What is the initial investigation for a patient presenting with bilateral Sudden Sensorineural Hearing Loss (SSNHL)?

Bilateral SSNHL Investigation Protocol

Bilateral sudden sensorineural hearing loss demands immediate comprehensive workup including MRI with dedicated internal auditory canal protocol, extensive laboratory testing for infectious and autoimmune causes, and consideration of lumbar puncture—this is NOT idiopathic SSNHL until proven otherwise. 1, 2

Critical First Principle

Bilateral SSNHL represents only 1.7% of all SSNHL cases and should trigger urgent investigation for life-threatening systemic diseases. 1 Unlike unilateral idiopathic SSNHL, bilateral presentation is rarely idiopathic and carries a 44% mortality rate within 5 years, indicating more sinister underlying pathology. 3

Immediate Audiometric Confirmation

• Obtain complete audiometry with pure-tone air and bone conduction testing within 14 days of symptom onset to confirm ≥30 dB hearing loss at 3 consecutive frequencies bilaterally. 4, 2
• Include speech recognition threshold (SRT) and word recognition score (WRS) in the initial assessment. 5

Essential Imaging Studies

MRI (Mandatory)

• Order MRI of brain and internal auditory canals with dedicated IAC protocol immediately. 2 This detects vestibular schwannomas, meningeal enhancement suggesting meningitis, brainstem stroke, and structural lesions. 1, 2
• MRI identifies the cause in 69% of bilateral SSNHL cases. 3

CT Scanning (Contraindicated)

• Do NOT order routine CT of the head—this is a strong recommendation against based on systematic reviews. 4, 2

Comprehensive Laboratory Workup

Infectious Disease Panel

• Syphilis serology (mandatory for all bilateral SSNHL). 2
• HIV testing to exclude immunocompromised states. 2
• Viral serologies including herpes simplex virus, varicella zoster virus. 1, 2
• Lyme disease testing in endemic areas. 2
• Tuberculosis and fungal studies if clinical suspicion exists. 2
• Blood cultures if fever or systemic symptoms present—endocarditis can cause bilateral SSNHL. 6

Autoimmune/Inflammatory Workup

• Antinuclear antibodies (ANA) titer for autoimmune inner ear disease and Cogan's syndrome. 1, 2
• Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) for inflammatory conditions. 2
• Angiotensin-converting enzyme (ACE) level for sarcoidosis. 2
• Antiphospholipid antibodies for hypercoagulable states. 2

Metabolic and Hematologic Studies

• Complete blood count with differential to assess for hematologic malignancy or infection. 2
• Serum protein electrophoresis for hyperviscosity syndromes. 2
• Lactate level to screen for MELAS (mitochondrial encephalomyopathy). 1, 2
• Lead level for toxic exposure. 1, 2
• Thyroid function tests based on clinical context. 2
• Coagulation studies if vascular etiology suspected. 2

Lumbar Puncture Indications

Perform lumbar puncture with CSF analysis if:

• Meningitis is suspected clinically (headache, fever, altered mental status). 2
• MRI shows meningeal enhancement. 2
• No other cause identified and clinical suspicion remains high for infectious or inflammatory meningitis. 1, 3

Additional Vestibular Testing

• Caloric testing shows abnormalities in 81% of bilateral SSNHL cases, indicating both cochlear and vestibular involvement. 3
• Cervical and ocular VEMP testing demonstrates 100% abnormality rates in bilateral cases, confirming extensive inner ear pathology. 3

Critical Historical Red Flags to Elicit

• Recent COVID-19 infection or symptoms—bilateral SSNHL occurs more frequently in unvaccinated patients with severe COVID-19. 5, 7
• Antecedent fluctuating hearing loss suggesting Ménière's disease or autoimmune inner ear disease. 4
• Focal neurologic symptoms: headache, confusion, diplopia, dysarthria, focal weakness, ataxia, facial weakness indicating brainstem stroke or meningitis. 4, 1
• Recent head trauma, acoustic trauma, or barotrauma. 4, 1
• Concurrent eye symptoms: pain, redness, photophobia suggesting Cogan's syndrome. 4, 8
• Ototoxic medication exposure: aminoglycosides, cisplatin. 1, 8
• Constitutional symptoms: fever, weight loss, night sweats suggesting systemic infection or malignancy. 6

Common Pitfalls to Avoid

• Never assume bilateral SSNHL is idiopathic—this requires proof of exclusion of all other causes. 1
• Do not treat with corticosteroids alone without comprehensive workup—while steroids may be initiated within 2 weeks of onset, they must not delay or replace diagnostic investigation. 1, 2
• Do not obtain routine laboratory tests for unilateral SSNHL, but bilateral presentation mandates extensive testing—this distinction is critical. 4, 2

Underlying Etiologies by Frequency

Based on research evidence, causes of bilateral SSNHL include:

• Neoplasm (31% in one series): meningeal carcinomatosis, lymphoma. 3
• Stroke (31%): brainstem infarction affecting bilateral auditory pathways. 3
• Meningitis (6%): bacterial, viral, fungal, or inflammatory. 3
• Autoimmune: Cogan's syndrome, autoimmune inner ear disease. 1, 8
• Infectious: endocarditis, COVID-19, syphilis. 6, 5, 7
• Unknown/idiopathic (31%)—but only after exhaustive workup. 3