Sudden hearing loss (SHL), also known as sudden sensorineural hearing loss (SSNHL), refers to a rapid onset of sensorineural hearing loss. It is considered a syndrome rather than an independent disease. According to the diagnostic criteria of the American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS), sudden hearing loss is defined as an abrupt, unexplained sensorineural hearing loss occurring within 72 hours, with a hearing reduction of ≥30 dB HL in at least three contiguous frequencies.
It may be accompanied by tinnitus or vertigo either simultaneously or sequentially. Between 32% and 65% of patients experience spontaneous recovery, while some cases may recur. Current literature on the incidence of SHL shows variability across countries, and there is a lack of large-scale epidemiological data. For example, in the United States, the reported incidence of SHL is 5–27 per 100,000 people annually (as of 2019), with approximately 4,000–25,000 new cases yearly. In Japan, the incidence was reported as 27.5 per 100,000 in 2001, while German guidelines indicate an incidence of 160–400 per 100,000 in 2011. Trends show a year-on-year increase in all countries. SHL can occur at any age but is most common between the ages of 45 and 65, with an increasing trend among younger populations. There is no significant difference in incidence between males and females. Clinically, unilateral occurrences are more frequent, though simultaneous or sequential bilateral involvement can also occur.
Etiology and Pathogenesis
Both localized and systemic factors may contribute to SHL. However, only 10–15% of patients present with a clearly identifiable cause during the acute phase, while an additional one-third of cases are diagnosed or hypothesized after long-term follow-up. Mental stress, high pressure, emotional fluctuations, irregular living habits, and sleep disorders are considered significant triggering factors. The etiology of SHL is complex and influenced by various factors. Common causes include the following:
Vascular Disorders
The labyrinthine artery is a terminal artery and primarily the sole blood supply to the inner ear. Its pathologies significantly impair inner ear function. Vascular disruptions such as thrombosis, hemorrhage, vasospasm, embolism, or vascular compression can lead to oxygen and blood supply deficiencies to the inner ear, causing degeneration of the spiral organ’s sensory structures and adjacent vestibular organs, resulting in sudden hearing loss and vertigo. Animal studies have shown that inner ear ischemia lasting six seconds results in the disappearance of cochlear potentials, and ischemia lasting 30 minutes or more causes irreversible changes despite the restoration of blood flow. Additionally, embolism in the distal branches of the anterior inferior cerebellar artery or posterior inferior cerebellar artery can lead to ischemic necrosis in their distribution areas (including the labyrinthine artery), resulting in acute vestibular and auditory dysfunction. Such cases are prone to being misdiagnosed as peripheral vestibular diseases.
Infections
Viral neuritis or cochleitis is thought to be the most common cause of SHL. The anatomical proximity of the vestibular and cochlear organs underpins the viral infection theory. Clinical observations suggest that approximately 28% of SHL patients report a history of upper respiratory tract infection within the preceding month. Serological testing often indicates elevated antiviral antibody titers. Various viruses have been implicated in this condition, such as mumps virus, cytomegalovirus, herpes virus, varicella-zoster virus, influenza virus, parainfluenza virus, rhinovirus, adenovirus type III, Epstein-Barr virus, and Coxsackie virus.
Other infectious causes include meningitis, syphilis, and acquired immunodeficiency syndrome (AIDS). Viral particles enter the inner ear through one of three pathways:
- Hematogenous Spread: Viral particles directly enter the inner ear's circulatory system via the bloodstream, leading to cochlear circulatory disturbances or endolymph labyrinthitis.
- Meningeal Pathway: Viruses invade the perilymphatic space from the subarachnoid space via the cribriform plate at the fundus of the internal auditory canal or via the cochlear aqueduct, resulting in perilymph labyrinthitis. This explains why cochlear symptoms often follow meningitis. Varicella-zoster virus is a primary pathogen associated with perilymph labyrinthitis.
- Round Window Pathway: Viruses causing non-purulent otitis media may invade the inner ear via the round window.
Autoimmune Diseases
The inner ear is not an immune-privileged organ. The endolymphatic sac not only absorbs and clears endolymph but also has immune functions, making it the primary site for antigen processing and immune response in the inner ear. Upon antigenic stimulation, peripheral lymphocytes may accumulate in the inner ear, proliferating locally and producing antibodies. The inner ear is regulated by the systemic immune system, and low levels of lymphocytes and antibodies in the bloodstream can cross the inner ear-blood labyrinth barrier, triggering inflammatory responses. Studies by Mayot et al. have shown significant reductions in CD4+ T lymphocytes and CD3+ T lymphocytes, along with an increase in CD8+ T lymphocytes in the peripheral blood of SHL patients, suggesting a strong link between SHL and immune suppression. Immunological mechanisms in the inner ear are now widely recognized by researchers worldwide.
Autoimmune diseases associated with SHL include Cogan's syndrome, systemic lupus erythematosus, temporal arteritis, and polyarteritis nodosa.
Tumors
Approximately 10.2% of patients with acoustic neuroma present with sudden hearing loss as their initial symptom, which underscores the importance of evaluating for mass lesions in the cerebellopontine angle region when diagnosing sudden hearing loss. Acoustic neuroma may cause sudden hearing loss due to tumor-related bleeding or edema of surrounding tissues compressing the cochlear nerve, leading to conduction block, or due to acute cochlear ischemia resulting from arterial compression by the tumor.
Ototoxic Hearing Loss
Common ototoxic drugs include aminoglycoside antibiotics (such as streptomycin, gentamicin, and amikacin), loop diuretics, antitumor agents (such as cisplatin and nitrogen mustard), as well as other substances like dextropropoxyphene, piroxicam, naproxen, and harmful inhaled gases (such as carbon monoxide and hydrogen sulfide). These agents may cause sudden hearing loss.
Congenital Developmental Abnormalities
Conditions such as enlarged vestibular aqueduct syndrome, which can manifest as sudden hearing loss following minor trauma (referred to as "one slap deafness"), are examples.
Membranous Labyrinth Abnormalities
Labyrinthine Membrane Rupture
This refers to ruptures of the round or oval window membrane, often accompanied by rupture of the cochlear membrane. These ruptures can lead to sudden hearing loss, vertigo, and tinnitus.
Endolymphatic Hydrops
Some patients with sudden hearing loss and vertigo symptoms show signs of endolymphatic hydrops. Studies indicate that 5%–6.6% of patients with sudden hearing loss eventually develop Meniere's disease. Sudden hearing loss may even be the initial symptom of Meniere's disease. Additionally, the effectiveness of diuretics in some cases of sudden hearing loss lends support to the role of endolymphatic hydrops.
It is worth noting that if a clear causative factor is identified during diagnosis and treatment, the diagnosis should be revised to reflect the specific underlying condition, such as acoustic neuroma or enlarged vestibular aqueduct syndrome.
Clinical Manifestations
Most patients report excessive fatigue, mental depression, anxiety, emotional agitation, exposure to cold, or a history of upper respiratory tract infections prior to onset. Common symptoms include the following:
Sudden Onset of Hearing Loss
Hearing loss may present as the first symptom and typically progresses to its lowest point within minutes, hours, or a day, while a minority of patients may experience a decline over three days. The condition is most commonly unilateral.
Tinnitus
Approximately 70% of cases involve tinnitus, occurring before, during, or after the hearing loss. Patients may experience high-pitched tinnitus in one or both ears, often accompanied by hearing loss. The duration varies, and in some cases, tinnitus persists even after hearing recovery with treatment.
Ear Fullness or Pressure
About 50% of patients report a feeling of fullness or pressure around the affected ear, sometimes accompanied by abnormal sensations around the ear.
Vertigo or Dizziness
Approximately 28%–57% of cases involve vertigo or dizziness, particularly in patients with severe hearing loss. Some may exhibit spontaneous nystagmus. Vertigo may occur before or after the hearing loss, with rotational vertigo being most common. A small proportion of patients report sensations of rocking or unsteadiness, which may be accompanied by cold sweats, nausea, or vomiting. Studies have shown that patients with additional vestibular damage exhibit more severe hearing loss, and the presence of vertigo is associated with poorer prognosis.
Hyperacusis or Recruitment
Sensitivity to certain sounds or abnormal amplification of sound perception may occur.
Other Symptoms
Some patients experience psychological symptoms such as anxiety or sleep disturbances, which may impact quality of life.
Examinations
Routine Examinations
Otolaryngological Examination
This includes inspection of the skin around the ear, lymph nodes, external auditory canal, and tympanic membrane. Observations should focus on the presence of herpes, redness, swelling, cerumen, ear canal boils, or blisters in the surrounding area.
Tuning Fork Test
This includes the Rinne test, Weber test, and Schwabach test.
Pure Tone Audiometry
This tests both air and bone conduction thresholds at 250 Hz, 500 Hz, 1,000 Hz, 2,000 Hz, 3,000 Hz, 4,000 Hz, and 8,000 Hz.
Tympanometry and Acoustic Reflex Testing
This involves tympanogram evaluation and measurement of stapedial reflexes in both ipsilateral and contralateral conditions.
Assessment of Vertigo
In cases with vertigo, spontaneous nystagmus should be evaluated, and bedside positional tests may be conducted on a case-by-case basis based on patient history.
Additional Examinations
Further tests may be conducted depending on the clinical situation:
Advanced Audiological Tests
These include otoacoustic emissions, auditory brainstem response (ABR), electrocochleography, and speech audiometry (e.g., speech reception threshold and word recognition scores).
Imaging
Inner ear MRI is increasingly emphasized to rule out cerebellopontine angle lesions such as acoustic neuroma. With MRI now incorporated into recommended screening guidelines for sudden hearing loss, detection rates of abnormal inner ear signals have increased, highlighting its value in diagnosis. Temporal bone CT may also be considered when clinically indicated.
Laboratory Tests
Laboratory evaluations include complete blood count, blood biochemistry (such as glucose levels, lipid profiles, and homocysteine), coagulation tests (e.g., fibrinogen), C-reactive protein (CRP), among others.
Pathogen Testing
Serological tests aim to isolate viruses and measure antibody titers; pathogens typically evaluated include Mycoplasma, syphilis, herpes viruses, varicella-zoster virus, HIV, and others.
Vestibular and Balance Testing
For patients with accompanying vertigo requiring further diagnostic clarity and treatment, vestibular and balance function tests may be selected, depending on the specific clinical context. Common assessments include the Romberg test, positional nystagmus tests, caloric testing, fistula test, and videonystagmography (VNG). Tests such as cervical vestibular evoked myogenic potential (cVEMP), ocular vestibular evoked myogenic potential (oVEMP), and video head impulse test (vHIT) are also frequently used to assess vestibular function.
Classification
Sudden hearing loss is categorized into four types based on the frequency spectrum and degree of hearing loss: high-frequency loss, low-frequency loss, flat loss, and total deafness (including profound hearing loss).
Low-Frequency Loss
Hearing loss is confined to frequencies below or equal to 1,000 Hz, with a hearing threshold reduction of ≥20 dB HL at 250 Hz and 500 Hz.
High-Frequency Loss
Hearing loss is confined to frequencies above or equal to 2,000 Hz, with a hearing threshold reduction of ≥20 dB HL at 4,000 Hz and 8,000 Hz.
Flat Loss
Hearing loss is uniform across all frequencies. The average hearing threshold for frequencies between 250 Hz and 8,000 Hz (including 250 Hz, 500 Hz, 1,000 Hz, 2,000 Hz, 3,000 Hz, 4,000 Hz, and 8,000 Hz) is ≤80 dB HL.
Total Deafness
Hearing loss affects all frequencies. The average hearing threshold for the frequency range of 250 Hz to 8,000 Hz (including 250 Hz, 500 Hz, 1,000 Hz, 2,000 Hz, 3,000 Hz, 4,000 Hz, and 8,000 Hz) is ≥81 dB HL.
Diagnosis
The diagnostic process for sudden hearing loss requires the exclusion of life-threatening conditions such as stroke, nasopharyngeal carcinoma, and acoustic neuroma. In addition, it is essential to rule out common localized or systemic diseases, such as Meniere’s disease, various types of otitis media, and viral infections like mumps or herpes zoster oticus (Ramsay Hunt syndrome).
For bilateral sudden hearing loss, systemic conditions should be considered. Potential causes include immune-related diseases (e.g., autoimmune inner ear disease, Cogan syndrome), endocrine disorders (e.g., hypothyroidism), neurological conditions (e.g., intracranial space-occupying lesions, diffuse encephalitis, multiple sclerosis), infectious diseases (e.g., meningitis), hematologic conditions (e.g., polycythemia, leukemia, dehydration, sickle cell anemia), genetic disorders (e.g., enlarged vestibular aqueduct syndrome), trauma, drug toxicity, and noise-induced hearing loss.
Treatment
Sudden hearing loss should be treated as an otologic emergency, and clinical treatment plans should be comprehensively formulated to begin therapy as early as possible during the hearing loss. The choice of treatment should align with the potential underlying causes of the condition, and different drug combinations may be used accordingly. For patients with comorbidities such as hypertension, hyperlipidemia, or diabetes, internal medicine interventions should be incorporated to address the underlying conditions. For cases primarily related to blood flow disturbances, treatment focuses on vasodilation, reducing blood viscosity, and increasing oxygen partial pressure. Since most patients present with unknown causes, treatment predominantly follows empirical strategies. Several common treatment methods are outlined as follows:
General Treatment
Adequate rest, appropriate sedation, and a low-sodium diet may help alleviate potential endolymphatic hydrops. Symptomatic relief is indicated during acute episodes of vertigo. Management of underlying conditions, such as diabetes and hypertension, should be actively pursued.
Glucocorticoids
Glucocorticoids are considered first-line therapies for sudden hearing loss. Their effects include anti-inflammatory, antiviral, and immunosuppressive properties, which can reduce vascular endothelial edema and improve inner ear blood supply. Prednisone is commonly employed, with an adult dosage of 1 mg/kg daily (typically not exceeding 60 mg/kg). Monitoring for glucocorticoid-related side effects is critical during treatment. Key principles of glucocorticoid use include:
- Patients should not have contraindications to glucocorticoid use.
- Administration is often in a single dose in the morning.
- Dosage may be adjusted based on individual patient conditions.
Glucocorticoids can be administered via oral intake, intravenous injection, or intratympanic injection. Intratympanic steroid injection may be used as first-line therapy or as a salvage treatment, particularly for patients with systemic conditions or contraindications to oral steroids, as well as for treatment-resistant cases or those with suboptimal outcomes using conventional therapies.
Vasodilators
This category includes various drugs, such as calcium channel blockers, histamine derivatives, and traditional Chinese medicine formulations that promote blood circulation and remove stasis.
Thrombolytic and Anticoagulant Therapy
Altered hemorheology in patients with sudden hearing loss includes elevated plasma fibrinogen levels, increased erythrocyte aggregation, and higher plasma viscosity compared to control groups. These findings suggest a role for plasma fibrinogen in the pathogenesis of sudden hearing loss. Regular monitoring of coagulation function is necessary when using fibrinogen-lowering drugs, with treatment adjusted according to test results. Thrombolytic and anticoagulant drugs are contraindicated in patients with bleeding disorders, severe liver or kidney dysfunction, or hypertension, and their efficacy in this context lacks robust evidence from evidence-based medicine.
Blood Viscosity Reduction
Agents like low-molecular-weight dextran or colloidal solutions such as hydroxyethyl starch can reduce blood viscosity, improve erythrocytosis, and enhance capillary circulation.
Antiviral Therapies
Antiviral drugs may be considered in the presence of direct evidence of viral infection.
Hyperbaric Oxygen Therapy
Therapeutic effects are achieved by increasing oxygen partial pressure, enhancing tissue oxygenation, reducing edema, lowering blood viscosity, inhibiting viral activity, modulating immune responses, and suppressing excessive inflammatory reactions. The treatment aims to alleviate tissue hypoxia. Antioxidant agents may be co-administered during hyperbaric oxygen therapy to prevent damage caused by oxygen free radicals.
Additional Medications
Treatment may include the use of antioxidants, magnesium supplements, zinc supplements, and drugs that improve inner ear energy metabolism.
Symptomatic Treatment for Vertigo
For patients with vertigo, symptom-specific medications can be employed to control acute episodes and alleviate discomfort.
Psychological Interventions
Patients with sudden hearing loss and vertigo often experience anxiety. Appropriate counseling and psychological therapy can help relieve stress and fear, which may positively affect prognosis.
Alternative Therapies
For cases where hearing in the affected ear does not recover despite treatment, hearing aids may be considered. For patients with profound hearing loss, cochlear implantation can be evaluated after ruling out contraindications.