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🦻 HEARING LOSS — Complete MBBS ENT Guide

Based on Dhingra & Dhingra's Diseases of Ear, Nose and Throat + Logan Turner

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PART 1: CLASSIFICATION OF HEARING LOSS

1. Definition

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2. Introduction

• Hearing loss is one of the most common sensory disabilities worldwide.
• It is broadly classified based on the site of lesion in the auditory pathway.
• Understanding the type helps in diagnosis, audiological testing, and management.

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3. Classification

A. Based on Site of Lesion

B. Based on Degree (WHO Grading)

C. Based on Time of Onset

• Pre-lingual: Before development of speech (< 2 years)
• Post-lingual: After development of speech (> 2 years)

D. Based on Duration

• Sudden: Within 72 hours
• Gradual (Progressive): Slow onset over months/years

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4. Important Viva Points

• Rinne test - Negative in CHL; Positive in SNHL
• Weber test - Lateralizes to affected ear in CHL; to better ear in SNHL
• Absolute Bone Conduction (ABC) test - Reduced in SNHL
• Audiogram: CHL shows air-bone gap (>10 dB); SNHL shows no air-bone gap

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5. Important Exam Points

• Distinguish between conductive, sensorineural, and mixed hearing loss using tuning fork tests
• Degree of hearing loss classification is frequently asked in university exams

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🧠 Mnemonic for Classification

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PART 2: CONDUCTIVE HEARING LOSS (CHL)

1. Definition

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2. Introduction

• Sound normally travels: Pinna → External Auditory Canal (EAC) → Tympanic Membrane (TM) → Ossicles (Malleus-Incus-Stapes) → Oval Window → Cochlea
• Any obstruction or disruption in this pathway causes CHL.
• Maximum possible CHL = 60 dB (air-bone gap cannot exceed this)

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3. Etiology (Causes)

A. External Ear Causes

• Wax (cerumen) impaction - Most common cause overall
• Foreign body in EAC
• Otitis externa (swelling of EAC)
• Atresia of EAC (congenital absence)
• Exostoses / osteoma of EAC

B. Middle Ear Causes

• Acute Otitis Media (AOM) - Fluid/pus in middle ear
• Otitis Media with Effusion (OME / Glue ear) - Thick fluid in middle ear; most common cause of CHL in children
• Chronic Suppurative Otitis Media (CSOM) - Perforation of TM + discharge
• Tympanic membrane perforation (trauma, infection)
• Otosclerosis - Fixation of stapes footplate; most common cause of CHL with normal-looking TM in adults
• Cholesteatoma - Erosion of ossicles
• Ossicular discontinuity - Trauma or infection (most commonly necrosis of long process of incus)
• Tympanosclerosis - Calcium deposits in TM or middle ear
• Eustachian tube dysfunction
• Hemotympanum - Blood in middle ear after head trauma

C. Congenital Causes

• Congenital ossicular fixation
• Middle ear aplasia

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4. Audiological Features of CHL

• Pure Tone Audiogram: Air conduction (AC) thresholds elevated; Bone conduction (BC) thresholds normal → Air-bone gap (ABG) > 10 dB
• Speech Discrimination Score (SDS): Usually good (inner ear intact)
• Tympanometry: Flat (Type B) in middle ear effusion; Type As in otosclerosis
• Rinne: Negative (BC > AC) in CHL
• Weber: Lateralizes to affected ear

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5. Tuning Fork Tests Summary

CHL:
  Rinne   → NEGATIVE (BC > AC) in affected ear
  Weber   → Lateralizes to WORSE (affected) ear
  ABC     → Normal (inner ear OK)
  Schwabach → Normal

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6. Management of CHL

A. Medical Management

B. Surgical Management

C. Hearing Rehabilitation

• Hearing aids (if surgery not suitable)
• Bone Anchored Hearing Aid (BAHA) - for atresia, single-sided deafness
• Bone conduction hearing aid - for those who cannot use conventional aids

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7. Clinical Significance

• CHL is potentially treatable/reversible in most cases
• Otosclerosis is autosomal dominant, more common in women, worsens in pregnancy
• Carhart's notch = dip at 2000 Hz on BC audiogram in otosclerosis (not a true SNHL)

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8. Important Viva Points

• Maximum air-bone gap = 60 dB
• Surgery for otosclerosis = Stapedectomy (Shea, 1958)
• Type of graft for myringoplasty = Temporalis fascia (most common)
• Grommet is a ventilation tube for OME
• Tympanoplasty types = Wullstein's classification (Type I-V)

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🧠 Mnemonic for Causes of CHL

• Wax, Atresia, Foreign body, External otitis
• AOM, OME, CSOM, Otosclerosis, STM perforation, Tympanosclerosis, Hemotympanum

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PART 3: SENSORINEURAL HEARING LOSS (SNHL)

1. Definition

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2. Types of SNHL

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3. Etiology (Causes)

A. Congenital

• Genetic/hereditary - 50% of congenital SNHL
  • Autosomal recessive (most common: Connexin 26 mutation, GJB2 gene)
  • Autosomal dominant
  • X-linked (Alport syndrome)
• Non-genetic congenital:
  • TORCH infections in utero: Toxoplasmosis, Others (syphilis), Rubella (most important), CMV, Herpes simplex
  • Rubella - classic cause; causes cochlear hair cell damage
  • Kernicterus (severe neonatal jaundice) - damages cochlear nuclei
  • Birth asphyxia

B. Acquired

Infective

• Viral labyrinthitis: Mumps, measles, CMV (most common viral causes)
• Bacterial meningitis: Most common cause of acquired SNHL in children (post-meningitic deafness)
• Syphilis (congenital/acquired)

Traumatic

• Noise-induced hearing loss (NIHL) - occupational/recreational
• Temporal bone fracture (transverse fracture - severe SNHL)
• Perilymph fistula

Ototoxic Drugs

• Aminoglycosides, cisplatin, loop diuretics, quinine, salicylates (see Part 5)

Vascular

• Sudden SNHL - idiopathic (viral theory, vascular theory)
• Stroke, vertebrobasilar insufficiency

Degenerative

• Presbycusis - age-related SNHL (see Part 6)

Neoplastic

• Acoustic neuroma (Vestibular Schwannoma) - unilateral SNHL + tinnitus + vertigo (CPA tumor)

Autoimmune

• Autoimmune inner ear disease - bilateral fluctuating SNHL

Metabolic

• Hypothyroidism, diabetes mellitus, chronic renal failure

Meniere's Disease

• Endolymphatic hydrops → triad: Fluctuating SNHL + Tinnitus + Episodic vertigo

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4. Audiological Features of SNHL

• Audiogram: Both AC and BC elevated; No air-bone gap
• Speech Discrimination Score (SDS): Reduced (hair cells/nerve damaged)
• Recruitment phenomenon: Present in cochlear (sensory) SNHL
  • SISI (Short Increment Sensitivity Index) test: >70% = cochlear lesion
• Tympanogram: Normal (Type A) — middle ear is normal
• OAE (Otoacoustic Emissions): Absent in cochlear SNHL
• BERA (Brainstem Evoked Response Audiometry): Gold standard for neural SNHL

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5. Tuning Fork Tests in SNHL

SNHL:
  Rinne   → POSITIVE (AC > BC) [but both reduced]
  Weber   → Lateralizes to BETTER ear
  ABC     → Reduced (inner ear damaged)
  Schwabach → Reduced

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6. Management of SNHL

A. Medical Treatment

B. Hearing Rehabilitation

• Hearing aids (most widely used) - for mild to severe SNHL
  • Behind-the-ear (BTE), In-the-ear (ITE), etc.
• Cochlear implants - for profound bilateral SNHL (>90 dB) where hearing aids fail
  • Criteria: Bilateral profound SNHL, no benefit from hearing aids, patent cochlea
• Auditory Brainstem Implant (ABI) - for NF2 (bilateral acoustic neuromas)

C. Surgical Treatment

• Acoustic neuroma: Surgery (translabyrinthine, middle fossa, retrosigmoid approach) or radiotherapy (Gamma Knife)
• Cholesterol granuloma, labyrinthitis: Treat underlying cause

D. Prevention

• Avoid ototoxic drugs if possible; monitor drug levels
• Hearing protection in noisy environments
• Vaccination (rubella, mumps, measles)
• Regular audiometry in patients on ototoxic drugs

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7. Clinical Significance

• SNHL is generally irreversible (hair cells cannot regenerate in mammals)
• Recruitment = abnormal loudness growth; pathognomonic of cochlear lesion
• Retrocochlear lesion = Acoustic neuroma; diagnosed by BERA (prolonged wave I-V interval) and MRI

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8. Common Student Mistakes

• Confusing "positive Rinne" in SNHL with normal hearing (both have positive Rinne - but in SNHL, bone conduction is also reduced)
• Saying Weber lateralizes to the affected ear in SNHL (it goes to the BETTER ear in SNHL)

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🧠 Mnemonic for Causes of SNHL

• Vascular (sudden SNHL, stroke)
• Infective (mumps, meningitis, rubella, syphilis)
• Traumatic (noise, fracture)
• Autoimmune / Aging (presbycusis)
• Metabolic (hypothyroid, DM, renal failure)
• Inherited/Congenital (genetic, TORCH)
• Neoplastic (acoustic neuroma)
• Chemical (ototoxic drugs)
• Degenerative (Meniere's)

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PART 4: NOISE-INDUCED HEARING LOSS (NIHL)

1. Definition

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2. Introduction

• Hazardous noise is any sound > 85 dB for sustained periods
• Most common occupational hazard causing hearing loss
• Affects outer hair cells of the basal turn of cochlea (high-frequency region) first

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3. Classification of NIHL

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4. Pathology / Mechanism

• Intense noise → mechanical disruption and metabolic exhaustion of outer hair cells
• Free radical formation (reactive oxygen species) → hair cell apoptosis and necrosis
• Outer hair cells (OHC) are damaged first → high-frequency hearing affected first
• Basal turn of cochlea (detects high frequencies 3000-6000 Hz) is most vulnerable

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5. Characteristic Audiogram Finding

• 4000 Hz notch = Classic, pathognomonic finding on pure tone audiogram
• Initially: notch at 3000-6000 Hz (most pronounced at 4000 Hz)
• With continued exposure: notch deepens and spreads to adjacent frequencies

          dB Hearing Level
  0  |__________________________
  20 |      _____|_____
  40 |           \   /\
  60 |            \ /  \
  80 |             V    \       ← Notch at 4000 Hz
 100 |                   \
      500  1k  2k  4k  6k  8k Hz

Key point: The notch is at 4000 Hz, NOT at the frequency of the offending noise. This is because the cochlear region responding to 4000 Hz lacks adequate protection (farthest from oval window, poor blood supply, relatively less supported mechanically).

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6. Features

• Bilateral and symmetrical (usually)
• Tinnitus commonly accompanies NIHL
• Speech discrimination affected late in the disease
• Asymmetrical NIHL in shooters: ear opposite to shoulder that holds the gun is worse
• Loudest occupational noise: Mining > Metal work > Textile industry

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7. Risk Factors

• High sound level (dB SPL)
• Long duration of exposure
• Spectral composition of noise (high-frequency noise is more damaging)
• Individual susceptibility (blue eyes = less melanin = more susceptible; genetic factors)
• Ototoxic drugs + noise = synergistic damage

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8. Prevention of NIHL (Hearing Conservation Program - 4 Steps)

1. Assess noise levels using sound level meter and dosimeter
2. Control noise at source (engineering controls: barriers, mufflers, ear muffs)
3. Ear protection devices: ear plugs, ear muffs if noise cannot be reduced
4. Audiometric monitoring - regular pure tone audiometry for workers

• OSHA safe limit: 90 dB for 8 hours/day; every 5 dB increase halves safe duration
  • 90 dB → 8 hrs; 95 dB → 4 hrs; 100 dB → 2 hrs; 105 dB → 1 hr; 110 dB → 30 min; 115 dB → 15 min

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9. Management

• No effective treatment for established NIHL (hair cells cannot regenerate)
• Hearing aids for rehabilitation
• Prevention is the only cure
• TTS: rest from noise; usually recovers in 16-48 hours
• Free radical scavengers (Vitamin E, N-acetylcysteine, Magnesium) - under research for prevention

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10. Clinical Significance

• Medicolegal importance: Occupational deafness → compensation claims
• NIHL does NOT cause hearing loss beyond 40 dB in speech frequencies (until late)
• Most important diagnostic point: 4000 Hz notch on audiogram
• Acoustic trauma: TM perforation, ossicular chain disruption, SNHL

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🧠 Mnemonic

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PART 5: OTOTOXIC DRUGS

1. Definition

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2. Introduction

• Ototoxic drugs damage outer hair cells of the cochlea and/or the vestibular hair cells
• Mechanism: Free radical formation → oxidative stress → hair cell apoptosis/necrosis
• Cochleotoxic = mainly affects hearing; Vestibulotoxic = mainly affects balance
• Mitochondrial mutation A1555G → predisposes to aminoglycoside ototoxicity

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3. Classification and Examples

A. Aminoglycoside Antibiotics (Most Important)

Memory: "SNG" are vestibulotoxic (Streptomycin, Neomycin-less, Gentamicin) "ATNK" are cochleotoxic (Amikacin, Tobramycin, Neomycin, Kanamycin)

B. Antineoplastic (Chemotherapy) Drugs

• Cisplatin - most ototoxic chemotherapy agent; causes bilateral, high-frequency SNHL; cochleotoxic
• Carboplatin
• Bleomycin (less common)

C. Loop Diuretics

• Furosemide (Frusemide) - dose-dependent, often reversible cochleotoxicity
• Ethacrynic acid - most ototoxic diuretic; can cause permanent SNHL
• Bumetanide

Key: Ethacrynic acid + aminoglycoside together = severe, irreversible SNHL

D. Quinine and Antimalarials

• Quinine - tinnitus, high-frequency SNHL (often reversible at therapeutic doses)
• Chloroquine - less common

E. Salicylates

• Aspirin (high doses) - tinnitus and reversible SNHL
• Effect is dose-dependent and usually reversible on stopping the drug
• Tinnitus appears at serum levels > 20-40 mg/dL

F. Other Ototoxic Agents

• Vancomycin (especially with aminoglycosides)
• Quinine
• Iodine compounds (topical)
• Industrial solvents (toluene, styrene, xylene)
• Nitrogen mustard
• Heavy metals (mercury, lead, arsenic)
• Desferrioxamine (deferoxamine)

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4. Risk Factors for Ototoxicity

• High dose and prolonged duration
• Renal impairment (reduced drug clearance → higher blood levels)
• Pre-existing SNHL
• Combination of ototoxic drugs (synergistic)
• Noise exposure + ototoxic drugs
• Genetic predisposition (mitochondrial A1555G mutation)
• Extremes of age (elderly and neonates more susceptible)

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5. Clinical Features

• Tinnitus - often the first symptom (warning sign)
• High-frequency SNHL (bilateral, symmetrical usually)
• Vestibular toxicity: oscillopsia, disequilibrium, unsteady gait
• Symptoms may appear or worsen even AFTER stopping the drug (aminoglycosides)

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6. Monitoring and Prevention

• Serial audiometry (baseline + regular PTA during treatment)
• Monitor serum drug levels (aminoglycosides: trough < 2 mg/L; peak < 10 mg/L)
• Renal function tests - impaired kidneys → drug accumulation
• Use alternative drugs when possible
• Avoid combination of two ototoxic drugs
• Aspirin may protect against gentamicin ototoxicity (paradoxically)

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7. Management

• Stop the offending drug immediately when ototoxicity suspected
• Hearing rehabilitation: hearing aids, cochlear implants for severe loss
• No proven reversal agent available
• Vestibular rehabilitation exercises for vestibulotoxicity

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8. Important Viva Points

• Most ototoxic aminoglycoside: Neomycin (cochleotoxic)
• Most vestibulotoxic: Streptomycin and Gentamicin
• Most ototoxic diuretic: Ethacrynic acid
• Aspirin ototoxicity is reversible; aminoglycoside ototoxicity is irreversible
• Synergistic ototoxicity: Furosemide + aminoglycosides

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🧠 Mnemonic: Ototoxic Drugs

• Aminoglycosides (Gentamicin, Neomycin, Streptomycin, Amikacin)
• Loop diuretics (Furosemide, Ethacrynic acid)
• Oncology drugs (Cisplatin, Carboplatin)
• Quinine/Antimalarials
• Salicylates (Aspirin)
• Antibiotics (Vancomycin)
• Various chemicals (Solvents, heavy metals)
• Etc (Nitrogen mustard, Deferoxamine)

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PART 6: PRESBYCUSIS

1. Definition

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2. Introduction

• Most common cause of SNHL in the elderly (> 60 years)
• Affects both sexes; slightly more common and earlier in men
• Gradual onset; progressive over years
• Speech discrimination is disproportionately reduced (worse than expected from pure tone audiogram alone)

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3. Classification (Schuknecht's Classification - 4 Types)

Strial presbycusis is the most common type (Schuknecht)

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4. Etiology / Risk Factors

• Age - primary factor; degeneration of hair cells is inevitable
• Genetic factors - family history of early hearing loss
• Noise exposure - cumulative lifetime noise accelerates presbycusis
• Ototoxic drug history
• Vascular factors - hypertension, diabetes, atherosclerosis → poor cochlear blood supply
• Smoking - increases risk
• Noise + aging = synergistic damage (socioacusis + presbycusis)

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5. Pathology

• Outer hair cells are lost first (basal turn = high-frequency area most affected)
• Progressive loss of spiral ganglion neurons
• Stria vascularis atrophy → reduced endocochlear potential
• Stiffening of basilar membrane → impaired frequency resolution
• Central auditory processing also degenerates with age

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6. Clinical Features

• Gradual, bilateral, symmetrical SNHL (usually starts after age 60)
• High-frequency hearing loss first (difficulty hearing consonants - s, f, sh, th)
• Tinnitus - commonly present (high-pitched, bilateral)
• Difficulty understanding speech, especially in noisy environments
• Patient hears sounds but cannot understand words = "I can hear but cannot understand"
• Diplacusis (sounds heard differently in both ears) may be present
• Recruitment may be present (cannot tolerate loud sounds)
• No vertigo (vestibular function relatively preserved)

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7. Audiogram Pattern in Presbycusis

dB Hearing Level
   0 |─────────────────────────────
  20 |────────────────\
  40 |                 \
  60 |                  \         ← High-frequency slope
  80 |                   \
 100 |
     250  500  1k  2k  4k  8k Hz

• High-frequency sloping pattern (downsloping audiogram)
• No air-bone gap (pure sensorineural)
• Both AC and BC curves slope down at high frequencies

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8. Investigations

• Pure Tone Audiogram (PTA) - First and essential investigation
• Speech Audiometry - Speech discrimination score reduced
• BERA - To rule out retrocochlear pathology
• OAE - Absent
• MRI brain/IAM - To exclude acoustic neuroma or CNS pathology if unilateral

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9. Management

A. Reassurance

• Reassure patient that hearing will NOT deteriorate to total deafness
• Explain the gradual, progressive nature

B. Hearing Aids

• Main treatment for presbycusis
• Hearing aid technology has improved dramatically
• BTE (Behind-the-ear) or digital hearing aids preferred
• Amplification is most helpful for high-frequency sounds
• Both ears should ideally be fitted (binaural fitting)

C. Cochlear Implants

• For profound presbycusis not helped by hearing aids
• Elderly patients can benefit from cochlear implants

D. Communication Strategies

• Face the patient when speaking
• Speak slowly and clearly; avoid background noise
• Lip-reading training / auditory rehabilitation

E. Associated Tinnitus

• Reassurance (tinnitus in presbycusis is usually not severe)
• Tinnitus maskers, hearing aids (masking effect)

F. Treat Underlying Risk Factors

• Control hypertension, diabetes
• Stop smoking

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10. Clinical Significance

• Presbycusis is a diagnosis of exclusion - rule out treatable causes (acoustic neuroma, hypothyroidism, Meniere's disease, ototoxic drugs)
• Most underdiagnosed condition in the elderly
• Social isolation, depression, and cognitive decline are associated with untreated presbycusis

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11. Important Viva Points

• Most common type: Strial (metabolic) presbycusis
• Characteristic: "I can hear but cannot understand"
• Audiogram: Downsloping, no ABG
• Main treatment: Hearing aid
• Schuknecht classified presbycusis into 4 types (Sensory, Neural, Metabolic/Strial, Mechanical)

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🧠 Mnemonic for Schuknecht's 4 Types

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⭐ HIGH-YIELD EXAM PEARLS

1. 4 kHz notch = NIHL (pathognomonic)
2. Negative Rinne = CHL (BC > AC); Positive Rinne can be normal OR SNHL
3. Weber lateralizes to worse ear = CHL; to better ear = SNHL
4. Most ototoxic diuretic = Ethacrynic acid; most ototoxic aminoglycoside = Neomycin
5. Most common cause of CHL in children = OME (Glue Ear); in adults with normal TM = Otosclerosis
6. Carhart's notch = dip at 2000 Hz BC in otosclerosis (not true SNHL)
7. Cochlear implant = for bilateral profound SNHL >90 dB where hearing aids fail
8. Presbycusis = strial type is most common (Schuknecht); treat with hearing aids
9. Sudden SNHL treatment = high-dose oral/intratympanic steroids
10. Rubella = most important congenital cause of SNHL (TORCH infections)
11. Otosclerosis = autosomal dominant; stapedectomy is treatment; Carhart's notch
12. Acoustic neuroma = unilateral SNHL + tinnitus; poor speech discrimination; BERA shows prolonged I-V latency; MRI is diagnostic

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🎯 ONE-PAGE QUICK REVISION NOTES

• Types: CHL / SNHL / Mixed / Central / Functional
• CHL: ABG present; Rinne -ve; Weber → bad ear; SDS good
• SNHL: No ABG; Rinne +ve; Weber → good ear; SDS poor

• Neo = most cochleotoxic; Gent/Strep = most vestibulotoxic; Ethacrynic acid = most ototoxic diuretic

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🧠 MEMORY TRICKS

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❓ TOP 5 VIVA QUESTIONS WITH ANSWERS

A. 4000 Hz notch on pure tone audiogram - a dip in hearing threshold specifically at 4000 Hz, representing damage to outer hair cells in the basal turn of the cochlea.

A. Schuknecht's 4 types: (1) Sensory - OHC loss at basal turn; (2) Neural - spiral ganglion cell loss; (3) Metabolic/Strial - stria vascularis atrophy (most common); (4) Mechanical - basilar membrane stiffening. All produce downsloping high-frequency SNHL on audiogram.

A. Cochleotoxic: Neomycin (most), Amikacin, Kanamycin, Cisplatin, Ethacrynic acid, Furosemide, Aspirin. Vestibulotoxic: Streptomycin (most), Gentamicin. Both: Tobramycin.

A. CHL: Rinne negative (BC>AC), Weber to worse ear, ABC normal. SNHL: Rinne positive (AC>BC, but both reduced), Weber to better ear, ABC reduced.

A. Carhart's notch is a mechanical (not sensorineural) dip in bone conduction threshold at 2000 Hz seen in otosclerosis, due to fixation of stapes altering the mechanical resonance of the ossicular chain. It disappears after stapedectomy. It is a mechanical artifact, NOT true SNHL. Its clinical significance: (1) Confirms diagnosis of otosclerosis; (2) Corrects after successful stapedectomy.

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