Sudden sensorineural hearing loss (SSNHL) is an acute, unexplained hearing loss occurring within 72 h. Criteria for a SSNHL diagnosis varies; the latest Chinese guidelines define SSNHL as ≥20 dB of hearing loss in two consecutive frequencies (1) and the American Academy of Otolaryngology-Head and Neck Surgery defines SSNHL as hearing loss ≥30 dB at three consecutive frequencies (2). Clinically, SSNHL is categorized into unilateral SSNHL (USSNHL) and bilateral SSNHL (BSSNHL). The incidence of BSSNHL is considerably lower than that of USSNHL (3, 4). Based on the time interval between the onset of hearing loss in both ears, BSSNHL is further divided into simultaneous BSSNHL (Si-BSSNHL, onset in both ears within ≤3 days) and sequential BSSNHL (Se-BSSNHL, onset in both ears with an interval of >3 days) (4). Existing evidence suggests that BSSNHL differs from USSNHL in etiology, treatment, and prognosis (5). USSNHL is often idiopathic and has higher recovery rates (6). In contrast, BSSNHL is frequently associated with underlying systemic diseases, leading to more profound hearing loss and less favorable treatment outcomes (5–7). Given the heterogeneity of BSSNHL etiologies and its low incidence, BSSNHL remains relatively poorly understood (2). This study aims to retrospectively analyze the clinical data of patients diagnosed with BSSNHL and summarize the clinical characteristics, etiologies, treatment outcomes, and factors influencing prognosis.

BSSNHL is a rare otologic condition that can occur simultaneously or sequentially, presenting in only 0.57–14.5% of all patients with SSNHL (4–6, 9). In this study, the 100 patients diagnosed with BSSNHL without prior diagnosis of Meniere’s Disease constituted 8.09% (100/1235) of all patients admitted for SSNHL during the selected period. Clinical data from in-patients with BSSNHL was analyzed for the condition’s etiology and clinical characteristics, thereby offering insights into the diagnosis and treatment of BSSNHL.

Underlying etiologies were identified in 38/100 patients; of these, 7 patients (18.4%) were diagnosed with LVAS, the only otologic etiology in this patient group. Thirty-one patients were diagnosed with systemic conditions, including vascular diseases, autoimmune diseases, infectious diseases, neoplastic diseases, and uremia. Co-morbidities were not present in this study’s pediatric group.

Vascular diseases were the most common possible etiologies of BSSNHL in this study and were more prevalent in older patients with BSSNHL. The inner ear is supplied by blood from the labyrinthine artery, which branches into the cochlea-vestibular artery, cochlear artery, and the anterior vestibular artery (10). Existing literature indicates that cases of SSNHL with concurrent cerebral infarction are predominately caused by infarction in the anterior inferior cerebellar artery—from which the labyrinthine artery most commonly arises (11) with only a minority associated with infarctions in the posterior inferior cerebellar artery or vertebral-basilar artery (9). Evidently, thrombosis, vasospasm, bleeding, and other vascular diseases can disturb otovestibular microcirculation, causing auditory and vestibular functional impairment. Brain MRI and magnetic resonance angiography examinations often reveal corresponding ischemic lesions in the affected areas (9, 12). However, in the early stages of the disease, MRI examinations may indicate negative results and multiple evaluations may be necessary to identify the cause. Therefore, some patients with SSNHL and concurrent ischemia present with BSSNHL as the only symptom, whereas others experience symptoms more consistent with cerebral infarctions such as dizziness, nystagmus, ataxia, falls, and speech challenges. Clinical manifestations and prognosis are related to the location and severity of vascular lesions, with severe cases posing a potential threat to life (13). Therefore, in patients with BSSNHL, accompanying focal neurological symptoms or signs such as ataxia and speech challenges warrant a high level of vigilance for cerebral infarction (14).

Autoimmune diseases are a major cause of BSSNHL, with reported cases of bilateral sensorineural hearing loss associated with systemic lupus erythematosus, granulomatosis with polyangiitis, relapsing polychondritis, antiphospholipid syndrome, Behçet’s disease, and Sweet syndrome, among others (9, 15–18). In this study, the occurrence of BSSNHL in five patients may be attributed to autoimmune diseases, including one case of antiphospholipid syndrome, two cases of relapsing polychondritis, one case of Sjögren’s syndrome, one case of Vogt–Koyanagi–Harada disease, and one case of Cogan syndrome. A prospective cohort study found that the risk for SSNHL is significantly higher in patients with autoimmune diseases (i.e., multiple sclerosis, Behçet’s disease, antiphospholipid syndrome), which supports this study’s findings (19). The pathophysiology mechanism may involve vascular endothelial inflammation in the cochlea, leading to impaired blood supply (20).

LVAS is the most common cause of sensorineural hearing loss in children (21). Five (45%) pediatric patients in this study underwent high-resolution temporal bone computed tomography scans and were diagnosed with LVAS. Recurrence is possible, often with precipitating factors leading to inner ear pressure imbalance and disturbances in the internal environment before onset (21). Given the prevalence of LVAS, for pediatric patients presenting with BSSNHL, a detailed medical history examination and a high-resolution temporal bone CT examination are necessary to identify inner ear abnormalities.

The suspected BSSNHL etiology in two patients in this study is meningitis, a finding previously reported (22). Other identified infectious causes in the literature include COVID-19 (23) and Lyme disease (24). Three patients were diagnosed with HIV, a risk factor not previously extensively investigated as a risk factor for BSSNHL. Notably, the patient with an HIV presented with increased hearing loss after treatment. Clinicians should consider SSNHL as an additional potential complication in HIV patients.

Vestibular schwannomas, which are present in most patients with neurofibromatosis type 2, are highly prevalent in patients with SSNHL (1.12–4.0%) (25–27) compared to the general population. One patient (1%) in this study has been diagnosed with neurofibromatosis type 2, which is consistent with previous findings. To our knowledge, acute myeloblastic leukemia with maturation (AML-M2) has not been associated with SSNHL in previous studies. Given that one patient in our study is diagnosed with AML-M2 alongside BSSNHL, future research should investigate the role of non-vestibular neoplasms, such as AML-M2, in SSNHL pathophysiological and progression.

BSSNHL causes more significant hearing impairment and poorer treatment outcomes and prognosis compared to USSNHL, consistent with the findings of the present study (4, 5). The overall treatment efficacy in this study was 32%, which aligns with previous studies reporting a treatment efficacy for BSSNHL of 12.5–37.5% (7, 22, 28). Though consistent with previous studies, the relatively low efficacy and poorer prognosis in this study may also be associated with a higher proportion of patients with severe or profound sensorineural hearing loss. Among the 100 patients (200 ears), 75.5% (151/200) exhibited severe or profound sensorineural hearing loss, while only 24.5% (49/200) had mild or moderate sensorineural hearing loss. This indicates that patients with BSSNHL are more likely to experience a more severe degree of hearing loss. The study findings revealed significant differences in treatment efficacy and prognosis among different degrees of hearing loss (p = 0.004). The treatment efficacy for profound sensorineural hearing loss was lower than that of mild sensorineural hearing loss (p < 0.01) and moderate sensorineural hearing loss (p < 0.01), suggesting that the degree of hearing loss is a crucial factor influencing the efficacy and prognosis of BSSNHL. Most patients with effective treatment had idiopathic etiology (70%). Moreover, hearing loss usually presents with full-frequency descent, and patients with BSSNHL may have progressive hearing loss. It emphasizes the importance of paying attention to unexpected outcomes and poor prognosis during treatment to avoid further sequelae.

The most common accompanying symptom in BSSNHL patients is tinnitus, present in 65/100 of the BSSNHL patients in this study. The effect of tinnitus on BSSNHL prognosis varies in previous studies (28). In our study, tinnitus showed no statistically significant relevance to efficacy evaluation.

In this study, patients with BSSNHL and comorbidities such as hypertension, diabetes, and coronary heart disease accounted for 24, 14, and 9% of all participants, respectively. Furthermore, patients with these comorbidities had a poorer prognosis (p < 0.05). Aimoni et al. (29) indicated that cardiovascular diseases, diabetes, and metabolic disorders were risk factors for the onset of SSNHL and demonstrated an unfavorable impact on the prognosis, aligning with our study’s findings. It is important to note that other patient characteristics variables (i.e., age, gender) can confound the association between these chronic conditions and BSSNHL. Nonetheless, clinicians should be aware of hypertension, diabetes, and coronary heart disease as risk factors for poor treatment outcomes in BSSNHL patients.

In summary, BSSNHL exhibits distinctive clinical characteristics compared to USSNHL. BSSNHL usually manifests with severe sensorineural hearing loss which may progressively worsen. Vascular, autoimmune, infectious, neoplastic, and uremic were considered as potential pathogenesis in our study. BSSNHL is closely associated with underlying chronic conditions such as diabetes, hypertension, and coronary heart disease, all of which predict poor prognosis. Clinical diagnosis and treatment should involve a detailed medical history and early comprehensive systemic examinations to achieve a precise diagnosis and timely targeted treatment, preventing misdiagnosis or delayed treatment. Based on our findings, accounting for patient sex and other comorbidities can guide us toward the proper diagnostic tests to unravel the underlying cause of BSSNHL. Although early examination, diagnosis, and treatment are crucial, long-term and close follow-up visits are necessary for patients with an unclear etiology.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

This study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Board of the local ethics committee at the Second Xiangya Hospital of Central South University (protocol code: LYF20230181, November 2023). Written informed consent for participation in this study was provided by the participants or their legal guardians/next of kin.

JH: Investigation, Writing – original draft. LJ: Investigation, Writing – review & editing. JY: Formal Analysis, Writing – review & editing. AM: Formal Analysis, Writing – review & editing. ZP: Data curation, Writing – review & editing, Investigation. JF: Data curation, Writing – review & editing, Investigation. QY: Data curation, Writing – review & editing, Investigation. WL: Writing – review & editing, Supervision, Methodology, Funding acquisition.