literature, summarise the findings, and synthesise the evidence obtained from a range of study designs (15,16). 2Material and MethodsThe methodology of this scoping review was conducted in accordance with the 6-stages framework developed by Arksey and O'Malley (15): (1) identifying the research question(s), (2) identifying relevant studies using appropriate keywords, (3) selecting relevant studies through iterative scanning of titles, abstracts, and full-texts, (4) extraction and charting the data, (5) collating, summarising and reporting of the results, (6) clinician review. The review is reported following the PRISMA-S guidelines (17) (see Supplemental Appendix Table 1 for PRISMA-ScR Checklist).2.1Identifying the research question(s)For this purpose, research questions (listed above) were developed in consensus with the team members based on existing knowledge of the field and literature.2.2Identifying relevant studies2.2.1Eligibility criteriaRecords were included if they reported studies/cases in which adults (≥18 years old) reported experiencing non-blast related TBI as an adult with associated hearing impairments, and hearing outcomes and assessment were reported (including self-reported auditory outcomes). Records were eligible if they reported symptoms or assessments pre-treatment and originated from cohort studies, case series, and case studies, as well as grey literature sources, particularly dissertations and theses. All included records were published in the English language and have full-text. Cases that did not meet our inclusion criteria were removed from the case series studies. Records were excluded if the studies were reporting adults who may have experienced blast-related TBI, TBI in childhood, whiplash injuries, or non-TBI conditions (e.g., strokes, acoustic neuroma) or they did not clearly define TBI or provide evidence of structural injury or functional deterioration due to TBI. Records involving participants with pre-existing audiological impairments before the TBI, where the aetiology of TBI was not reported, and/or records whose primary aim was to determine the reliability and validity of tests were excluded. Review articles (including systematic reviews), book chapters, randomised control trials, qualitative research studies and any sources reporting personal/expert opinions were excluded.2.2.2Search strategy The research strategy was developed by the research team and was supported by a medical information specialist (Dr Farhad Shokraneh). The search was conducted following Cochrane Handbook (18) and Cochrane's MECIR (19) and PRESS guideline for peer-reviewing the search strategies (20). Electronic databases were searched including Embase, MEDLINE, ProQuest Dissertations & Theses A&I, PsycINFO, Science Citation Index Expanded and SPORTDiscus in May 2022. The search strategy included keywords on TBI, auditory and vestibular conditions (a separate review was conducted for vestibular outcomes). These were reviewed and revised following a primary search (see Supplemental Appendix Table 2 for search strategy). Specific search term strategies were applied in each search engine, searching article topics, titles, abstracts, and keywords. Filters were applied to retrieve articles in the English language and human participant studies only, when possible. There was no restriction in the search period. To seek further eligible documents for inclusion, manual searches of the reference lists and most common journals (determined using the interquartile rule for outliers) in which eligible records had been sourced were conducted. The final database and manual searches were conducted in September 2024.2.3Study selectionRecords identified through electronic databases were exported with citation, title and abstract into EndNote (version X9), where duplicates were removed, before records were imported into Rayyan (21) for screening. Records were independently screened by four researchers (KB, OP, LE, KF), starting with the title and abstract, before moving onto the full text. Lead researcher (KB) screened all records. The records obtained as a result of the manual search were subjected to full-text screening. When disagreements arose regarding the inclusion or exclusion of any given record, the reviewers discussed their reasons until agreement was reached or a third reviewer was consulted to reach a majority decision. 2.4Extraction and charting of the dataA data extraction form was developed in Microsoft Excel and piloted on five included records and was subsequently modified following team discussions. Data from each record were extracted by lead researcher (KB) and checked by KF. Data were extracted on study characteristics, study population, TBI characteristics, audiological complaints and assessments/outcomes, and limitations (Box 1). *** insert Box 1. here ***2.5Collating, summarising and reporting resultsExtracted data were collated and categorised based on the objectives of our research. Similar findings were grouped into categories such as auditory outcomes, severity of TBI, aetiology, and gender effects. Data were then summarised to identify common patterns and significant variations in auditory outcomes.2.6Clinician reviewAfter the categories were identified, categorised outcomes were also examined by clinician LE.3ResultsFigure 1 illustrates the process of record identification and selection. Electronic searches resulted in an initial set of 19.019 records. Duplicates were removed and of the remaining 12.424 records, 11.901 were excluded because the title and abstract indicated that the articles did not meet the eligibility criteria. Manual searches identified a further 12 potential articles which were subjected to full-text screening. Of the remaining 535 records, 474 records were excluded at the full-text screening. Most commonly the studies excluded did not report TBI or clearly define TBI, included participants under 18 years old and did not report TBI aetiology. Full-text records could not be located for 31 records. None of these records could be traced, regardless of support from the University of Nottingham librarian. The electronic and manual searches created a final list of 61 eligible full-text records for data collection. *** insert Figure 1. here ***3.1Study CharacteristicsTable 1 provides an overview of the study and participant characteristics. As shown by Table 1, the majority of records were reporting case reports/case series (51/61) (11,22–71) and were mainly conducted in the United States (n=23), the United Kingdom (n=5), Japan (n=4), and Korea (n=3). Articles were published from 1956 to 2023. *** insert Table 1. here ***3.2Participant CharacteristicsAcross 61 records, 507 participants were included. Of these, 396 were in the patient group, whilst for four studies, 111 participants were in the control groups (either without TBI or without both TBI and auditory symptoms) (72–75). Pre-TBI health status of participants was not reported consistently across studies (Table 1). Assessment time since injury varied widely across studies (Table 1). In 39 studies, follow-up/s' assessments were performed after the initial time of injury before any treatment was offered (22–27,30,32–40,42–44,46–51,53–55,57,58,63–65,67–71,76).3.3Overview of auditory impairments following non-blast related TBIMany different symptoms such as hearing loss, tinnitus, and hyperacusis were reported across the studies. These symptoms were assessed using a variety of tests, including peripheral and central auditory function assessments and patient-reported outcome measurements (PROMs) which are briefly described below. A summary of these tests and PROMs are presented in Supplemental Appendix Table 3 and the results are shown in Table 2.*** insert Table 2. here ***3.3.1Otoscopic assessmentIn 27 (44%) studies, otoscopic assessment, a clinical procedure used to inspect the external auditory canal, tympanic membrane (eardrum), and middle ear (77), was conducted (11,22,23,30,35–37,39,44,46,51–53,55,56,58,59,61–63,65,67,70–73,78). Some studies described otoscopic assessments as ENT, otologic, or clinical examinations (see Table 2). Eight studies presented clinical findings related to the tympanic membrane or external auditory canal without mentioning explicitly otoscopic assessment (e.g., intact eardrum) (25,32,38,43,54,60,64,69) and 2 studies stated that otoscopy was performed, however the results were not reported (72,73). In 18 (67%) out of the 27 records, the otoscopic assessment results or clinical findings indicated a normal eardrum (11,22,23,25,36,37,43,44,46,53,55,56,58,63,65,70,71,78), whilst 16 (59%) studies noted at least one of the following symptoms: serous effusion, dried blood, blood, bloody otorrhea, cerebrospinal otorrhea, haemotympanum or haemorrhage (22,30,32,35,38,39,51,52,54,59–62,64,67,69). These symptoms were detected in the right ear in most of studies (10/16) (22,30,35,38,39,52,59–62).3.3.2Pure Tone (Behavioural) Audiometry (PTA)PTA refers to the assessment of thresholds determined by the lowest intensity at which an individual responds to sound at least 50% of the time (79). PTA was the most commonly used audiological assessment method with 56 studies reporting it (11,22–43,45–67,69,71–73,75,76,78,80–82). In four case studies, it was not explicitly stated whether PTA was conducted, but hearing loss was reported (49,76), audiometer screening was performed (33) or audiometric findings were presented (31). Normal hearing was reported for 19 (34%) out of 56 studies. Of these 19 studies, ten reported that hearing was normal or normal group mean bilaterally post-TBI (11,33,38,41,47,59,72,73,75,82), whilst 9 studies reported normal hearing in at least one ear or in one case (22,23,26,29,39,51,56,65,80). Of these, two studies (11,82), provided an accepted range for normal hearing (≤ 25 dB HL). The remaining 17 studies (17/19) provided no explanation, but nine (9/17) did demonstrate normal hearing with audiogram results of patients or groups mean thresholds (≤ 20 dB HL or 25 dB HL) (22,26,29,39,41,47,65,72,73).Based on PTA assessment, the most commonly reported type of hearing loss post-TBI (n=25, 45%) was SNHL (22,23,25–27,36,39,42,45,46,48,50,52,53,58,61–67,78,80,81). Among these, twelve (12/25) were identified as severe or profound SNHL (22,26,36,39,46,58,61–63,65,78,81), with two case reports observing severe or profound SNHL in follow-up assessments (36,46). Nine (9/25) were reported as mild or slight SNHL (22,23,42,48,64,66,67,78,81), with one case report observing mild SNHL in a follow-up assessment (67). In six studies (6/25), moderate SNHL was reported (27,53,63,66,67,81), with one study noting this in a follow-up assessment (67). Following this, CHL (n=12, 21%) was most reported (22,24,25,30,32,35,43,44,49,54,62,64), whilst MHL (n=7, 12.5%) was the least reported type of hearing loss (24,26,30,39,51,60,67). Three studies had no response to the stimulus in PTA assessment at all (37,40,55). In two other studies, no response was initially observed; however, SNHL was detected in the follow-up assessment before treatment (36,46). In another study, MHL was observed in the initial PTA in left ear, however hearing worsened during follow-up, and no response was detected (39). In four studies, the type of hearing loss changed during follow-up assessments, and there were cases where hearing partially improved (22,25,30,67). In ten studies, following PTA the degree (severity) of hearing loss or only hearing loss was reported, without reporting the type of hearing loss (in studies involving more than one case, at least one case) (22,28–30,34,50,56,69,71,76). Eight out of ten studies reported severe to total (profound) hearing loss post-TBI (22,28,30,34,50,56,69,76). One study stated that four frequencies (0.5 to 4 kilohertz (kHz)) were used to determine the average of hearing loss (71), whilst seven studies have not described the classification method used to determine the degree of hearing loss (i.e., mild, moderate and/or severe hearing loss) (22,28,30,34,50,71,76).3.3.3Site-of-lesion TestsSite of lesions tests performed via audiometry are used to distinguish cochlear and retro-cochlear abnormalities (83). Four studies utilised 3 of the site-of-lesion tests (Békésy, Tone Decay and Alternate Binaural Loudness Balance (ABLB) test) (22,28,50,66). Tone decay indicated findings in favour of retro-cochlear pathology in a patient with bilateral SNHL (66). Another study (50) that performed the Békésy test, reported a type I finding that indicated neither cochlear nor retro-cochlear pathology, even though the patient had bilateral SNHL. In one case study, the ABLB test showed no recruitment at low frequencies with severe hearing loss (28), whilst another study reported recruitment around 500 Hz in case 5 with SNHL in left ear (22).3.3.4Tuning Fork (TF) Test (Weber and/or Rinne)The TF test is used for screening and determining the type of hearing loss, confirming PTA results (84,85). Nine studies used the Rinne and/or Weber TF tests (22,34,39,40,46,64–66,70). In eight studies, the TF test results were consistent with the PTA results as seen in Table 2 (22,34,39,40,46,64–66), whilst the remaining study did not perform PTA (70).3.3.5Impedance Audiometry (Tympanometry and Acoustic reflex thresholds)Tympanometry objectively evaluates middle ear function (86,87). The acoustic reflex thresholds (ART) assess auditory pathway integrity up to the superior olivary complex (SOC) via stapedius muscle reflex (88). In ten studies, both tympanometry and ART measurements were performed (11,34,37,40,43,50,54,57,67,71), in eight only tympanometry was performed (36,46,55,64,66,72,73,78) and in one study only ART measurement was conducted (39). Of the 18 studies (18/61) that performed tympanometry, normal (Type A) results were obtained from 12 (11,34,36,37,40,46,50,55,57,71,73,78). The details of ART results performed ipsilaterally and/or contralaterally are presented in Table 2.3.3.6Basic and Advanced Speech Audiometry Speech audiometry examines the ability to process speech in auditory centres, starting from the outer ear and ending with the cortex, using speech signals. Of 61 included records, both basic (e.g., speech reception threshold, speech discrimination score) and advanced (e.g., speech-in-noise tests) tests were performed in 2 (3%) studies (11,73), while basic speech audiometry test(s) were performed in 13 (21%) studies (26,32,37,39,47,50,53,56,57,63,66,71,78), including six studies using Speech Discrimination Score (SDS) (26,32,50,57,66,71), four using Speech Reception Threshold (11,37,53,73) and four using Speech Recognition Threshold (11,32,47,73). The results of the tests varied depending on the patients or cases, from normal to no response at all (Table 2). A common result was not identified. Studies with follow-up assessments reported improvement in SDS results over time for one case with bilateral mild to moderate hearing loss (71), whilst another reporting worsening of SDS in one case with bilateral SNHL (50). More advanced, QuickSIN test was used in two studies (11,73), Words-in-Noise (WIN) test was used in one of those studies (73). In both studies, although the average hearing was normal post-TBI, mild signal-to-noise-ratio (SNR) loss or an abnormal result in at least one ear was observed in the QuickSIN results. Similarly, in the WIN test, abnormal results were reported in at least one ear across 8 participants (73).3.3.7Otoacoustic Emissions (OAEs) and Suppression TestOAEs provide an objective assessment of the functionality of the outer hair cells in the cochlea (89). Only 6 (10%) studies out of the 61 records used OAEs. In 2 studies, both Distortion Product OAE (DPOAE) and Transient Evoked OAE (TEOAE) measurements were employed (37,71), in three DPOAE was measured (47,50,66), and one study measured TEOAE (72). In studies where only DPOAE was performed, the DPOAE was obtained in normal hearing (47), whilst it was absent or very poor in cases of SNHL (50,66), consistent with the hearing conditions of patients. In one case study, bilateral responses were observed in both TEOAE and DPOAE (up to 3 kHz or 4kHz) in a patient with mild hearing loss (71), whilst another case study observed bilateral responses of TEOAE and DPOAE (only absent at 2 kHz) despite no response being obtained in either PTA or the ipsi-contralateral ART (37). In a study comparing a control group to a TBI group with/without auditory complaints (e.g., tinnitus, difficulty of hearing in noise, hyperacusis), where hearing was within normal limits in all groups, it was observed that the TEOAE amplitudes of the entire TBI group were lower than those of the control group. However, the amplitudes of the TBI group with auditory complaints were higher than those without auditory complaints (72). In one study, an OAEs suppression test referred to as medial olivocochlear suppression effect (MOSE) test, which allows for the evaluation of the efferent system (90), indicated that an absent effect of the auditory efferent system in one or both ears of the TBI patients with auditory complaints (72).3.3.8Electrophysiological TestsElectrophysiological tests performed with auditory potentials enable the evaluation of the auditory pathway from the auditory nerve to more central regions in the brain (91). Out of the 61 records, 16 (26%) studies used electrophysiological tests, with 15 studies using Auditory Brainstem Response (ABR/BAEP) (15/16) (34,36,37,40,41,46,47,50,53,55,58,66,68,71,80), one study using electrocochleography (ECOG) (38) and five using additional tests, including middle latency responses (MLR/MLAEPs) (41,47), late latency responses (LLR) (66,71), and mismatch negativity (MMN) and P300 (66).Out of the 15 studies, 13 showed that ABR findings were consistent with PTA results (34,36,37,41,46,47,50,53,55,58,66,71,80). For instance, in cases of bilateral profound SNHL, either bilateral unobtainable ABRs were observed (58) or, depending on the degree of hearing loss, waves I and III were obtained, but no peak in wave V was observed (66). In cases with normal hearing normal ABR results (47), or prolonged latency in wave V were obtained (41). However, in one of these studies, ABR, PTA and ART results were not obtained consistently, whilst results of OAEs were present (37) (refer to the OAEs section). In one study, ABR results were in a normal waveform and no response was obtained in PTA, whilst ART results were present bilaterally (40). Another study by Shibata (2020) reported cortical deafness due to delayed traumatic intracerebral haematoma using Magnetic Resonance Imaging (MRI), but 1 month later a normal response was observed in the ABR performed (68). Furthermore, in two case studies (50,71), improvement in ABR results in follow-up assessments corresponded to improvement in the degree of hearing loss obtained in PTA (71), whilst deterioration in ABR results corresponded to worsening in the degree of hearing loss (50). The details of other electrophysiological test results are presented in Table 2.3.3.9Central Auditory TestsCentral auditory system assessments facilitate the evaluation of auditory processes such as the processing, interpretation, and discrimination, enabling the assessment of the central levels of the auditory pathway (92). Out of 61 records, 5 (8%) studies performed various central auditory tests (11,33,41,47,73), despite normal hearing reported in PTA, abnormal results were observed in at least one central auditory test (Table 2). The age of participants in these studies ranged from 22 to 71 years.3.3.10PROMsPROMs (93) were used in 8 (13%) out of the 61 studies (11,57,71,73,74,76,82,94) for assessment of hearing (11,71,73,74,82), tinnitus (57,71,74,76,82) and hyperacusis (11,74,82). PROMs used include Hearing Handicap Inventory for Adults (HHI-A) (11,71,74,82), the speech spatial and qualities of hearing scale (73), the screening checklist for Auditory Processing in Adults (71), Tinnitus Handicap Inventory (THI) (71,74,82), a Likert scale for tinnitus amplitude (76), the Tinnitus Questionnaire (TQ) and numeric rating scale (NRS) for loudness, discomfort, annoyance, ignorability, and unpleasantness (57) and Hyperacusis Questionnaire (HQ) (11,74,82) were performed. In one study, the post-concussion symptom scale (PCSS) was used (94). In most studies (5/8), more than one PROM was used (11,57,71,74,82).Hearing impairment was observed in all studies in which HHI-A was reported (11,71,74,82). In particular, although normal hearing was detected in PTA in two studies, mild to severe (82) or substantial impairment (11) was observed because of HHI-A. Similarly, for the 16 (26%) studies reporting complaints of tinnitus (11,24,26,28–30,39,47,51,57,59,61,71,72,76,78), in the 3 studies using THI a range from slight to catastrophic score was reported (71,74,82). In the case study where TQ and NRS were used (57), the tinnitus severity grade was reported as extreme, and the patient considered tinnitus to be a very big problem in the NRS. Furthermore, tinnitus was reported to worsen from the time of TBI to initial consultation. Three (5%) studies reported complaints of hyperacusis (11,60,72), out of which 2 studies found that hyperacusis was the most reported symptom among individuals with TBI using HQ. Both studies reported significant sensitivity based on HQ results (74,82). In the case study where HQ was used, the patient found all sounds too loud and reported substantial impairment (11). Hyperacusis was also reported in the study using PCSS (94). Detailed results of other PROMs are presented in Table 2.3.4Effect of severity of non-blast related TBI on auditory outcomes The majority of studies have not clearly stated the severity of TBI (38/61) (22–30,35,36,38–45,48,50–56,60–62,65–70,76,81). Of the remaining, 10 studies included severe TBI (24,30,33,37,46,57,58,64,71,80), 6 included mild TBI (11,47,63,73,74,78), 7 studies reported concussion (i.e., mild TBI) (29,31,32,59,75,81,94), 2 observed moderate/severe TBI (34,49) and 2 included a range from mild to severe TBI patients (72,82) (see Table 1 more details on severity e.g., criteria of severity).In the 2 studies with a range of mild to severe TBI (72,82), normal hearing (≤ 20 dB HL or 25 dB HL) was observed and tinnitus and/or hyperacusis were reported. In Knoll et al., (2020) (82), tinnitus was the commonly reported symptom in both mild-TBI and moderate-severe-TBI groups. However, the mean for THI was higher in the moderate-severe-TBI group indicating more severe score than the mild TBI group (Table 2). In 6 studies where TBI severity was classified only as mild (11,47,63,73,74,78), abnormal results were observed in at least one central auditory test despite normal hearing in three studies (11,47,73), the remaining studies did not perform central hearing tests (63,78,82). In one study for mild TBI, a severe hearing impairment was reported using HHI-A (74). In patients exposed to mild TBI, Jang, Bae and Seo, (2019) (63) observed moderate and severe SNHL, whilst Jafarzadeh et al., (2022) (78) reported mild to profound SNHL. Tinnitus was observed in four studies involving mild TBI (11,47,74,78), whilst two studies reported both hyperacusis and tinnitus (11,74). HQ results of these studies are explained in the PROMs section earlier. In the remaining three studies (11,47,78), tinnitus was reported; however, no formal assessment was conducted. Notably, one of these studies, the reported tinnitus resolved a several months later (47). Also, different outcomes were observed in each of the studies reporting concussion such as normal hearing, mild CHL and profound SNHL (29,31,32,59,75,81). Hyperacusis was observed after concussion (94), and complaint of tinnitus were reported in another study (59).SNHL was reported in 4 out of 10 studies reporting severe TBI (46,58,64,80). This group also exhibited a range of outcomes from normal hearing to total deafness as well as CHL. There were tinnitus complaints in three case studies in severe TBI (30,57,71). In 2 studies evaluating tinnitus in this group, catastrophic score was detected in THI for mild hearing loss (71), and extreme tinnitus severity was observed in TQ in normal hearing between 0.125-2 kHz, with a steep decline toward higher frequencies on both sides (57). Hyperacusis was not indicated in any of the studies that included only severe TBI. In four of those studies, abnormal results were observed in at least one component of ABR (e.g., wave V) at the brainstem level, despite normal hearing or varying types or degrees of hearing loss (37,46,58,71). Figure 2 shows the distribution of auditory outcomes across studies according to TBI severity.*** insert Figure 2. here ***In summary, the severity of TBI may not consistently predict auditory outcomes and both mild and severe TBI can result in significant auditory impairments and abnormal central auditory test results.3.5Effect of aetiology of non-blast related TBI on auditory outcomes In terms of aetiology, the majority of studies (36/61) reported motor vehicle accidents (MVA) at least one participant or case (Table 1) (11,22–25,28–30,32–34,37,38,40,42,43,49,53,54,58,60,62–64,66,67,69,71–74,76,80–82,94).To examine the effect of aetiologies related to TBI, they were classified into five categories: MVA, falls, sports-related injuries, assaults, and others. In six studies involving multiple participants, different aetiologies, from MVA to assault, were included (72–74,80,82,94). For these studies results are reported together under all aetiologies. In one of the 6 studies (73) the group mean showed normal hearing, but a small range of hearing loss was reported in at least one ear in three participants with TBI. However, the degree classification of hearing loss was not explained (Table 2) (73). For these participants, two had an aetiology of fall, whilst one was due