Can sudden sensorineural hearing loss (SSNHL) occur bilaterally, affecting both ears, or is it limited to one side?

Sudden Sensorineural Hearing Loss: Unilateral vs. Bilateral Presentation

Sudden sensorineural hearing loss (SSNHL) is predominantly unilateral, but bilateral presentation can occur and is rare—representing only 1.7% of all SSNHL cases—and when bilateral hearing loss occurs, it should immediately trigger an urgent search for serious underlying systemic diseases rather than being treated as idiopathic SSNHL. 1, 2, 3

Key Clinical Distinction

Most patients with idiopathic SSNHL have unilateral hearing loss; bilateral loss is rare and should prompt consideration of other causes. 1

• Idiopathic SSNHL is rarely bilateral or recurrent and is not associated with other focal neurologic symptoms or signs 1
• The sudden onset of bilateral sensorineural hearing loss is relatively rare and should raise concern for specific underlying causes 1
• Bilateral SSNHL has a fundamentally different disease profile than unilateral SSNHL, with higher morbidity and mortality (35% mortality rate) 3

When Bilateral SSNHL Occurs: Underlying Causes to Investigate

The presence of bilateral sudden hearing loss excludes idiopathic SSNHL by definition and mandates urgent investigation for life-threatening conditions. 1, 3

Infectious Causes

• Meningitis (infectious, inflammatory/autoimmune, or neoplastic) with headache, fever, other cranial nerve palsies, and abnormal cerebrospinal fluid 1
• Viral infections including herpes simplex virus, varicella zoster virus (Ramsay Hunt syndrome), HIV, and others 1
• Bacterial infections, mycoplasma, Lyme disease, tuberculosis, syphilis, and fungal infections 1

Autoimmune/Inflammatory Conditions

• Autoimmune inner ear disease, often with hearing fluctuation and vertigo 1
• Cogan's syndrome, which may show bilateral involvement at onset 1

Toxic Causes

• Ototoxic medications (aminoglycosides, cisplatin) with vestibular loss and oscillopsia 1
• Lead poisoning with learning disabilities and other stigmata 1

Traumatic Causes

• Significant head trauma with temporal bone fractures 1
• Barotrauma 1
• Recent acoustic trauma 1

Metabolic/Genetic Disorders

• MELAS (metabolic encephalopathy, lactic acidosis, and stroke-like episodes) with confusion, elevated serum lactic acid, MRI white matter changes 1
• Genetic disorders (syndromic or nonsyndromic) 1

Vascular Causes

• Bilateral SSNHL patients have higher incidence of vascular disease 2
• Stroke or structural lesions (though rarely presenting as isolated SSNHL) 1

Sequential/Metachronous Bilateral SSNHL

SSNHL can also present sequentially, affecting one ear first and then the contralateral ear at a separate time, though this remains uncommon. 4, 5

• Sequential bilateral SSNHL occurs in temporally discrete episodes (>6 months apart) affecting both ears 5
• The mean interval between episodes is 9.6 years (range 1-22 years) 5
• Sequential profound bilateral SSNHL may manifest without any specific warning signs, occurring as soon as 37 days after initial unilateral presentation 4
• Out of 558 SSNHL patients, only 11 (2%) had metachronous SSNHL 5

Clinical Characteristics of Bilateral vs. Unilateral SSNHL

Bilateral SSNHL patients differ significantly from unilateral cases in age, severity, and recovery patterns. 2, 3

• Bilateral SSNHL patients are older at onset and have a bimodal age distribution compared to unilateral cases 2, 3
• Bilateral cases typically present with more profound hearing loss 3
• Usually bilateral SSNHL is asymmetric rather than symmetric 2
• Vestibular symptoms are less common in bilateral SSNHL than in unilateral cases 3
• Recovery rate for bilateral SSNHL is 67% compared to 52% for unilateral cases, though this difference is not statistically significant 2
• In bilateral SSNHL patients showing improvement, both ears typically respond to treatment 2
• Bilateral SSNHL has poorer overall recovery and higher mortality (35%) compared to unilateral disease 3

Critical Management Algorithm

When encountering bilateral sudden hearing loss, immediately initiate the following urgent workup: 1, 6

1. Complete audiometry to confirm bilateral sensorineural hearing loss (≥30 dB loss at 3 consecutive frequencies) 1, 6

2. Dedicated MRI of internal auditory canals with high-resolution 3D T2 sequences (FIESTA/CISS) and T1 sequences with gadolinium contrast 6

   • MRI reveals abnormalities in 57% of SSNHL cases, with 11% directly related to hearing loss 6
   • Essential for detecting meningeal enhancement, eighth nerve inflammation, vestibular schwannoma, and other pathology 6

3. Comprehensive history and physical examination specifically assessing for: 1

   • Antecedent fluctuating hearing loss on one or both sides
   • Concurrent severe bilateral vestibular loss with oscillopsia
   • Focal neurologic symptoms (headache, confusion, diplopia, dysarthria, focal weakness, ataxia, facial weakness)
   • Recent head trauma, acoustic trauma, or barotrauma
   • Eye symptoms (pain, redness, lacrimation, photophobia)
   • Fever, headache, or signs of systemic illness

4. Laboratory workup including: 6, 2

   • Antinuclear antibody (ANA) titer (higher incidence of positive ANA in bilateral cases)
   • Syphilis serology (FTA-abs)
   • Consider lumbar puncture for CSF analysis if meningitis suspected

5. Initiate treatment with systemic corticosteroids immediately if within 2 weeks of onset, while completing diagnostic workup 6, 2

Critical Pitfalls to Avoid

Never assume bilateral sudden hearing loss is idiopathic—this is a medical emergency requiring urgent investigation for life-threatening conditions. 1, 3

• Do not treat bilateral SSNHL with corticosteroids alone without comprehensive workup for underlying systemic disease 7
• Do not fail to warn patients with unilateral SSNHL about the rare possibility of subsequent contralateral involvement 4
• Do not order routine CT of the head—MRI with dedicated IAC protocol is the appropriate imaging modality 1, 6
• Recognize that bilateral SSNHL has a 35% mortality rate and requires urgent, thorough investigation 3