What causes sudden sensorineural hearing loss?

Causes of Sudden Sensorineural Hearing Loss

In 85-90% of cases, sudden sensorineural hearing loss (SSNHL) is idiopathic at presentation, with presumptive attribution to vascular, viral, or multiple etiologies despite thorough evaluation. 1, 2

Primary Causes of SSNHL

Idiopathic (Most Common)

• Accounts for 85-90% of cases at initial presentation 1
• Diagnosis of exclusion after ruling out identifiable causes
• May be attributed to:
  • Vascular mechanisms
  • Viral infections
  • Multiple or combined etiologies

Identifiable Causes (10-15% of cases)

Vascular Causes

• Stroke (particularly vertebrobasilar territory)
  • Risk of stroke at 5 years is 1.64 times higher in SSNHL patients 2
  • Requires immediate evaluation when suspected
• Vascular occlusion of the internal auditory artery
• Hyperviscosity syndromes

Infectious/Inflammatory

• Viral infections (commonly implicated)
• Meningitis (infectious, inflammatory, neoplastic) 1
• Lyme disease (associated with erythema chronicum migrans, abnormal CSF) 1
• Syphilis (abnormal FTA-abs test, fluctuating bilateral hearing loss) 1
• COVID-19 infection (emerging evidence) 3

Autoimmune

• Autoimmune inner ear disease (may have fluctuating hearing) 1
• Cogan syndrome 1

Neurological/Neoplastic

• Vestibular schwannoma (acoustic neuroma) 2
• Other cerebellopontine angle tumors

Other Specific Causes

• Ménière's disease (most common cause of fluctuating unilateral hearing loss) 2
• Ototoxic medications 1
• Trauma (head trauma, barotrauma, temporal bone fractures) 1
• Herpes zoster oticus (Ramsay-Hunt syndrome) 1
• Genetic disorders (may be syndromic or nonsyndromic) 1
• Metabolic disorders (e.g., MELAS syndrome) 1
• Substance abuse (e.g., heroin injection) 4
• Vaccine-related (rare reports following COVID-19 vaccination) 5

Warning Signs Requiring Urgent Evaluation

Bilateral Sudden Hearing Loss

• Rare and concerning presentation
• Associated with specific causes requiring immediate attention:
  • Meningitis
  • Autoimmune disorders
  • Ototoxic medication exposure
  • Systemic infections
  • Stroke (requires immediate workup) 2

Neurological Red Flags

• Focal weakness
• Dysarthria
• Hemiataxia
• Encephalopathy
• Severe headaches
• Diplopia
• Downbeating or gaze-evoked nystagmus 1

Diagnostic Approach

1. Identify timing of onset (within 72 hours defines SSNHL)
2. Confirm sensorineural nature with audiometric testing (30-dB loss at 3 consecutive frequencies)
3. Assess for bilateral involvement, recurrent episodes, or focal neurologic findings
4. MRI is preferred when evaluating for retrocochlear pathology 2
5. Avoid routine CT scans of the head/brain in initial evaluation 2

Clinical Pearls and Pitfalls

• Long-term follow-up is essential as some underlying causes may only become evident later 1, 2
• The primary presenting symptom is often a "full or blocked ear" which may delay diagnosis due to its non-specific nature 1
• History of fluctuating hearing suggests non-idiopathic causes such as Ménière's disease 1, 2
• Approximately half of patients recover spontaneously, usually within about 2 weeks 6
• Immediate steroids offer the best chance of hearing recovery when indicated 3

Despite extensive research, the exact pathophysiological mechanisms of idiopathic SSNHL remain incompletely understood, with histopathologic studies showing various patterns of cochlear injury including atrophy of the organ of Corti, loss of cochlear neurons, and degeneration of various inner ear structures 7.