Introduction
Sudden sensorineural hearing loss (SSNHL) is a critical otologic emergency characterized by rapid-onset hearing loss in one or both ears, occurring within 72 hours. SSNHL is commonly defined as hearing loss of at least 30 dB across three consecutive frequencies on pure-tone audiometry (PTA). Despite its prevalence, the exact etiology of SSNHL remains unclear, and is often classified as idiopathic. The postulated pathomechanisms include viral infections, microvascular impairments, and autoimmune processes.
Corticosteroids are widely considered the primary treatment for SSNHL owing to their anti-inflammatory properties and potential to reduce cochlear edema. These drugs, which are administered systemically [1] or via intratympanic injection [1,2], target the recovery of auditory function by mitigating cochlear inflammation. Despite their widespread use, the efficacy of corticosteroid therapy remains a topic of debate, with outcomes varying significantly among patients [2].
In addition to corticosteroid therapy, adjuvant [3] and rescue therapies [4], such as hyperbaric oxygen therapy (HBOT), have gained traction as potential options for improving hearing recovery in patients with SSNHL [1,5,6]. HBOT involves inhalation of 100% oxygen under increased atmospheric pressure to enhance oxygenation in ischemic cochlear tissues and promote auditory function restoration. The current clinical guidelines, such as those from the American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS), recommend HBOT as a secondary intervention for patients with suboptimal outcomes after primary treatment [7].
Research on the efficacy of HBOT in SSNHL has yielded mixed results. While some studies suggest that early initiation of HBOT may lead to improved hearing recovery [8], others report no significant difference compared to conventional treatments [9]. The timing of HBOT administration is particularly critical; earlier treatment initiation is generally associated with better outcomes. Key prognostic factors influencing SSNHL recovery include patient age, severity of initial hearing loss, and time elapsed between symptom onset and treatment initiation [1,10].
In clinical practice, many patients with SSNHL do not receive immediate treatment due to delayed presentation, varying accessibility to medical facilities, and individual circumstances. These delays necessitate observational studies encompassing diverse patient presentations and treatment timelines to better understand the impact of timing and therapy combinations on hearing outcomes.
Therefore, this study aimed to evaluate the effectiveness of various treatment modalities for SSNHL, including systemic corticosteroids, intratympanic injection, and HBOT. Furthermore, it sought to assess the efficacy of HBOT and the influence of its timing on hearing recovery in a prospective cohort of patients presenting to our institution. By analyzing real-world data from a diverse population of patients with SSNHL, this study provides valuable insights to aid clinicians in optimizing treatment strategies and improving patient outcomes.
Subjects and Methods
Study population
This prospective, registry-based, observational study was conducted at the Department of Otorhinolaryngology-Head and Neck Surgery, Asan Medical Center, Seoul, Korea, between November 2018 and May 2023. Patients presenting with unilateral SSNHL were evaluated, and their eligibility for study participation was determined on the same day via PTA assessment. Eligible patients were provided a detailed explanation of the study, and only those who voluntarily provided written informed consent form were enrolled.
The inclusion criteria were adult patients aged 19 years or above, who were diagnosed with unilateral SSNHL characterized by rapid sensorineural hearing loss of at least 30 dB across three consecutive frequencies within 72 hours of onset. Patients who had already received initial treatment at other institutions were also included.
The exclusion criteria were patients with evidence of conductive hearing loss or a history of middle ear surgery.
This study was approved by the Institutional Review Board of Asan Medical Center (IRB No. 2018-1164). This study was performed in accordance with the ethical standards enshrined in the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards. Informed consent form was obtained from all participants before study commencement. Consent to publish was included in the informed consent form. All participants agreed to participate in this study with their own decision.
Treatment protocols
Patients with SSNHL were treated according to the 2019 AAO-HNS guidelines [7]. The treatment modalities included systemic corticosteroid therapy, intratympanic dexamethasone injection, and HBOT, delivered either as standalone or combination therapies. The specific treatment protocol was determined based on the presentation time and individual clinical circumstances.
Systemic corticosteroid therapy comprised methylprednisolone (1 mg/kg/day, up to a maximum of 60 mg/day) administered for 9 days, followed by a tapering regimen over the subsequent 5 days, for a total treatment duration of 14 days. Intratympanic corticosteroid was administered in the form of dexamethasone disodium phosphate (5 mg/mL; Ilsung Pharmaceuticals), with sufficient volume (0.4–0.5 mL) to fill the tympanic cavity. A minimum of four injections were administered per patient.
HBOT was offered as adjunctive therapy based on the clinical judgment of the physician and patient preference. Each HBOT session involved inhalation of 100% oxygen at 2.0–2.5 atmospheres absolute (ATA) for 90 minutes daily over 7 consecutive days.
Data collection
Hearing assessments were conducted between 2 and 3 months after the onset of hearing loss. PTA thresholds were used to calculate the pure-tone average by averaging the hearing thresholds (dB HL) at 500 Hz, 1 kHz, 2 kHz, and 3 kHz. The degree of hearing recovery was classified using the modified AAO-HNS guideline as follows [11]: 1) complete recovery (CR): pure-tone average difference ≤10 dB and speech discrimination score (SDS) difference ≤10%; 2) partial recovery (PR): pure-tone average ≤50 dB and SDS ≥50% (serviceable hearing); 3) slight recovery (SR): improvement of ≥10 dB in the pure-tone average and ≥10% in the SDS but not reaching serviceable hearing; and 4) no recovery (NR): improvement of <10 dB in the pure-tone average and <10% in the SDS. Hearing evaluations were performed at three time points, viz., on the day of presentation, 1 to 2 months after treatment, and 2 to 3 months after treatment (Table 1).
Statistical analysis
The Mann–Whitney U test was used to compare the treatment outcomes between groups. Logistic regression analysis was employed to evaluate the influence of the prognostic factors. Data were presented as the mean±standard deviation; p-values of less than 0.05 were considered statistically significant. Statistical analyses were conducted using R software (version 4.4.1; R Foundation for Statistical Computing).
Results
Clinical characteristics
During the study period, 111 patients were enrolled. Six patients were lost to follow-up because they did not undergo the post-treatment hearing assessments, leaving 105 patients who completed all procedures, including the follow-up evaluations.
The clinical characteristics of the patients are summarized in Table 2. The mean age was 53 years, with 57 male patients (54.3%) and 48 female patients (45.7%). The right ear was affected in 40 patients (38.1%) and the left in 65 patients (61.9%). Comorbidities included diabetes mellitus in 27 patients (25.7%) and hypertension in 35 patients (33.3%). Accompanying symptoms included tinnitus (81%), aural fullness (62.9%), and vertigo (28.6%).
The mean duration from symptom onset to treatment initiation was 5.2 days, with a range of 0–37 days. The mean duration from symptom onset to the initiation of HBOT was 12.3 days, ranging from 1 to 45 days. The total follow-up period averaged 8.6 months.
Systemic steroid therapy was administered to all patients except for three who could not receive steroids due to underlying medical conditions. Intratympanic dexamethasone injections were administered in 102 patients (97.1%), with a mean of 5.7 injections per patient. HBOT was administered to 93 patients (88.6%).
Hearing recovery outcomes
Hearing recovery was assessed using PTA between 2 and 3 months after symptom onset. Recovery was classified as CR in 16 (15.2%), PR in 22 (21.0%), SR in 44 (42.0%), and NR in 23 patients (22.0%) (Table 3).
Patients were divided into two groups based on the timing of treatment initiation: those who started treatment within 2 weeks of symptom onset (n=89) and those who began treatment 2 weeks after symptom onset (n=16). As shown in Table 3, no statistically significant difference in treatment outcomes was observed between the two groups (Mann–Whitney U test, p=0.2685).
Even after adjusting for the effects of diabetes and hypertension via multiple regression analysis, the timing of treatment initiation did not show a statistically significant association with treatment outcomes (p=0.4695).
When patients with initial PTA values between 40 dB and 70 dB (n=20) were analyzed, no significant difference in treatment outcomes was observed between those treated within 2 weeks and those treated after 2 weeks (14 vs. 6; p=0.3194). However, in patients with initial pure-tone average values >70 dB (n=79), those treated within 2 weeks showed significantly better hearing outcomes compared to those treated after 2 weeks (72 vs. 7; p=0.004695) (Table 3).
When all 105 patients were stratified into those with good prognosis (CR and PR) and poor prognosis (SR and NR), the timing of initial treatment initiation was not found to influence prognosis (odds ratio: 0.9859, p=0.5388, logistic regression analysis).
Outcomes of HBOT
Among patients who received HBOT, the time from symptom onset to the initiation of HBOT was associated with prognosis. Longer delays before HBOT initiation were correlated with a higher likelihood of poor prognosis (odds ratio: 1.0594, p=0.0249, logistic regression analysis).
When patients who received HBOT were divided into those who started treatment within 2 weeks (n=82) and after 2 weeks (n=11), the former exhibited significantly better outcomes (p=0.0011, Mann–Whitney U test) (Table 4).
Among patients with initial pure-tone average values >70 dB who underwent HBOT (n=75), those treated within 2 weeks of symptom onset showed significantly better hearing recovery compared to those treated after 2 weeks (p=0.0000187) (Table 5). However, no significant difference in outcomes was observed between patients treated within 1 week and those treated within 2 weeks (p=0.47).
In patients who started treatment within 2 weeks and received HBOT (n=68), the outcomes were further stratified based on whether HBOT was initiated within (n=48) or after 2 weeks (n=20). The group that commenced HBOT within 2 weeks demonstrated significantly better outcomes (p=0.0004279) (Table 6).
Discussion
This prospective, observational study investigated the treatment outcomes of patients with SSNHL presenting at various time points, focusing mainly on the timing and efficacy of adjunctive HBOT. These results highlight the importance of early treatment initiation, demonstrating that earlier intervention leads to better outcomes. These findings are consistent with existing guidelines, such as that devised by the AAO-HNS, which recommends starting treatment within 2 weeks of symptom onset [7].
Numerous studies have explored the application of HBOT in SSNHL treatment, showing its effectiveness as salvage and primary therapy [4,8,12-14]. Our findings corroborate these reports, revealing that HBOT initiated within 2 weeks of symptom onset resulted in significantly better hearing recovery than delayed initiation. Furthermore, early initiation of HBOT was associated with greater improvements in hearing thresholds. These findings align with the hypothesis that the timing of HBOT is critical to the treatment outcomes, emphasizing the importance of early intervention. Previous studies have similarly demonstrated that early HBOT administration yields superior outcomes [12-14]. Moreover, patients with more severe initial hearing loss have reportedly derived greater benefit from HBOT [13]. The results of this study also support these observations, as patients with initial hearing thresholds worse than 70 dB who received HBOT exhibited better recovery outcomes.
Although there may be variations in the specific timing recommendations across studies, our results and prior evidence collectively suggest that initiating HBOT early after symptom onset yields the best outcomes. Early HBOT may be especially beneficial for patients with severe initial hearing loss. This study observed the advantage of early HBO initiation in patients with hearing thresholds worse than 70 dB. These findings underscore the importance of prompt and proactive application of HBOT in managing severe SSNHL. However, the optimal treatment protocol may vary depending on individual patient factors and should be determined in conjunction with steroid therapy [12].
The improved outcomes associated with early HBOT support the hypothesis that timely oxygen delivery to the cochlea can mitigate ischemic or hypoxic damage. These results align with previous studies suggesting that the efficacy of HBOT is time-dependent [12-14], further underscoring the critical importance of early intervention.
In this study, while the post-treatment hearing improvement was statistically more remarkable in the HBOT group, the final hearing thresholds in this group were worse than those in the non-HBOT group. These results may reflect a tendency to administer HBOT earlier in patients with more severe hearing loss. Therefore, it can be inferred that for patients with initial hearing thresholds worse than 70 dB, early initiation of HBOT is particularly beneficial, advocating for its active use in this patient population.
Despite the benefits of early HBO therapy, not all patients who received this treatment experienced full hearing recovery [9,15]. This variability in response may be attributable to factors such as the severity of initial hearing loss, underlying etiology, and patient comorbidities. Therefore, further studies are needed to refine patient selection criteria, optimize the timing of HBOT, and establish standardized protocols for SSNHL treatment.
Internationally recognized guidelines for SSNHL, such as the AAO-HNS guideline [7], recommend initiating treatment within 2 weeks of symptom onset. Beyond this time frame, the guidelines do not suggest additional treatment options. The same applies to national guidelines in South Korea [16]. However, as observed in this study, it is rare for patients with SSNHL presenting to tertiary care hospitals to seek medical attention immediately after symptom onset. Some patients do not visit primary care facilities, especially those with less severe hearing loss. Thus, there is a need for appropriate guidelines to manage patients who fall outside the established recommendations. Developing guidelines for such cases could alleviate patient anxiety and facilitate early auditory rehabilitation.
Despite its valuable contributions, this study has some limitations. The observational design limits the ability to draw causal inferences, as the decision to administer HBOT and its timing were not randomized but determined by clinical judgment. This design introduces the potential for selection bias, particularly since patients with worse initial hearing loss were more likely to receive early HBOT. Additionally, the findings of this study were derived from a single-center cohort, which may limit generalizability to other populations or clinical settings.
Future research should address these limitations through randomized controlled trials to confirm the findings and establish causality. Further studies are also needed to optimize HBOT protocols, including the ideal timing and patient selection criteria, and to explore alternative treatments for patients who present beyond the 2-week window. Developing expanded and nuanced guidelines that account for these factors could improve outcomes for a broader range of patients with SSNHL.
This study reinforces the importance of early treatment initiation in SSNHL, particularly the critical role of HBOT within the first 2 weeks of symptom onset. It also highlights the differential benefits of HBOT based on initial hearing loss severity, since patients experiencing greater baseline deficits derived the most significant benefits. While existing treatment guidelines remain a valuable framework, this study identified practical gaps and suggested the need for expanded protocols to address delayed presentations. These findings contribute to a deeper understanding of SSNHL management and underscore the need for continued research to optimize treatment strategies and improve patient outcomes.
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