Sensorineural Hearing Loss

Sensorineural hearing loss (SNHL) results from a lesion in the cochlea (sensory) or the cochlear nerve and central auditory pathway (neural). It is the most heavily examined hearing-loss topic in NEET PG because it ties together audiology, ototoxic pharmacology, Meniere disease and acoustic neuroma into one cluster.

Definition & basic classification

Hearing loss is broadly conductive (outer/middle ear, sound conduction problem) or sensorineural (cochlea or VIII nerve, perceptive problem); a combination is mixed. SNHL itself is split anatomically into:

• Sensory (cochlear) — hair cells / organ of Corti. Examples: presbycusis, noise-induced, ototoxic, Meniere disease.
• Neural (retrocochlear) — spiral ganglion, VIII nerve, brainstem. Example: acoustic neuroma (vestibular schwannoma), auditory neuropathy.
• Central — cortical/subcortical pathway lesions.

High-yield: The single most important clinical question in SNHL is cochlear vs retrocochlear, because retrocochlear lesions (acoustic neuroma) need imaging and are surgical. Recruitment, SISI, tone decay and reflex decay are the differentiating tests.

Feature	Conductive HL	Sensorineural HL
Site	External/middle ear	Cochlea / VIII nerve
Rinne	Negative (BC > AC)	Positive (AC > BC)
Weber	Lateralises to diseased ear	Lateralises to healthy/better ear
Absolute Bone Conduction	Normal	Reduced
Audiogram	Air–bone gap present	AC = BC, no gap, sloping curve
Max loss	~60 dB	Can be total
Speech discrimination	Good once audible	Often poor (esp. retrocochlear)

Tuning fork tests — the bedside backbone

Always use a 512 Hz fork (256 Hz over-feels vibration, 1024 Hz decays fast).

• Rinne test: AC vs BC. Positive (AC > BC) = normal or SNHL. Negative (BC > AC) = conductive loss >15–20 dB. False-negative Rinne = severe unilateral SNHL ("dead ear") where the patient hears BC in the opposite cochlea — a classic exam trap.
• Weber test: Lateralises to the better-hearing ear in SNHL and to the worse ear in conductive loss.
• Absolute Bone Conduction (ABC): Examiner's (normal) BC vs patient's BC. Reduced ABC = cochlear reserve loss = SNHL.

Approach to a "deaf ear" with negative Rinne: Rinne negative → mask the other ear → if still negative = true conductive; if becomes positive = false-negative = severe SNHL of the test ear.

Etiology & pathophysiology

Cochlear (sensory) causes

• Presbycusis — age-related, bilateral, symmetrical, high-frequency sloping loss with poor speech discrimination (especially in noise — "I hear but don't understand"). Schuknecht types: sensory, neural, strial (metabolic), cochlear conductive.
• Noise-induced (NIHL) — chronic loud exposure. Characteristic 4000 Hz notch (acoustic dip / "Carhart notch" is for otosclerosis; do not confuse). Hair cells of basal turn damaged first. Initially a temporary threshold shift, later permanent.
• Ototoxic drugs — see dedicated section.
• Meniere disease — endolymphatic hydrops, fluctuating low-frequency SNHL.
• Sudden SNHL, infections (mumps, measles, congenital rubella, CMV, meningitis), autoimmune inner ear disease, head trauma/labyrinthine concussion, perilymph fistula.

Retrocochlear (neural) causes — acoustic neuroma, neurofibromatosis-2 (bilateral vestibular schwannomas), meningioma, auditory neuropathy spectrum disorder.

High-yield: Presbycusis = high-frequency loss; Meniere = low-frequency loss (early, fluctuating); NIHL = 4 kHz notch. These three audiogram shapes are repeatedly tested.

Audiogram interpretation

Pure-tone audiometry (PTA) plots threshold (dB HL) vs frequency (250–8000 Hz) for AC and BC.

dB HL (0.5/1/2 kHz avg)	Grade of impairment
≤25	Normal
26–40	Mild
41–55	Moderate
56–70	Moderately severe
71–90	Severe
>90	Profound

• SNHL: AC and BC overlap (no air–bone gap), curve slopes down toward high frequencies.
• Conductive: BC normal, AC depressed → air–bone gap.
• Carhart's notch (BC dip at 2000 Hz) → otosclerosis (a conductive condition, but classically tested alongside).

Special audiological tests — cochlear vs retrocochlear

These four are the highest-yield differentiators.

Test	Cochlear lesion	Retrocochlear lesion
Recruitment (loudness balance / ABLB)	Present (abnormal growth of loudness)	Absent
SISI (Short Increment Sensitivity Index)	High score (70–100%)	Low score (0–20%)
Tone decay (Carhart)	Minimal (<20 dB)	Marked (>30 dB) — abnormal adaptation
Acoustic (stapedial) reflex decay	Normal	Positive decay (reflex fades)
Speech discrimination score (SDS)	Moderately reduced	Disproportionately poor / rollover

High-yield: Recruitment positive + high SISI + speech discrimination roughly matching PTA = cochlear. Tone decay + reflex decay + rollover (SDS worse than PTA predicts) = retrocochlear (acoustic neuroma).

Speech audiometry terms

• Speech Reception Threshold (SRT): lowest dB at which 50% of spondee words are repeated; correlates with PTA.
• Speech Discrimination Score (SDS): % of phonetically balanced words correctly repeated at comfortable loudness. Rollover phenomenon (SDS falls as intensity rises) strongly suggests a retrocochlear lesion.

Objective tests

• Otoacoustic emissions (OAE): test outer hair cell function; absent in cochlear SNHL and useful for newborn screening. Present OAE with absent ABR = auditory neuropathy.
• BERA / ABR (Brainstem Evoked Response Audiometry): clicks evoke waves I–V.
  • Mnemonic for generators — "E COLI": Eighth nerve (I), Cochlear nucleus (II/III region), Olivary (superior olive, III), Lateral lemniscus (IV), Inferior colliculus (V).
  • Wave V is the most robust and used for threshold estimation in infants/malingerers.
  • Acoustic neuroma: prolonged wave I–V interval, increased interaural wave V latency difference (>0.2 ms) — classic finding.
• Tympanometry: normal Type A in SNHL (middle ear is healthy); helps exclude conductive overlap.

High-yield: MRI with gadolinium of the internal acoustic meatus (IAM) is the investigation of choice for acoustic neuroma (detects intracanalicular tumours BERA may miss). BERA is the best screening test, MRI is the gold standard.

Ototoxic drugs — a NEET PG mainstay

Ototoxicity damages cochlear (outer hair cells, basal turn first → high-frequency loss first) and/or vestibular structures.

Drug class / drug	Cochleotoxic	Vestibulotoxic	Reversible?
Streptomycin, Gentamicin	+	+++ (vestibular predominant)	Usually permanent
Amikacin, Kanamycin, Neomycin, Tobramycin	+++ (cochlear predominant)	+	Permanent
Cisplatin	+++ (dose-related, high-freq)	+	Permanent
Loop diuretics (furosemide, ethacrynic acid)	+ (stria vascularis)	+	Reversible
Salicylates / aspirin	+ (tinnitus, reversible)	–	Reversible
Quinine	+ (cinchonism)	–	Reversible
Erythromycin, Vancomycin	+	–	Often reversible

Key facts:

• Aminoglycosides cause irreversible ototoxicity by destroying hair cells (and accumulate in endolymph; risk rises with renal failure and high trough levels).
  • Most vestibulotoxic: streptomycin, gentamicin.
  • Most cochleotoxic: neomycin, amikacin, kanamycin.
  • Maternal aminoglycoside / mitochondrial A1555G mutation → susceptibility to deafness with even single dose.
• Cisplatin ototoxicity is dose-dependent, cumulative, bilateral and permanent; high-frequency first. Monitor with serial audiometry. Amifostine/sodium thiosulfate are protective.
• Loop diuretics act on the stria vascularis; toxicity is usually reversible but potentiates aminoglycoside damage (synergy).
• Salicylates/quinine → reversible tinnitus and SNHL.

High-yield mnemonic — ototoxic drugs: "A VANCE Loop Quietly Saves Cis" → Aminoglycosides, VANComycin, Erythromycin, Loop diuretics, Quinine, Salicylates, Cisplatin. For aminoglycosides: "Streptomycin & Gentamicin spin (vestibular); Neomycin, Amikacin, Kanamycin deafen (cochlear)."

Meniere disease (idiopathic endolymphatic hydrops)

A cochlear SNHL classically asked as a clinical vignette.

• Pathology: distension of the endolymphatic system (endolymphatic hydrops) due to over-production or defective absorption of endolymph.
• Classic tetrad: (1) episodic vertigo (20 min–24 h), (2) fluctuating low-frequency SNHL, (3) tinnitus, (4) aural fullness.
• Early audiogram: rising (low-frequency) SNHL; later flat. Recruitment present, SISI high (cochlear).
• Electrocochleography: raised SP/AP (summating/action potential) ratio >0.4–0.5.
• Glycerol test: transient hearing improvement after osmotic dehydration supports hydrops.

Management flow: Lifestyle (low-salt diet, caffeine/alcohol/stress avoidance) → medical: betahistine, diuretics (thiazide/acetazolamide), vestibular sedatives for acute attack (prochlorperazine) → intratympanic steroids → intratympanic gentamicin (chemical labyrinthectomy) for intractable cases with serviceable hearing loss → surgery (endolymphatic sac decompression, vestibular neurectomy preserving hearing, or labyrinthectomy if hearing already lost).

High-yield: Acute Meniere attack vertigo lasts minutes to hours (vs seconds in BPPV, days in vestibular neuritis). Hearing loss is low-frequency and fluctuating early — the differentiating feature.

Acoustic neuroma (vestibular schwannoma)

• Benign Schwann-cell tumour of the superior vestibular division of CN VIII, arising at the IAM. Most common cerebellopontine angle (CPA) tumour.
• Bilateral acoustic neuromas = Neurofibromatosis type 2 (NF2).
• Symptoms (order): progressive unilateral SNHL + tinnitus → disequilibrium → CN V (corneal reflex loss, facial numbness) → CN VII (late) → cerebellar/raised ICP.
• Disproportionately poor speech discrimination and rollover; positive tone decay and reflex decay.
• BERA: delayed wave V / interaural latency difference. MRI with gadolinium = investigation of choice.
• Management: observation (small, elderly), stereotactic radiosurgery (gamma knife), or microsurgical excision.

High-yield: Hitselberger sign = hypoaesthesia of the posterior-superior external auditory canal (CN VII sensory branch) in acoustic neuroma. Vertigo is mild/absent because the slow growth allows central compensation — a favourite distractor against vestibular neuritis.

Sudden Sensorineural Hearing Loss (SSNHL)

• Definition: ≥30 dB SNHL over ≥3 contiguous frequencies within 72 hours. An otological emergency.
• Mostly idiopathic (?viral, vascular, autoimmune). Must rule out acoustic neuroma (MRI) in unilateral cases.
• Management: early systemic +/- intratympanic corticosteroids (best within 2 weeks), supportive care; consider hyperbaric oxygen.

Congenital & paediatric SNHL

• Universal Newborn Hearing Screening: OAE then automated ABR; identify by 3 months, intervene by 6 months ("1-3-6 rule").
• TORCH (especially congenital CMV — leading non-genetic cause; rubella), genetic (Connexin-26 / GJB2 mutation — commonest non-syndromic), Pendred (SNHL + goitre, enlarged vestibular aqueduct), Waardenburg (white forelock, heterochromia), Usher (SNHL + retinitis pigmentosa), Jervell–Lange-Nielsen (SNHL + prolonged QT), Alport (SNHL + nephritis).

Management & rehabilitation of SNHL

Most SNHL is not curable; the focus is amplification and rehabilitation.

Stepwise approach: Identify & treat reversible causes (stop ototoxic drug, steroids in SSNHL/autoimmune) → hearing aids for residual hearing → cochlear implant when aids fail → auditory brainstem implant if cochlear nerve absent → aural rehabilitation/speech therapy.

• Hearing aids: for mild–severe SNHL with usable residual hearing; digital, behind-the-ear, in-the-canal.
• Cochlear implant (CI): for bilateral severe-to-profound SNHL not benefiting from aids. Electrode stimulates the spiral ganglion/cochlear nerve, bypassing dead hair cells. Best results when implanted early (ideally <2–3 years in prelingual deafness); prelingual implantation before language acquisition is critical.
• Bone-anchored hearing aid (BAHA): for single-sided deafness or conductive/mixed loss, not pure SNHL.
• Auditory brainstem implant: when cochlear nerve is absent (e.g., NF2 after tumour removal).

High-yield: Cochlear implant requires an intact cochlear nerve; in NF2 with bilateral nerve loss, an auditory brainstem implant is used instead. Prelingual children must be implanted early to develop speech.

Complications

• Permanent disability, impaired speech and language development in children, social isolation and depression in adults.
• Disabling tinnitus and vertigo (Meniere).
• In acoustic neuroma: facial palsy, brainstem compression, hydrocephalus, death if untreated.
• Ototoxicity: irreversible bilateral deafness and oscillopsia/ataxia (vestibular).

Key differentials

• SNHL vs conductive loss: Rinne/Weber, air–bone gap, ABC (see table).
• Cochlear vs retrocochlear: recruitment, SISI, tone decay, reflex decay, SDS/rollover, BERA, MRI.
• Vertigo causes: Meniere (mins–hrs + low-freq SNHL) vs BPPV (secs, positional, no HL) vs vestibular neuritis (days, no HL) vs acoustic neuroma (chronic unilateral SNHL, mild imbalance).
• Presbycusis vs NIHL: age vs exposure history; sloping high-freq vs 4 kHz notch.

Recently asked / exam angle

• Most vestibulotoxic aminoglycoside (streptomycin/gentamicin) vs most cochleotoxic (neomycin/amikacin/kanamycin) — recurring single-best-answer.
• Reversible vs irreversible ototoxicity: aminoglycosides & cisplatin = permanent; loop diuretics, salicylates, quinine = reversible.
• Audiogram-shape recognition: 4 kHz notch = NIHL; high-frequency slope = presbycusis; low-frequency = Meniere; 2 kHz Carhart notch = otosclerosis.
• Investigation of choice for acoustic neuroma = Gd-MRI of IAM; best objective screen = BERA (interaural wave V latency).
• OAE tests outer hair cells / newborn screening; present OAE + absent ABR = auditory neuropathy.
• SISI high & recruitment present = cochlear; tone decay & reflex decay = retrocochlear.
• Definition of SSNHL (≥30 dB over ≥3 frequencies within 72 h) and steroid treatment.
• Cochlear implant criteria and the need for an intact cochlear nerve; ABI in NF2.

Rapid revision

1. SNHL = Rinne positive, Weber to better ear, no air–bone gap, reduced ABC.
2. False-negative Rinne = severe unilateral SNHL (dead ear).
3. Presbycusis = bilateral high-frequency SNHL with poor speech discrimination in noise.
4. NIHL = bilateral 4000 Hz notch; basal-turn outer hair cell damage.
5. Meniere = endolymphatic hydrops; fluctuating low-frequency SNHL + vertigo (mins–hrs) + tinnitus + aural fullness; SP/AP ratio raised.
6. Recruitment + high SISI = cochlear; tone decay + reflex decay + rollover = retrocochlear.
7. OAE = outer hair cell function & newborn screening; BERA wave V = threshold & retrocochlear screen.
8. Acoustic neuroma: unilateral progressive SNHL + tinnitus, Hitselberger sign, Gd-MRI of IAM is investigation of choice; bilateral = NF2.
9. Aminoglycosides & cisplatin = irreversible; loop diuretics, salicylates, quinine = reversible ototoxicity.
10. Most vestibulotoxic = streptomycin/gentamicin; most cochleotoxic = neomycin/amikacin/kanamycin.
11. SSNHL = ≥30 dB over ≥3 frequencies in ≤72 h → treat early with steroids, rule out acoustic neuroma.
12. Cochlear implant needs an intact cochlear nerve and should be done early in prelingual deafness; ABI when the nerve is absent (NF2).