Physiology · Year 1 · from Physiology

Case 2: Sensorineural Hearing Loss - Cochlear Dysfunction

Clinical Image

Source: Wikimedia Commons - Hearing aid - CC BY-SA 3.0

Patient Presentation

A 68-year-old male presents to his primary care physician with progressive difficulty hearing over the past 5 years. He reports trouble understanding speech, especially in noisy environments like restaurants, and frequently asks people to repeat themselves. His wife complains that he turns the television volume too loud. He has worked in construction for 40 years and was exposed to loud machinery without hearing protection. He denies vertigo, ear pain, discharge, or recent infections. He reports constant high-pitched ringing in both ears (tinnitus).

Demographics

  • Age: 68 years
  • Sex: Male
  • Occupational history: 40 years in construction with significant noise exposure

Chief Complaint

Progressive bilateral hearing loss, difficulty understanding speech, and tinnitus

Physical Examination

  • Otoscopy: External auditory canals clear, tympanic membranes intact bilaterally with normal landmarks
  • Whispered voice test: Unable to hear whispered words at arm's length
  • Weber test: Sound does not lateralize (midline) - symmetric hearing loss
  • Rinne test: Air conduction > bone conduction bilaterally (positive Rinne - consistent with sensorineural loss)
  • Cranial nerves: Otherwise normal including facial nerve function

Workup

  • Pure tone audiometry:
  • Bilateral symmetric sensorineural hearing loss
  • High-frequency sloping pattern (characteristic of presbycusis with noise-induced contribution)
  • 4 kHz notch present bilaterally (acoustic notch - pathognomonic of noise exposure)
  • Speech discrimination: Reduced, especially in background noise
  • Tympanometry: Type A (normal) bilaterally, excluding conductive component
  • Acoustic reflex: Present but elevated thresholds
  • MRI internal auditory canals: No evidence of vestibular schwannoma (acoustic neuroma ruled out)

Diagnosis

Mixed Presbycusis and Noise-Induced Sensorineural Hearing Loss with Tinnitus

Treatment

  1. Hearing aids: Digital programmable bilateral hearing aids
  • Customized to audiogram pattern (more amplification at high frequencies)
  • Directional microphones to improve speech-in-noise understanding
  1. Hearing assistive technologies: FM systems, captioned telephone
  2. Communication strategies training: Face-to-face communication, quiet environments
  3. Tinnitus management:
  • Hearing aids (often reduce tinnitus perception)
  • Sound therapy (white noise, environmental sounds)
  • Cognitive behavioral therapy if distressing
  1. Hearing protection education: Prevent further damage (earplugs, avoid loud environments)
  2. Aural rehabilitation: Consider speech-reading training
  3. Consider cochlear implant evaluation if profound loss not benefiting from aids

Physiological Principles Demonstrated

  • Tonotopic organization: The basilar membrane is organized by frequency - high frequencies at the base (near oval window), low frequencies at the apex. Noise damage and aging preferentially affect the high-frequency basal region, explaining the characteristic high-frequency hearing loss pattern.
  • 4 kHz notch: The external auditory canal resonates at approximately 2-4 kHz, amplifying these frequencies. Combined with basilar membrane mechanics, the 4 kHz region is most vulnerable to noise damage, creating the characteristic audiometric notch.
  • Hair cell mechanotransduction: Sound vibrations deflect stereocilia on hair cells, opening mechanically-gated ion channels. The unusual high K+ concentration of endolymph creates the driving force for depolarization. Hair cells do not regenerate in humans; damage is permanent.
  • Outer vs. inner hair cells: Outer hair cells amplify sound through electromotility (cochlear amplifier). Their loss reduces sensitivity and frequency selectivity, explaining difficulty with speech discrimination in noise. Inner hair cells are the primary sensory receptors transmitting signals to the auditory nerve.
  • Weber and Rinne tests: In sensorineural loss, both air and bone conduction are impaired (inner ear or nerve problem), but air conduction remains better than bone (positive Rinne). Weber does not lateralize in symmetric loss. In conductive loss, Weber lateralizes to the affected ear, and bone conduction > air conduction (negative Rinne).
  • Speech-in-noise difficulty: High-frequency consonants (s, f, th, sh) carry much of the information distinguishing words. High-frequency hearing loss reduces speech discrimination, especially in background noise where the auditory system cannot use frequency-selective filtering.

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