Disorders of Hearing¶
Chapter 36 | Part 2: Cardinal Manifestations and Presentation of Diseases
KEY CLINICAL POINTS¶
- Hearing loss affects nearly 10% of adults and one-third of individuals >65 years; it can be conductive (external/middle ear pathology), sensorineural (inner ear/neural), or mixed
- Presbycusis (age-related hearing loss) is the most common cause of sensorineural hearing loss in adults and is associated with increased cognitive decline, falls, and mortality
- Conductive hearing losses are generally surgically correctable, while sensorineural losses are managed with amplification devices or cochlear implants
- Genetic causes account for >50% of childhood hearing impairment, with GJB2 (connexin 26) mutations responsible for nearly 20% of all childhood deafness
- Noise-induced hearing loss is preventable through education and use of hearing protection; OSHA mandates hearing conservation programs at exposures ≥ 85 dB over 8 hours
1. PHYSIOLOGY OF HEARING¶
The function of the external and middle ear is to amplify sound and facilitate conversion of mechanical energy into electrical signals by inner-ear hair cells (mechanotransduction).
1.1 Sound Transmission Pathway¶
Sound waves enter the external auditory canal → set tympanic membrane (eardrum) in motion → moves malleus, incus, and stapes → movement of stapes footplate causes pressure changes in fluid-filled inner ear → traveling wave in basilar membrane of cochlea. The tympanic membrane and ossicular chain serve as an impedance-matching mechanism, boosting sound energy nearly 200-fold. Without this mechanism, 99.9% of acoustical energy would be reflected.
1.2 Cochlear Hair Cells¶
Two types of hair cells in the cochlea: inner and outer hair cells. Both are mechanoreceptors that detect mechanical energy and aid conversion to electrical signals. Inner hair cells: primarily afferent innervation (sensory). Outer hair cells: primarily efferent innervation, outnumber inner cells 6:1 (20,000 vs 3,500). Outer hair cell motility creates a 'cochlear amplifier' explaining the cochlea's exquisite sensitivity and frequency selectivity.
1.3 Mechanotransduction¶
Stereocilia of hair cells in organ of Corti rest on basilar membrane and contact tectorial membrane. Traveling wave deformation stretches tip links between stereocilia → ion channel opening → potassium influx → hair cell depolarization → neurotransmission. Frequency specificity: high-frequency tones cause maximal basilar membrane displacement near cochlear base; low-frequency sounds cause maximal displacement toward apex.
1.4 Central Auditory Pathway¶
Frequency specificity maintained throughout: dorsal and ventral cochlear nuclei → trapezoid body → superior olivary complex → lateral lemniscus → inferior colliculus → medial geniculate body → auditory cortex. Intensity encoded by neural activity in individual neurons, number of active neurons, and specific neurons activated. Speech processing is lateralized: left auditory cortex specializes in speech recognition/production; right hemisphere processes emotional and tonal aspects. Left hemisphere dominance for speech: 95-98% right-handed persons, 70-80% left-handed persons.
2. CLASSIFICATION OF HEARING LOSS¶
Hearing loss is classified by the anatomic site of pathology and mechanism of impairment.
Common Causes of Hearing Loss by Type¶
| Conductive Hearing Loss | Mixed Hearing Loss | Sensorineural Hearing Loss |
|---|---|---|
| Acute otitis media | Acute otitis media | CNS infection (meningitis) |
| Cerumen impaction | Cholesteatoma | CNS tumors |
| Cholesteatoma | Inner ear dehiscence/third window | Cerebellopontine angle tumors |
| Eustachian tube dysfunction | Inner ear malformation | Endolymphatic hydrops (Ménière's) |
| Inner ear dehiscence/third window | Middle ear tumors | Endolymphatic sac tumor |
| Middle ear tumors | Otosclerosis | Inner ear malformation |
| Ossicular discontinuity | Stapes gusher syndrome | Perilymphatic fistula |
| Ossicular fixation | Temporal bone trauma | Labyrinthitis |
| Otosclerosis | Multiple sclerosis | |
| Serous otitis media | Noise-induced hearing loss | |
| Temporal bone trauma | Presbycusis | |
| Tympanic membrane perforation | Radiation therapy | |
| Sudden hearing loss | ||
| Stroke | ||
| Temporal bone trauma |
2.1 Conductive Hearing Loss¶
Results from lesions in the auricle, external auditory canal, or middle ear that impede sound transmission from the external environment to the inner ear.
2.2 Sensorineural Hearing Loss¶
Results from lesions impairing mechanotransduction in the inner ear or transmission of the electrical signal along the eighth nerve to the brain.
2.3 Mixed Hearing Loss¶
Combination of conductive and sensorineural hearing loss from pathology affecting both middle and inner ear.
2.4 Central Auditory Processing Disorders (CAPD)¶
Disorders affecting the central auditory pathway from cochlea to auditory cortex, resulting in poor sound localization, lateralization, speech comprehension, and deterioration of auditory performance in background noise.
3. CONDUCTIVE HEARING LOSS - ETIOLOGY & PATHOPHYSIOLOGY¶
Factors that obstruct sound transmission or dampen acoustic energy result in conductive hearing loss.
3.1 External Ear Causes¶
Obstruction by cerumen, debris, foreign bodies; swelling of canal lining; atresia; neoplasms of the canal.
3.2 Tympanic Membrane Pathology¶
Perforations from trauma, acute otitis media (AOM), or chronic otitis media. Small perforations often heal spontaneously; larger defects require surgical intervention. Tympanoplasty is >90% effective for repair.
3.3 Middle Ear Pathology¶
Ossicular chain disruption (necrosis of long process of incus in trauma/infection); fluid, scarring, or neoplasms in middle ear. Eustachian tube dysfunction is extremely common and may predispose to AOM or serous otitis media (SOM). Balloon dilation can relieve acquired inflammatory obstruction.
3.4 Cholesteatoma¶
Benign tumor of stratified squamous epithelium in middle ear or mastoid. Slowly growing, destroys bone and normal ear tissue. Pathogenesis theories: traumatic immigration of squamous epithelium through retraction pocket, implantation through perforation/surgery, metaplasia from chronic infection. Signs: chronically draining ear unresponsive to antibiotics, perforation with cheesy white squamous debris, aural polyp, conductive hearing loss from ossicular erosion. CT shows bony destruction. Treatment: surgical removal and ossicular reconstruction.
3.5 Otosclerosis¶
Fixation of stapes causing low-frequency conductive hearing loss. Equal incidence in men and women. Inheritance: autosomal dominant with incomplete penetrance; may be manifestation of osteogenesis imperfecta. Onset: late teens to forties; accelerated during pregnancy in women. Treatment: hearing aid or stapedectomy (outpatient surgery). Cochlear otosclerosis (extension beyond stapes footplate) can cause mixed or sensorineural hearing loss; fluoride therapy is of uncertain value.
3.6 Third Window Pathology¶
Inner ear malformations creating a pathologic third window (in addition to normal oval and round windows) cause 'inner-ear conductive hearing loss' through acoustic energy dissipation. Causes include: Superior semicircular canal dehiscence syndrome (erosion of otic bone over superior semicircular canal), Large jugular bulb or jugular bulb diverticulum eroding into vestibular aqueduct or posterior semicircular canal, Lateral semicircular canal dysplasia, Large vestibular aqueduct, Incomplete partition (stapes gusher syndrome). Symptoms of superior semicircular canal dehiscence: conductive hearing loss mimicking otosclerosis, vertigo from loud sounds (Tullio phenomenon), vertigo from Valsalva maneuvers or tragus pressure, aural fullness, pulsatile tinnitus, hearing eye/neck movements. Diagnosis: low VEMP threshold, inner-ear erosion on CT. Treatment: surgical repair for recalcitrant vertigo/dizziness.
4. SENSORINEURAL HEARING LOSS - ETIOLOGY & PATHOPHYSIOLOGY¶
Results from damage to the mechanotransduction apparatus of the cochlea or disruption of electrical conduction from inner ear to brain.
4.1 Hair Cell Damage Causes¶
Intense noise exposure, viral infections, ototoxic drugs (salicylates, quinine and analogues, aminoglycoside antibiotics, loop diuretics [furosemide, ethacrynic acid], cisplatin and chemotherapeutic agents), temporal bone fractures, meningitis, cochlear otosclerosis, Ménière's disease, aging, congenital inner ear malformations, genetic predisposition.
4.2 Noise-Induced Hearing Loss¶
Acute or prolonged loud noise exposure causes temporary or permanent threshold shifts depending on intensity and duration. Permanent loss is due to hair cell injury/death. Characteristic 'noise notch' at 3000-4000 Hz on audiometry. Hidden hearing loss (cochlear synaptopathy): normal pure tone audiometry but complaints of difficulty hearing in background noise; due to loss of auditory synapses on hair cells. Prevention: earplugs or earmuffs essential.
4.3 Presbycusis (Age-Related Hearing Loss)¶
Most common cause of sensorineural hearing loss in adults. Affects >50% of adults aged >75 years in the US. Early stages: symmetric, gentle to sharply sloping high-frequency loss. Progression: involves all frequencies with significant loss of clarity. Features: loss of phoneme discrimination, recruitment (abnormal loudness growth), difficulty with speech in noise. Associated with: increased cognitive impairment, faster cognitive decline, increased falls, diminished quality of life, increased morbidity and mortality. Treatment: hearing aids (improve signal-to-noise ratio, shown to reduce cognitive decline and fall risk); cochlear implants when word recognition score <50%.
4.4 Ménière's Disease¶
Characterized by: episodic vertigo (essential for diagnosis), fluctuating sensorineural hearing loss, tinnitus, aural fullness. Note: Absence of vertigo with fluctuating hearing loss, tinnitus, and fullness suggests cochlear hydrops, not Ménière's disease. Epidemiology: annual incidence 0.5-7.5 per 1000; peak onset fifth decade. Histology: endolymphatic system distention (endolymphatic hydrops) with vestibular and cochlear hair cell degeneration. Etiology: endolymphatic sac dysfunction from infection, trauma, autoimmune disease, inflammatory causes, tumor, or idiopathic (most common). Hearing pattern: typically low-frequency unilateral sensorineural loss (though any pattern possible). Diagnosis: abnormal VEMP test may detect contralateral ear involvement; MRI to exclude retrocochlear pathology. Treatment: 2 g/day low-salt diet (mainstay); adjuncts include diuretics, short-course oral glucocorticoids, intratympanic glucocorticoids, intratympanic gentamicin. Surgical options for refractory cases: endolymphatic sac decompression, labyrinthectomy, vestibular nerve section (>90% success for vertigo). No effective therapy for hearing loss, tinnitus, or aural fullness.
4.5 Central Nervous System Causes¶
Neoplastic, vascular, demyelinating, infectious, degenerative diseases, or trauma affecting central auditory pathways. Characteristic: speech comprehension loss far exceeds pure tone loss. Audiologic findings: normal OAEs with abnormal ABR (auditory neuropathy pattern). Vestibular schwannoma: asymmetric sensorineural hearing loss with greater speech understanding deterioration than pure tone loss; may present with tinnitus, imbalance (rarely vertigo), cranial neuropathy (facial, trigeminal nerves with larger tumors). Multiple sclerosis: acute unilateral or bilateral hearing loss; pure tone stable but speech understanding fluctuates. Labyrinthine infarction: acute hearing loss and vertigo from posterior circulation stroke (usually anterior inferior cerebellar artery); may herald basilar artery infarction. HIV: can produce both peripheral and central auditory pathology.
4.6 Trauma¶
Temporal bone fractures may cause conductive, sensorineural, or mixed hearing loss. Fracture sparing inner ear: conductive loss from tympanic membrane rupture or ossicular disruption (surgically correctable). Fracture involving inner ear: profound hearing loss and severe vertigo; perilymphatic fistula may occur requiring surgical repair. Associated facial nerve injury common. CT: best for assessing temporal bone fracture, ear canal, ossicular chain, inner ear involvement. CSF leaks usually self-limited; prophylactic antibiotics of uncertain value.
5. CENTRAL AUDITORY PROCESSING DISORDERS (CAPD)¶
Following sound reception by inner ear hair cells, the central auditory pathway refines, analyzes, modifies, organizes, and interprets peripheral auditory input.
5.1 Causes¶
Loss of auditory nerve synapses from loud noise exposure, auditory/vestibular nerve tumors, CNS demyelinating disorders, brainstem diseases, cerebral stroke, aging.
5.2 Clinical Features¶
Poor sound localization, lateralization, and speech comprehension; deterioration of auditory performance in background noise. Poor understanding of speech in noise is a common elderly complaint (due to peripheral loss and central processing deterioration).
5.3 Treatment¶
Enhance signal-to-noise ratio through amplification of sounds of interest while reducing background noise. Auditory training and compensatory strategies to minimize CAPD impact.
6. TINNITUS¶
Perception of sound when no environmental sound exists. May be buzzing, roaring, ringing, or pulsatile (synchronous with heartbeat). Often associated with conductive or sensorineural hearing loss.
6.1 Etiology¶
Usually determined by finding the cause of associated hearing loss. May be first symptom of vestibular schwannoma. Pulsatile tinnitus requires vascular evaluation to exclude: vascular tumors (glomus jugulare), aneurysms, dural arteriovenous fistulas, stenotic arterial lesions. Pulsatile tinnitus may also occur with: SOM, superior semicircular dehiscence, inner-ear dehiscence. Most commonly due to jugular bulb abnormalities (large jugular bulb or diverticulum). In absence of MRA/MRV or CT angiography pathology: attributed to turbulent venous blood flow through transverse sinus, sigmoid sinus, and jugular bulb.
6.2 Management¶
Minimize caffeine, avoid high-dose NSAIDs, reduce stress. Masking with background music or white noise. Hearing aids help with tinnitus suppression. Tinnitus maskers: devices presenting a more pleasant sound than tinnitus; often followed by hours of tinnitus inhibition. Antidepressants may help patients cope.
7. GENETIC CAUSES OF HEARING LOSS¶
More than half of childhood hearing impairment is hereditary. Hereditary hearing impairment (HHI) can also manifest later in life.
Common Syndromic Forms of Hereditary Hearing Impairment¶
| Syndrome | Associated Features |
|---|---|
| Usher's syndrome | Retinitis pigmentosa and hearing loss |
| Waardenburg's syndrome | Pigmentary abnormality and hearing loss |
| Pendred's syndrome | Thyroid organification defect and hearing loss |
| Alport's syndrome | Renal disease and hearing loss |
| Jervell and Lange-Nielsen syndrome | Prolonged QT interval and hearing loss |
| Neurofibromatosis type 2 | Bilateral vestibular schwannomas |
| MELAS | Mitochondrial encephalopathy, lactic acidosis, stroke-like episodes |
| MERRF | Myoclonic epilepsy and ragged red fibers |
| PEO | Progressive external ophthalmoplegia |
7.1 Classification¶
Nonsyndromic HHI (~65%): hearing loss is the only abnormality. Syndromic HHI (~35%): hearing loss associated with anomalies in other organ systems. Nonsyndromic inheritance patterns: Autosomal recessive (DFNB): 70-80%, Autosomal dominant (DFNA): 15-20%, X-linked (DFNX): <5%, Mitochondrial: <5%.
7.2 Nonsyndromic HHI Genes¶
Over 150 loci mapped. Gene categories: structural proteins (MYH9, MYO7A, MYO15, TECTA, DIAPH1), transcription factors (POU3F4, POU4F3), ion channels (KCNQ4, SLC26A4), gap junction proteins (GJB2, GJB3, GJB6). Some genes (GJB2, TECTA, TMC1) cause both dominant and recessive forms. General pattern: Dominant genes: onset in adolescence/adulthood, variable severity, progressive. Recessive genes: congenital, profound.
7.3 GJB2 (Connexin 26)¶
Responsible for nearly 20% of all childhood deafness; half of genetic childhood deafness is GJB2-related. Two frameshift mutations (35delG, 167delT) account for >50% of cases, but full gene sequencing required. 167delT mutation highly prevalent in Ashkenazi Jews (~1 in 1765 homozygous and affected). Phenotype varies within families (modifier genes suspected). Digenic inheritance: single GJB2 mutation + single GJB6 (connexin 30) mutation can cause hearing loss.
7.4 Other Genetic Factors¶
Several nonsyndromic genes associated with progressive hearing loss. Genetics contribute to presbycusis (genetic susceptibility + environmental noise exposure). Aminoglycoside ototoxicity sensitivity: maternally transmitted mitochondrial mutation. Noise-induced hearing loss susceptibility may be genetically determined.
7.5 Syndromic Forms¶
Over 400 syndromic forms exist.
8. CLINICAL FEATURES & APPROACH TO THE PATIENT¶
Goals: determine (1) nature of hearing impairment (conductive vs sensorineural vs mixed), (2) severity (mild, moderate, severe, profound), (3) anatomy (external ear, middle ear, inner ear, central pathway), and (4) etiology.
Signs and Symptoms Suggestive of Hearing Loss¶
| Symptom/Sign |
|---|
| Saying 'huh' frequently |
| Reduced clarity of hearing |
| Difficulty understanding conversations in background noise |
| Family complaining of hearing loss |
| Tinnitus |
| Turning up radio or television volume |
| Sensitivity to noises |
| Fullness in the ear |
| Avoiding social settings |
8.1 History¶
Elicit: duration of deafness, unilateral vs bilateral, onset (sudden vs insidious), progression (rapid vs slow). Associated symptoms: tinnitus, vertigo, imbalance, aural fullness, otorrhea, headache, facial nerve dysfunction, head/neck paresthesias. Risk factors: head trauma, ototoxin exposure, occupational/recreational noise exposure, family history.
8.2 Symptom Patterns¶
Sudden unilateral hearing loss ± tinnitus: viral inner ear infection, vestibular schwannoma, or stroke. Unilateral hearing loss (sensory or conductive): reduced hearing, poor sound localization, difficulty in background noise. Gradual progression: otosclerosis, noise-induced hearing loss, vestibular schwannoma, Ménière's disease. Small vestibular schwannoma: asymmetric hearing loss, tinnitus, imbalance (rarely vertigo); larger tumors add cranial neuropathy. Ménière's disease: episodic vertigo, tinnitus, aural fullness. Superior semicircular canal dehiscence: sound-induced vertigo, autophony, hearing eye/neck movements. Hearing loss with otorrhea: chronic otitis media or cholesteatoma.
8.3 Physical Examination¶
Auricle, external ear canal, tympanic membrane examination. In elderly: use wall-mounted suction or cerumen loops (canal skin fragile); avoid irrigation. Tympanic membrane: topography more important than light reflex; examine pars tensa (lower 2/3) and pars flaccida (upper 1/3) for retraction pockets. Insufflation: assess tympanic membrane mobility and compliance. Inspect nose, nasopharynx, upper respiratory tract. Unilateral serous effusion or unexplained otalgia: fiberoptic examination of nasopharynx and larynx to exclude neoplasm. Cranial nerve evaluation: special attention to facial and trigeminal nerves (affected by cerebellopontine angle tumors).
8.4 Tuning Fork Tests (512 Hz)¶
Rinne test: compares air conduction to bone conduction. Normal/sensorineural loss: air conduction louder than bone conduction. Conductive loss ≥ 30 dB: bone conduction louder than air conduction. Weber test: stem placed on midline of head. Unilateral conductive loss: tone perceived in affected ear. Unilateral sensorineural loss: tone perceived in unaffected ear. Lateralization requires ≥ 5 dB difference between ears.
9. LABORATORY ASSESSMENT OF HEARING¶
Minimum audiologic assessment includes pure tone audiometry (air and bone conduction), speech reception threshold, word recognition score, tympanometry, acoustic reflexes, and acoustic-reflex decay.
9.1 Pure Tone Audiometry¶
Performed by audiologist in sound-attenuated chamber using audiometer. Tests frequencies 250-8000 Hz at specific intensities. Air-conduction thresholds: via headphones. Bone-conduction thresholds: vibrating tuning fork or oscillator on head. Masking: broad-spectrum noise to nontest ear when hearing loss present. Decibel (dB) = 20 × log(patient threshold pressure / normal threshold pressure). 6 dB change = doubling of sound pressure; 20 dB change = tenfold change. Loudness doubles with approximately each 10 dB increase.
9.2 Audiometric Patterns¶
Conductive loss (large mass component, e.g., effusion): elevated high-frequency thresholds. Conductive loss (stiffness component, e.g., early otosclerosis): elevated low-frequency thresholds. Sensorineural loss (presbycusis): higher frequencies affected more than lower. Ménière's disease: low-frequency sensorineural loss (though any frequency possible). Noise-induced: 'noise notch' at 3000-4000 Hz, greater than at higher frequencies. Vestibular schwannoma: characteristically high-frequency, but any pattern possible.
9.3 Speech Audiometry¶
Speech reception threshold (SRT): intensity at which 50% of two-syllable words are correctly repeated. Word recognition score (discrimination): percentage of phonetically balanced one-syllable words correctly repeated at 25-40 dB above SRT. Normal/conductive loss: 88-100% discrimination. Sensorineural loss: variable discrimination loss. Neural lesions: greater discrimination deficits than cochlear lesions. Vestibular schwannoma clue: greater than expected discrimination loss with mild asymmetric sensorineural loss. 'Rollover phenomenon' (deterioration at higher intensities): suggests eighth nerve or central pathology.
9.4 Tympanometry¶
Measures middle ear impedance to sound. Tympanogram: graphic representation of compliance change with pressure change. Type A: maximal compliance at atmospheric pressure, decreases with pressure change (normal or sensorineural loss). Type Ad: very high compliance peak (ossicular discontinuity). Type As: reduced compliance peak (otosclerosis). Type B: compliance unchanged with pressure (middle ear effusion). Type C: maximal compliance with negative ear canal pressure (Eustachian tube obstruction).
9.5 Acoustic Reflex¶
Intense tone elicits stapedius muscle contraction; compliance change detected during tympanometry. Helps determine hearing loss etiology and localize facial nerve paralysis. Differentiates otosclerosis (absent reflex) from inner-ear third window (present reflex). Normal/elevated thresholds with sensorineural loss: suggests cochlear origin. Absent reflex with sensorineural loss: not helpful for localization. Acoustic reflex decay: helps differentiate sensory from neural loss; adapts with time in neural loss (e.g., vestibular schwannoma).
9.6 Otoacoustic Emissions (OAEs)¶
Generated by outer hair cells; measured with microphones in external auditory canal. May be spontaneous or sound-evoked. Presence indicates intact outer hair cells of organ of Corti. Used to assess auditory thresholds and distinguish sensory from neural hearing loss.
9.7 Evoked Responses¶
Electrocochleography: measures earliest evoked potentials from cochlea and auditory nerve. Cochlear microphonic (outer hair cells), summating potential (inner hair cells), whole nerve action potential (first-order neurons). Clinical use: Ménière's disease diagnosis (elevated summating potential to action potential ratio). Brainstem auditory-evoked responses (BAERs/ABRs): five distinct potentials from eighth nerve, cochlear nucleus, superior olivary complex, lateral lemniscus, inferior colliculus. Uses: differentiate site of sensorineural loss, assess patients who cannot give reliable voluntary thresholds, diagnose retrocochlear pathology (vestibular schwannoma), intraoperative hearing monitoring, determine brain death.
9.8 Vestibular-Evoked Myogenic Potential (VEMP)¶
Investigates otolith and vestibular nerve function using high-level acoustic stimulus. cVEMP (cervical): vestibulocollic reflex; afferents from saccule via inferior vestibular nerve; recorded from sternocleidomastoid muscle. May be diminished/absent in Ménière's disease, vestibular neuritis, BPPV, vestibular schwannoma. May have lower threshold in superior canal dehiscence, inner-ear dehiscence, perilymphatic fistula. oVEMP (ocular): primarily involves utricle and superior vestibular nerve; recorded from extraocular muscles; abnormal in superior vestibular neuritis.
10. IMAGING STUDIES¶
Choice depends on whether evaluating bony anatomy or auditory nerve/brain.
10.1 CT Temporal Bone¶
Fine 0.3-mm axial and coronal cuts. Ideal for: external auditory canal caliber, ossicular chain integrity, middle ear or mastoid disease, inner ear malformations, bone erosion in chronic otitis media/cholesteatoma. Pöschl reformatting (plane of superior semicircular canal): required to identify dehiscence or absent bone over superior semicircular canal.
10.2 MRI¶
Superior for: retrocochlear pathology (vestibular schwannoma, meningioma, other cerebellopontine angle lesions), brainstem demyelinating lesions, brain tumors.
10.3 Overlapping Uses¶
Both CT and MRI equally capable of identifying inner-ear malformations and assessing cochlear patency for cochlear implant evaluation.
11. MANAGEMENT & TREATMENT¶
General principle: conductive hearing losses are amenable to surgical correction; sensorineural hearing losses are usually managed medically.
11.1 Treatment of Conductive Hearing Loss¶
Ear canal atresia: surgical repair or bone-anchored hearing aid (BAHA). Tympanic membrane perforation: outpatient tympanoplasty. Otosclerosis: stapedectomy (>95% success). Middle ear effusion: tympanostomy tubes for prompt hearing return. Cholesteatoma: surgical removal and ossicular reconstruction. Hearing aids: effective and well-tolerated for conductive losses.
11.2 Hearing Aids¶
Rehabilitate mild, moderate, and severe sensorineural losses. Current generation: nearly invisible, digital, individually programmable, multiple/directional microphones. Enhance signal-to-noise ratio by amplifying sounds close to listener. Limitation: amplify sound but cannot restore clarity lost with presbycusis. Shown to reduce cognitive decline and fall risk. Cost is significant obstacle; bilateral amplification usually recommended. Over-the-counter (OTC) hearing aids: FDA-approved category to reduce cost and increase accessibility (similar to reading glasses).
11.3 Unilateral Deafness Solutions¶
Problems: difficulty with sound localization, reduced clarity in background noise. CROS hearing aid: microphone on deaf side transmits to receiver on hearing side. BAHA: hearing aid on screw integrated into skull on deaf side; vibrates skull to transfer signal to hearing ear. BICROS hearing aid: for profound deafness on one side with some loss in better ear; microphone on deaf side plus hearing aid in better ear. Limitation: CROS and BAHA provide benefit but do not restore hearing in deaf ear. Cochlear implants: increasingly used for single-sided deafness; restores hearing, reduces tinnitus, improves sound localization and background noise performance, reduces fatigue.
11.4 Assistive Devices¶
Based on principle of speaker closer to microphone than noise source. Types: infrared transmission, FM transmission, electromagnetic loop transmission to hearing aid telecoil. Telecoils: hearing aids can use properly equipped telephones. Bluetooth technology: revolutionized connectivity between hearing aids and smartphones/devices.
11.5 Cochlear Implants¶
Neural prostheses converting sound energy to electrical energy to stimulate auditory nerve directly. Rationale: in profound hearing loss, hair cells lost but ganglionic cells of eighth nerve preserved. Components: electrodes inserted into cochlea through round window, speech processors extracting acoustic elements, transcutaneous transmission system. Criteria: severe to profound hearing loss with open-set sentence cognition ≤ 40% under best-aided conditions. Outcomes: >600,000 recipients worldwide. Sound perception helps with speech reading, open-set word recognition, voice modulation. Within 3-6 months, adult patients understand speech without visual cues. Nearly 75% can converse on telephone with current multichannel implants. Bilateral implantation: commonly performed, especially in children; better performance in noise, sound localization, less fatigue than monaural hearing.
11.6 Hybrid Cochlear Implants¶
Indicated for high-frequency hearing loss in patients with normal low-frequency hearing who don't benefit from hearing aids but are not candidates for conventional cochlear implants. Design: shorter electrode, atraumatic insertion preserving low-frequency hearing. Function: patient uses natural low-frequency 'acoustic' hearing plus implant for 'electrical' high-frequency hearing. Outcomes: better speech discrimination in quiet and noise than with hearing aids alone.
11.7 Brainstem Auditory Implants¶
For patients born without cochlea or with bilateral eighth nerve destruction (e.g., NF2). Placed near cochlear nucleus. Outcomes: provide sound awareness; speech understanding remains elusive.
11.8 Emerging Molecular Therapies¶
Understanding genetic basis, hair cell regeneration signals, and ototoxicity pathways opens molecular therapy opportunities. Approaches: gene therapy, stem cell therapy, RNA-based therapies, CRISPR/Cas9 gene editing. Recent development: adeno-associated virus-mediated otoferlin delivery restored hearing in congenitally deaf children. Future potential: gene replacement, gene suppression, genome editing to protect, preserve, or regenerate inner ear structures.
11.9 Communication Strategies¶
Reduce unnecessary environmental noise (radio, television) to enhance signal-to-noise ratio. Lip reading aids comprehension: speaker's face should be well-illuminated and visible. Speech should be loud and clear, but be aware of recruitment in sensorineural loss. Optimal communication requires full attention from both parties.
12. PREVENTION¶
Many forms of hearing loss are preventable through appropriate measures.
Decibel (Loudness) Level of Common Environmental Noise¶
| Source | Decibel (dB) |
|---|---|
| Weakest sound heard | 0 |
| Whisper | 30 |
| Normal conversation | 55-65 |
| City traffic inside car | 85 |
| OSHA monitoring requirement begins | 90 |
| Jackhammer | 95 |
| Subway train at 200 ft | 95 |
| Power mower | 107 |
| Power saw | 110 |
| Painful sound | 125 |
| Jet engine at 100 ft | 140 |
| 12-gauge shotgun blast | 165 |
| Loudest sound that can occur | 194 |
12.1 Conductive Hearing Loss Prevention¶
Prompt antibiotic therapy of adequate duration for acute otitis media. Tympanostomy tubes for middle ear effusions lasting ≥ 12 weeks.
12.2 Ototoxicity Prevention¶
Aminoglycoside-induced hearing loss and vestibular dysfunction can largely be prevented by careful monitoring of serum peak and trough levels.
12.3 Noise-Induced Hearing Loss Prevention¶
Epidemiology: 10 million Americans have noise-induced hearing loss; 20 million exposed to hazardous workplace noise. Prevention: avoid loud noise exposure or use earplugs or fluid-filled ear muffs. High-risk activities: electrical equipment for wood/metalworking, target practice, hunting with small firearms. All internal-combustion and electric engines require user protection: snow blowers, leaf blowers, snowmobiles, outboard motors, chainsaws. Education should begin before teenage years.
12.4 OSHA Requirements¶
Hearing conservation programs required when 8-hour exposure averages ≥ 85 dB.
13. KEY POINTS & CLINICAL PEARLS¶
13.1 Diagnostic Pearls¶
Chronically draining ear unresponsive to antibiotics suggests cholesteatoma. Conductive hearing loss with normal ear canal and intact tympanic membrane suggests otosclerosis or third window pathology. Acoustic reflex present with conductive loss indicates third window; absent indicates otosclerosis. Greater than expected discrimination loss with mild asymmetric sensorineural loss suggests vestibular schwannoma. Unilateral serous effusion or unexplained otalgia warrants fiberoptic examination to exclude nasopharyngeal neoplasm. Sudden hearing loss may be viral infection, vestibular schwannoma, or stroke. Pulsatile tinnitus requires vascular workup.
13.2 Treatment Pearls¶
Stapedectomy for otosclerosis has >95% success rate. Tympanoplasty for tympanic membrane perforation has >90% success rate. Cochlear implants are indicated when word recognition score falls below 50% despite optimal hearing aids. Bilateral cochlear implants provide better noise performance, localization, and less fatigue. Cochlear implants are now used for single-sided deafness and provide benefits beyond CROS/BAHA. Hearing aid use reduces cognitive decline and fall risk in elderly.
13.3 Prevention Pearls¶
Virtually all noise-induced hearing loss is preventable through education and hearing protection. Education about hearing protection should begin before the teenage years. Monitor aminoglycoside levels to prevent ototoxicity. Hearing conservation programs mandatory for workplace noise ≥ 85 dB over 8 hours.
13.4 Genetic Counseling Points¶
GJB2 mutations account for nearly 20% of all childhood deafness and half of genetic childhood deafness. 35delG and 167delT mutations account for >50% of GJB2 cases, but full gene sequencing required. 167delT is highly prevalent in Ashkenazi Jews (~1/1765 homozygous). Phenotype can vary within families even with same mutation. Consider genetic testing in all children with unexplained hearing loss.