Sudden Sensorineural Hearing Loss — Recognition and Workup

Sudden sensorineural hearing loss is the otological emergency. A patient who wakes up deaf in one ear, or who notices their hearing dropping over the course of a day, needs to be seen urgently — not reassured that it will clear up, not given decongestants, not told to come back in two weeks. The window for treatment (if corticosteroids are indicated) is days to weeks from onset; delayed presentation significantly worsens outcomes. Every clinician from GP to emergency physician needs to recognise this presentation and act on it.

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Definition

The AAO-HNS clinical practice guideline defines sudden sensorineural hearing loss as:

≥30 dB sensorineural hearing loss across ≥3 consecutive audiometric test frequencies occurring within 72 hours

The 72-hour criterion distinguishes sudden from rapidly progressive hearing loss. The ≥30 dB threshold ensures that minor fluctuations and measurement variability are excluded. SSNHL is unilateral in the vast majority of cases; bilateral simultaneous SSNHL is a distinct and more alarming entity requiring investigation for autoimmune, vascular, or haematological causes.

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Epidemiology

• Incidence: approximately 5–30 per 100,000 per year (wide variation due to case ascertainment)
• Any age, but peak incidence in the 50s
• No consistent sex predominance
• Idiopathic (ISSNHL) in approximately 85–90% of cases — no cause found despite investigation
• A secondary cause is found in approximately 10–15%: these include acoustic neuroma, multiple sclerosis, autoimmune inner ear disease, Lyme disease, syphilis, lymphoma, mumps, vestibular neuritis

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Aetiology — What Is Causing It?

In most patients, no cause is identified — hence “idiopathic” SSNHL. The proposed mechanisms include:

Viral: The most accepted hypothesis. Direct viral cochleitis (herpes simplex virus type 1 reactivation in the spiral ganglion, similar to Bell’s palsy mechanism; mumps; cytomegalovirus) or post-viral immune-mediated cochlear damage. Evidence is circumstantial — viral titres and temporal bone studies suggest association but causation is not established.

Vascular: Ischaemia of the labyrinthine artery (a terminal branch with no collateral supply). The cochlea is exquisitely sensitive to ischaemia — even brief interruption of its blood supply causes rapid hair cell death. Microvascular events, vasospasm, or thromboembolism may all play a role.

Autoimmune: Antibodies against inner ear antigens (particularly the 68 kDa antigen, now identified as HSP70) cause cochlear damage. This mechanism is more likely in bilateral or rapidly progressive presentations.

Rupture of labyrinthine membranes: A Reissner’s membrane or round window membrane rupture can produce sudden hearing loss — more likely in the context of exertion, Valsalva, or barotrauma (air travel, diving).

In practice, the distinction matters because it influences investigation — though most patients receive empirical corticosteroid treatment regardless of the presumed cause.

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Red Flags — When SSNHL May Not Be Idiopathic

The following features should lower the threshold for MRI and extended workup:

• Unilateral tinnitus or hearing loss that preceded the acute event — acoustic neuroma classically produces unilateral progressive SNHL before the sudden “drop” event
• Bilateral simultaneous sudden hearing loss — autoimmune, vasculitis, haematological cause
• Associated neurological symptoms (diplopia, ataxia, dysarthria, facial numbness) — central cause (stroke, MS, CPA tumour)
• Young patient or known autoimmune disease — autoimmune inner ear disease
• Recent viral illness (mumps, measles, varicella) — direct viral cochleitis
• Travel history or tick exposure — Lyme disease (Borrelia)
• Sexual history / high-risk behaviour — syphilis (otosyphilis can mimic ISSNHL)
• Occupational noise or barotrauma history — perilymph fistula

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Investigation

Audiogram — Urgent

The first investigation is urgent pure-tone audiometry to confirm and quantify the loss. Without an audiogram, the diagnosis cannot be made. If a patient presents describing sudden hearing loss and has a normal audiogram, the diagnosis is excluded.

The audiometric pattern in ISSNHL can be:

• Flat — all frequencies equally affected
• High-frequency — high-frequency loss with relatively preserved lows (most common)
• Low-frequency — low-frequency predominance (similar to early Menière’s; some authorities regard single-episode low-frequency SSNHL as a forme fruste of Menière’s)
• Total (coffinlid) pattern — profound loss across all frequencies; worst prognosis

The depth and pattern of loss at presentation are prognostic: deeper losses have lower spontaneous recovery rates.

MRI Internal Auditory Canals

MRI with gadolinium of the internal auditory canals (IAC) should be performed in all patients with SSNHL to exclude acoustic neuroma (vestibular schwannoma). Acoustic neuromas present as unilateral SNHL (often sudden) in approximately 10% of cases. The MRI may be deferred until after initial treatment is started, but should not be omitted.

Bloods

Basic workup: FBC, ESR, CRP, fasting glucose, lipid profile, VDRL/RPR (syphilis), and in the appropriate clinical context, ANA, ANCA, anti-cochlear antibodies (HSP70), Lyme serology.

Tympanometry

To confirm that there is no conductive component (Type A expected; Type B would indicate middle ear effusion or perforation as an alternative or contributing diagnosis).

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Prognosis

Spontaneous recovery occurs in approximately 45–65% of patients (variable across studies). Recovery is more likely if:

• Loss is mild-moderate rather than severe-profound
• Low-frequency pattern (better prognosis than high-frequency or flat)
• Younger patient
• Treatment started early (within 2 weeks of onset)
• Absence of vertigo (presence of vertigo associated with worse prognosis — suggests greater damage to the labyrinth)
• Absence of diabetes, cardiovascular disease

Complete recovery means return to within 10–15 dB of the pre-morbid threshold. Partial recovery is more common. Approximately 15–20% have minimal recovery.

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Key Numbers

Parameter	Value
Definition: threshold	≥30 dB SNHL across ≥3 consecutive frequencies
Definition: onset window	Within 72 hours
Idiopathic proportion	~85–90%
Spontaneous recovery rate	~45–65%
Acoustic neuroma causing SSNHL	~1–3% of SSNHL presentations
Bilateral simultaneous SSNHL	Rare; warrants urgent systemic workup
Treatment window	Days to weeks from onset
Tinnitus association	~85% of SSNHL cases
Vertigo association	~30%; associated with worse prognosis

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Frequently Asked Questions

Is sudden hearing loss ever conductive rather than sensorineural? Yes — sudden hearing loss should always be assessed with otoscopy and tuning forks before confirming sensorineural. A large wax bolus suddenly lodging against the drum, acute otitis media, a spontaneous haemotympanum, or a traumatic drum perforation can all produce sudden hearing loss. These are conductive, not sensorineural — and are managed very differently. Otoscopy and Weber/Rinne tests will distinguish: true SSNHL has a normal drum, normal tympanogram, no air-bone gap on audiometry, and Weber lateralising to the better ear.

Why is an acoustic neuroma (vestibular schwannoma) the most important differential? Acoustic neuromas can produce sudden hearing loss as their presenting symptom in approximately 10% of cases — usually from sudden haemorrhage into the tumour or temporary loss of blood supply to the cochlea. If an acoustic neuroma is missed and treated empirically as ISSNHL, the patient recovers (partially), is discharged, and the tumour continues to grow — potentially causing progressive hearing loss, tinnitus, facial nerve involvement, and eventually brainstem compression. MRI is therefore not optional.

Why does the prognosis worsen with delayed treatment? Hair cell death following the ischaemic or inflammatory insult is progressive — the window during which surviving but dysfunctional cells can be rescued with corticosteroids is finite. Once hair cells die, no treatment can restore them. The evidence for corticosteroids in ISSNHL is that they improve recovery rates when given early; their benefit beyond 4–6 weeks is minimal. Patients who present late — often because they assumed the hearing loss would “clear up on its own,” or because an earlier clinician was not alarmed — have significantly lower odds of recovery.