Disorders of Hearing¶
Chapter 36 | Part 2: Cardinal Manifestations and Presentation of Diseases · Part 2 – Cardinal Manifestations & Presentation · Chapter 36
Key Clinical Points¶
- Conductive hearing loss (CHL) results from middle/outer ear obstructions; sensorineural hearing loss (SNHL) results from inner ear or auditory nerve damage.
- Presbycusis is the most common cause of SNHL in adults, characterized by symmetric, high-frequency loss.
- Ménière's disease requires episodic vertigo; its absence suggests cochlear hydrops instead.
- Noise-induced hearing loss typically presents with a 'noise notch' at 3000–4000 Hz.
- GJB2 (Connexin 26) mutations account for nearly 20% of childhood deafness cases.
- Tympanometry results: Type A (normal), Type B (effusion), Type C (negative pressure/Eustachian tube dysfunction).
- Rinne test: Positive (AC > BC) in normal/SNHL; Negative (BC > AC) in CHL.
- Weber test: Lateralizes to affected ear in CHL; lateralizes to unaffected ear in SNHL.
- Hidden hearing loss (cochlear synaptopathy) involves loss of auditory synapses despite normal pure tone audiometry.
- Cochlear implants are used for severe-to-profound SNHL where traditional hearing aids are insufficient.
DEFINITION & CLASSIFICATION¶
• General Overview: Hearing loss is one of the most common sensory disorders; ~10% of adults have some hearing loss. _One-third of individuals aged >65 years require hearing aids.
• Classification by Anatomical Site (Table 1 & Table 2): Conductive Hearing Loss: • Definition: Lesions in the auricle, external auditory canal, or middle ear that impede sound transmission. • Causes: Acute otitis media, cerumen impaction, cholesteatoma, Eustachian tube dysfunction, ossicular fixation/discontinuity, otosclerosis, and tympanic membrane perforation. Sensorineural Hearing Loss: • Definition: Lesions impairing mechanotransduction in the inner ear or transmission along the eighth nerve. • Causes: CNS infections (meningitis), tumors (vestibular schwannoma), endolymphatic hydrops (Ménière's), noise-induced, presbycusis, and stroke. Mixed Hearing Loss: • Definition: Combination of conductive and sensorineural components. • Causes: Otosclerosis, cholesteatoma, head trauma, or middle ear tumors.
Physiology of Hearing¶
• Mechanism: Sound → Tympanic membrane → Ossicular chain (malleus, incus, stapes) → Oval window → Cochlea. • Impedance Matching: _The eardrum and ossicles boost sound energy nearly 200-fold to overcome the transition from air to fluid. _Without this mechanism, ~99.9% of acoustical energy would be reflected.
• Cochlear Mechanics: Inner hair cells: Primarily afferent innervation. _Outer hair cells: Approximately 20,000 (vs. 3,500 inner) provide cochlear amplification and frequency selectivity. _Basilar Membrane: High-frequency tones peak near the base; low-frequency sounds peak toward the apex.
• Central Processing: Frequency specificity is maintained through the auditory pathway to the cortex. _Speech Processing: Lateralized (Left hemisphere: 95–98% of right-handers, 70–80% of left-handers). Right Ear Advantage: Exists for dichotic listening tasks in many individuals.
EPIDEMIOLOGY¶
• Prevalence: _Nearly 10% of the adult population has some hearing loss. _One-third of adults >65 years require hearing aids.
• Presbycusis: _Most common cause of SNHL in adults. _Affects over half of adults aged >75 years. _Characterized by symmetric, high-frequency loss (Figure 2)."
ETIOLOGY & PATHOPHYSIOLOGY¶
• Genetic Causes of Hearing Loss: _Hereditary Hearing Impairment (HHI) affects >50% of children. _Nonsyndromic HHI: ~2/3 are nonsyndromic; 70–80% are autosomal recessive (DFNB); 15–20% are autosomal dominant (DFNA). _Syndromic HHI: Over 400 forms, including Usher's (retinitis pigmentosa), Waardenburg's (pigmentary abnormality), and Alport's (renal disease).
• GJB2 (Connexin 26): _Responsible for nearly 20% of all cases of childhood deafness. _Two mutations (35delG, 167delT) account for >50% of cases; full gene sequencing is required for diagnosis.
• Noise-Induced Hearing Loss: Acute exposure → hair cell injury/death → 'noise notch' at 3000–4000 Hz. _Hidden Hearing Loss (cochlear synaptopathy): Loss of auditory synapses on hair cells; results in difficulty hearing in background noise despite normal pure tone audiometry. _Prevention: Use of earplugs or earmuffs is recommended to avoid acoustic trauma."
CLINICAL FEATURES¶
• General Symptoms: _Reduced hearing, poor sound localization, and difficulty in background noise. _Tinnitus: Perception of sound without external source; pulsatile tinnitus requires vascular evaluation. _Aural Fullness: Often associated with Ménière's or Eustachian tube dysfunction.
• Ménière's Disease: Definition (Harrison's 22e): "episodic vertigo, fluctuating sensorineural hearing loss, tinnitus, and aural fullness". _Diagnostic Note: Absence of vertigo is inconsistent with Ménière's; suggests cochlear hydrops. Progression: Tinnitus and fullness may be absent initially but appear as the disease worsens.
• Specific Conditions: Vestibular Schwannoma: Asymmetric hearing loss, tinnitus, and imbalance (rarely vertigo). Larger tumors may cause facial or trigeminal nerve palsy. _Sudden Unilateral Loss: May indicate viral infection, vestibular schwannoma, or stroke.
• Patient Indicators (Table 2): _Saying 'huh' frequently; turning up volume on TV/radio; avoiding social settings; family history of hearing loss."
DIAGNOSTIC APPROACH¶
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Audiologic Assessment: • Perform Rinne test to differentiate conduction: _Positive (AC > BC) → Normal or SNHL. _Negative (BC > AC) → Conductive loss. • Perform Weber test to determine lateralization: _Lateralizes to affected ear → Conductive loss. _Lateralizes to unaffected ear → Sensorineural loss.
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Tympanometry: • Assess middle ear compliance and pressure: _Type A → Normal middle ear compliance. _Type B → Indicates effusion. _Type C → Indicates negative pressure (Eustachian tube dysfunction).
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Audiogram Analysis: _Evaluate for 'noise notch' at 3000–4000 Hz to identify noise-induced damage. _Assess high-frequency loss to determine presbycusis or other SNHL causes (Figure 2)."
MANAGEMENT & TREATMENT¶
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Ménière's Disease Management: • Identify and treat symptoms of vertigo, tinnitus, and fullness.
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Otosclerosis Management: • Perform stapedectomy to address conductive loss from fixed stapes (Figure 3). Procedure: Place a piston prosthesis from the incus to the stapes footplate.
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Cholesteatoma Management: • Surgical intervention for bone destruction or middle ear masses.
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Cerumen Impaction Management: • Removal of wax to resolve conductive hearing loss.
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Severe Sensorineural Hearing Loss: • Provide hearing aids to improve signal-to-noise ratio. Advanced Case: Provide cochlear implants (Figure 4) for patients with minimal residual hearing.
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Noise Prevention: • Recommend earplugs or earmuffs to prevent 'hidden' and overt hearing loss."
PROGNOSIS & COMPLICATIONS¶
• Complications of Hearing Loss: Elderly patients: Increased risk of cognitive decline, falls, and decreased quality of life. _Social Impact: Patients may avoid social settings due to difficulty in background noise. _Safety: Risk of injury from reduced awareness of environmental sounds (e.g., traffic)."
KEY PEARLS & HIGH-YIELD POINTS¶
• Clinical Pearls: _Ménière's Diagnosis: Must have vertigo; no vertigo → think cochlear hydrops. _Tympanometry Quick-Ref: A=Normal, B=Fluid (Effusion), C=Pressure (Eustachian tube). _Noise Notch: 3000–4000 Hz is the hallmark of acoustic trauma. _GJB2: Key gene for ~20% of childhood deafness; requires full sequence analysis. Hearing Aids: Improve signal-to-noise ratio, which is critical for cognitive health in the elderly. _Environmental Noise (Table 3):* _Normal conversation: 55–65 dB. _OSHA limit: 90 dB. _Jet engine at 100 ft: 140 dB. _Loudest possible sound: 194 dB."
Reference Tables¶
TABLE 36-1 Common Causes of Conductive, Mixed, and Sensorineural Hearing Loss Conductive Hearing Loss Acute otitis…¶
Harrison's 22e, p.246
| Conductive Hearing Loss | ||||||
|---|---|---|---|---|---|---|
| Acute otitis media | ||||||
| Cerumen impaction | ||||||
| Cholesteatoma | ||||||
| Eustachian tube dysfunction | ||||||
| Inner ear dehiscence or “third window” | ||||||
| Middle ear tumors (facial nerve tumors, glomus tympanicum, glomus jugulare) | ||||||
| Ossicular discontinuity | ||||||
| Ossicular fixation | ||||||
| Otosclerosis | ||||||
| Serous otitis media | ||||||
| Temporal bone trauma | ||||||
| Tympanic membrane abnormalities | ||||||
| Tympanic membrane perforation | ||||||
| Mixed Hearing Loss | ||||||
| Sensorineural Hearing Loss | ||||||
| CNS infection (meningitis) | ||||||
| CNS tumors | ||||||
| Cerebellopontine angle tumors (vestibular schwannoma, meningioma) | ||||||
| Endolymphatic hydrops (Ménière’s disease) | ||||||
| Endolymphatic sac tumor | ||||||
| Inner ear malformation | ||||||
| Perilymphatic fistula | ||||||
| Labyrinthitis | ||||||
| Multiple sclerosis | ||||||
| Noise-induced hearing loss | ||||||
| Presbycusis | ||||||
| Radiation therapy | ||||||
| Sudden hearing loss | ||||||
| Stroke | ||||||
| Temporal bone or head trauma | ||||||
| Left | ||||||
| Right | Left | |||||
| 5 | 0 dB SRT | 55 dB | ||||
| 64% Disc | . 70% |
TABLE 36-2 Signs and Symptoms Suggestive of Hearing Loss Saying “huh” a great deal Reduced clarity of hearing…¶
Harrison's 22e, p.248
- Saying “huh” a great deal
- Reduced clarity of hearing
- Difficulty understanding conversations in background noise
- Family complaining of hearing loss
- Tinnitus
- Turning the volume up on radio or television
- Sensitivity to noises
- Fullness in the ear
- Avoiding social settings
TABLE 36-3 Decibel (Loudness) Level of Common Environmental Noise SOURCE Weakest sound heard Whisper Normal…¶
Harrison's 22e, p.251
| SOURCE | DECIBEL (dB) |
|---|---|
| Weakest sound heard | 0 |
| Normal conversation | 55–65 |
| OSHA Monitoring Requirement Begins | 90 |
| Subway train at 200 ft | 95 |
| Power saw | 110 |
| Jet engine at 100 ft | 140 |
| Loudest sound that can occur | 194 |