Hearing

Incidence & Prevalence

  • Hearing loss = 3rd most common health problem in the U.S.
  • 28–36 million28\text{–}36\,\text{million} Americans (≈ 17%17\% of the population) have some degree of hearing loss.
  • 1 / 1000 infants present with severe–profound loss; 2 – 3 / 1000 children are born deaf or hard-of-hearing.
  • ≥50%50\% of profound, early-onset deafness is genetic.
  • 3 / 4 children experience otitis media by age 3.
  • 12 / 1000 people with impairment are
  • 15 % (≈26 million) of Americans 20–69 yrs have high-frequency loss secondary to noise exposure.
  • 9 / 10 deaf children are born to hearing parents.
  • U.S. prevalence has doubled in 30 yrs; incidence higher in men.

Core Terminology

  • Oto = ear; Audio = sound; Rhino = nose; Laryngo = throat; ENT = ear/nose/throat.
  • Unilateral/Bilateral; Acute/Chronic; Static/Progressive.
  • Ways of hearing: Air conduction vs. Bone conduction.
  • Common classroom acronyms: HI (hearing-impaired), HOH (hard-of-hearing), d/Deaf (cultural distinction).

Sub-Disciplines of Audiology

  • Pediatric • Medical • Rehabilitative/Dispensing • Educational • Industrial.

Properties of Sound

  • Pitch = frequency (Hz). Human range 20–20,000 Hz20\text{–}20{,}000\,\text{Hz}.
    • <20 Hz20\,\text{Hz} perceived as vibration; >20,000 Hz20{,}000\,\text{Hz} = ultrasonic.
  • Loudness = intensity (dB). Formula dB=10log⁡<em>10(II</em>0)\text{dB}=10\log<em>{10}\left(\dfrac{I}{I</em>0}\right).
  • Decibel reference examples:
    • 60 dB – conversation/dishwasher; 80 dB – alarm clock; 90 dB – hair dryer/blender/lawnmower; 100 dB – MP3 full volume.
    • 110 dB concerts; 120 dB jet take-off; 130 dB ambulance; 140 dB gunshot/fireworks.
  • Exposure >85 dB for prolonged periods ⇒ risk for NIHL (noise-induced hearing loss).

Degrees of Hearing Loss (PTA – pure-tone average)

  • 0–25 dB0\text{–}25\,\text{dB} = Normal
  • 26–40 dB26\text{–}40\,\text{dB} = Mild
  • 41–55 dB41\text{–}55\,\text{dB} = Moderate
  • 56–70 dB56\text{–}70\,\text{dB} = Moderately-Severe
  • 71–90 dB71\text{–}90\,\text{dB} = Severe
  • >91\,\text{dB} = Profound
  • Audiograms plot Frequency (Hz, x-axis) vs. Intensity (dB HL, y-axis) and visualize configuration (sloping, rising, cookie-bite, etc.).

Assessment & Testing Modalities

  • Otoscopic exam (visual status of canal & TM).
  • Behavioral tests
    • Pure-tone audiometry (air & bone conduction thresholds).
    • Speech audiometry: Speech Reception Threshold (SRT), Word Recognition Score (WRS).
  • Physiologic/Objective
    • Immittance Audiometry: Tympanometry (ME compliance) + Acoustic reflexes.
    • Electrocochleography (ECochG) – cochlear electrical response.
    • Auditory Evoked Potentials: ABR/BAER for neural integrity.
    • Otoacoustic Emissions (OAEs) – outer-hair-cell function.
  • Movement measures (infant behavioral observation) & brain-wave recordings supplement.

Anatomy & Physiology of Hearing

Outer Ear

  • Components: Auricle/Pinna, External Auditory Canal, Tympanic Membrane (TM).
  • Functions: collect, resonate, and direct sound; protect ME.
  • TM: thin, oval; vibrates proportionally; ≈17× larger than oval window.

Middle Ear

  • Air-filled cavity from TM → oval window; houses ossicles: Malleus, Incus, Stapes.
  • Eustachian Tube equalizes pressure, opens during swallowing.

Inner Ear

  • Cochlea (hearing) + Vestibular System (balance).
  • Cochlea:
    • Fluid-filled; ≈13,000 outer & 3,500 inner hair cells.
    • Hair-cell movement → neural impulses via Cranial Nerve VIII.
    • Tonotopic mapping (base = high freq, apex = low freq).
  • Vestibular System: 3 semicircular canals; works with vision & proprioception for balance.

Energy Transduction Chain

  • Acoustic (outer ear) → Mechanical (TM & ossicular chain) → Hydraulic (fluid waves in cochlea) → Electrochemical (CN VIII) → Cortical perception.
  • 75%75\% of right-ear fibers cross to left hemisphere; remaining 25%25\% stay ipsilateral.

Functional Considerations

  • Sound localization (inter-aural time & intensity differences).
  • Tonotopic sensitivity; semicircular-canal orientation; Eustachian-tube patency.

Types & Etiologies of Hearing Loss

Conductive

  • Obstruction/dysfunction outer or middle ear.
  • Common causes: Cerumen impaction; TM perforation; Otitis media; Otosclerosis; Malformed pinna/canal; External otitis; Foreign objects.
  • Audiogram: air–bone gap, normal bone thresholds.

Sensorineural

  • Cochlear or CN VIII pathology.
  • Presbycusis (age-related); Noise-induced; Ototoxic drugs; Menière’s; Genetic syndromes; Tinnitus often co-occurs.
  • Audiogram: elevated air & bone thresholds, no gap; often high-frequency sloping.

Mixed

  • Combination conductive + sensorineural components.

Central Auditory Processing Disorders (CAPD)

  • Lesions or dysfunction along central pathways/cortex → agnosias, pure-word deafness, cortical deafness.

Specific Pathologies & Clinical Nuggets

  • Otitis Media: ME inflammation; treat antibiotics, PE tubes for recurrent cases.
  • Otosclerosis: stapes footplate fixation; 2× incidence in women; may require stapedectomy.
  • Noise-Induced HL: temporary or permanent threshold shift; time-intensity trade-off critical.
  • Presbycusis: symmetric, progressive high-frequency loss; speech-in-noise difficulty.
  • Tinnitus: perceived ringing/roaring; possible early indicator of otosclerosis or Menière’s; affects QOL.
  • Menière’s Disease: episodic vertigo, fluctuating SNHL, tinnitus.

Communication Impact

  • Variables: age of onset, severity/configuration, timeliness of rehab, co-morbidities.
  • Speech: inaudible sounds harder to acquire (final consonants, unstressed syllables); prosody (stress, rate, pitch) affected.
  • Language: Form – telegraphic (missing function words); Content – limited lexicon; Use – conversational challenges.

Causative Risk Factors

  • Excessive noise, infections, trauma, TM/inner-ear injury via foreign objects, ototoxic meds, normal aging, prematurity, hereditary factors, meningitis.

Prevention & Early Management

  • Ear protection, industrial hearing conservation, youth education, ASHA resources.
  • Universal Newborn Screening: ABR & OAEs enable detection <1 mo, intervention <6 mo.

Treatment & Rehabilitation

Medical/Surgical

  • Antibiotics, myringotomy + PE tubes, stapedectomy, tumor removal, cochlear implantation.

Amplification & Assistive Technology

  • Hearing Aids: BTE, ITE, ITC, CIC.
    • Components: Microphone → Amplifier → Receiver.
  • FM & Sound-Field systems; personal amplifiers.
  • Assistive Listening Devices (ALDs): telephone amplifiers, TV ears, loop systems.
  • Other devices: closed captioning, vibrotactile pagers, flashing alerts, vib-alarm clocks.

Cochlear Implants

  • Indication: severe–profound bilateral SNHL with limited benefit from aids.
  • Components: External microphone & speech processor; Internal receiver-stimulator + electrode array.
  • Function: Direct electrical stimulation of CN VIII; perceived sound differs from acoustic – requires post-implant auditory training.
  • ≈112,000 recipients worldwide; U.S. ≈23k adults + 15.5k children.

Aural (Re)habilitation Goals

  • Minimize communication barriers.
  • Auditory Training (detection → discrimination → identification → comprehension); necessitates prior amplification.
  • Programs: Auditory-Verbal Therapy (AG Bell), Berard/Tomatis integration, Speechreading.
  • Speech Development: increase vocalizations, expand phonetic repertoire, enhance intelligibility.
  • Language Therapy: complex syntax, pragmatics, narrative, literacy emphasis.

Sign Systems & Deaf Culture

  • Modalities: ASL, Signed Exact English (SEE), Manually-Coded English, Cued Speech.
  • Deaf Culture perspectives: deafness as identity not deficit; varied attitudes toward cochlear implants/oralism; CODA (Children of Deaf Adults).
  • Quote (Marschark): society frames deafness as lack rather than presence.

Psychosocial/Emotional Considerations

  • Families journey through grief stages; need resources, support, adjustment of expectations.

Balance Disorders

  • Vestibular pathology leads to vertigo; semicircular canal dysfunction considered in audiology evaluations.

Resources & References

  • ASHA: www.asha.org
  • American Academy of Audiology: www.audiology.org
  • Hearing Test Labs: www.hearingtestlabs.com/hearing.htm
  • Gallaudet University: https://www.gallaudet.edu/
  • Educational videos: “How Hearing Works,” “2-Minute Neuroscience – Vestibular,” Cranial Nerve VIII, etc.

Ethical & Practical Implications

  • Equity in newborn screening, access to amplification, cultural competence with Deaf community.
  • Noise regulation in occupational/leisure environments.
  • Insurance coverage for hearing aids & implants remains limited despite clear functional benefit.