Audiology Chapter 3: Human Ear, Hearing Loss, and Pure-Tone Hearing Tests

Learning Objectives

• Master goals for Chapter 3:
  • $3.1$ Explain general anatomy/physiology of the hearing mechanism & pathways of sound.
  • $3.2$ Define every type of hearing loss (HL).
  • $3.3$ Describe hearing-test methodology & strategies for reducing ambient noise.
  • $3.4$ Clarify patient & examiner roles.
  • $3.5$ Differentiate purposes of air-conduction (AC) vs. bone-conduction (BC) audiometry.
  • $3.6$ Outline clinical steps for AC/BC tests.
  • $3.7$ Accurately interpret pure-tone data.

Ear Anatomy & Energy Transduction

• 3 anatomical divisions:
  • Outer ear
  • Middle ear
  • Inner ear
• Conductive mechanism = outer + middle ears.
  • Tympanic membrane (TM) converts acoustic energy → mechanical vibrations.
  • Ossicular chain continues mechanical transmission.
• Sensory/Neural mechanism = inner ear + auditory nerve + central pathways.
  • Cochlea changes mechanical input → hydromechanical fluid wave.
  • Hair-cell transduction yields electrical impulses along the VIII cranial nerve.

Formal Types of Hearing Loss

• Conductive HL
  • Pathology in outer and/or middle ear.
  • Audiometric hallmark: abnormal AC, normal BC → air-bone gap (ABG).
• Sensorineural HL (a.k.a. sensory/neural HL)
  • Damage in inner ear or VIII nerve.
  • AC and BC equal & elevated.
• Mixed HL
  • Combination of conductive + sensorineural components.
• Central HL
  • Reduced auditory comprehension despite normal peripheral sensitivities (auditory processing disorder).
• Non-organic HL
  • Behavioural results suggest loss, but physiology appears normal.
  • Malingering = deliberate feigning.
  • Psychogenic HL = secondary to psychological disorder.
• Key term: attenuation = decrease in sound strength.

Pre-Test Procedures & Bed-Side Tools

• Case History
  • Most critical diagnostic tool; obtained before any measurement.
• Otoscopy
  • Visual exam of canal & TM to rule out obstruction, infection, perforation.
• Classic tuning-fork tests (mid-19th century onward):
  • Weber Test (lateralisation)
  • Fork on skull midline.
  • Normal/symmetrical ears → midline perception.
  • Asymmetric sensorineural loss → lateralises to better ear.
  • Conductive loss → lateralises to poorer ear (occlusion effect).
  • Rinne Test (AC vs BC loudness)
  • Compare mastoid placement (BC) vs tines at canal (AC).
  • Normal or sensorineural HL: AC perceived louder.
  • Conductive HL: BC perceived louder.

Pure-Tone Audiometry – Global Concepts

• Goal: determine hearing threshold = minimum intensity at which tone is perceived $50\%$ of the time.
• Modalities:
  • Air-Conduction: assesses outer → central pathway.
  • Bone-Conduction: bypasses conductive system, assesses cochlea + nerve.
  • Speech Audiometry: examines speech thresholds & word recognition.

Environmental Noise Control

• Crucial for accurate thresholds, especially in schools, industrial sites, tele-practice.
• Solutions:
  • Sound-treated booth (prefabricated).
  • Supra-aural headphones with cushion seal.
  • Insert earphones (foam) → best attenuation, infection control, less masking need.

Equipment Calibration

• Electroacoustic calibration (annual)
  • Earphones, inserts, BC vibrators coupled to sound-level meter.
  • Verify: output levels, distortion, attenuator linearity within tolerance.
• Bio-acoustic check (daily)
  • Clinician re-tests own thresholds vs archived data.
  • Listen for scratches, pops, cord intermittency.

Pure-Tone Air-Conduction Testing – Step-by-Step

1. Instructions
   • Clarify tone nature, response mode, ear order (start with better ear).
   • Example script: “You will hear beeps that get softer; raise your hand even if you only think you hear them.”
2. Familiarisation
   • Present tone $10\text{–}20\,\text{dB}$ above expected threshold (suprathreshold) using sweep-up.
3. Threshold Search (Hughson-Westlake / ASHA ascending method)
   • After a response → lower $10\,\text{dB}$.
   • After non-response → raise $5\,\text{dB}$.
   • Threshold = level with 2–3 positive responses on ascending runs.

• Stimulus parameters:
  • Pulsed tone, $0.5\,\text{s}$ duration, $1\,\text{s}$ inter-stimulus gap.
  • Irregular rhythm prevents guessing.
• Response modes: clicker button, hand raise, verbal (consider dexterity & feedback).

AC Testing Frequencies & Intensities

• Routine frequencies $250\text{–}8000\,\text{Hz}$
  • Optional $125\,\text{Hz}$ & extended high-freq to $20000\,\text{Hz}$ (research/monitoring ototoxicity).
• Test order: $1000, 2000, 3000, 4000, 6000, 8000, 1000$(re-test), $500, 250\,\text{Hz}$.
  • Mid-octave (e.g., $750, 1500\,\text{Hz}$) if adjacent octaves differ by $\ge 20\,\text{dB}$.
• Starting intensity $30\,\text{dB HL}$; if absent → $50\,\text{dB}$ then 10-dB steps.

Patient Instructions & Observations

• Keep hand raised until tone ceases → shows duration perception.
• Ensure patient cannot observe examiner’s switch movements (eliminate visual cues).
• Note false-positives (anticipatory responses) & false-negatives (delayed).
• Document reliability: good / fair / poor.

Bone-Conduction Audiometry Essentials

• Transducer typically on mastoid; forehead possible (different calibration).
• Underlying mechanisms:
  • Compressional vibration of cochlear shell.
  • Osseotympanic route (ear-canal air vibration).
  • Inertial ossicular lag.
• Procedure:
  • Begin $10\,\text{dB}$ above AC threshold.
  • Same 10-down/5-up technique.
  • Keep both ear canals unoccluded to avoid occlusion effect (artificial threshold improvement at $\le 1000\,\text{Hz}$ if ear is plugged).
• Air-Bone Gap interpretations:
  • $\text{AC} = \text{BC}$ → sensorineural deficit.
  • \text{BC} > \text{AC} by $\ge 15\,\text{dB}$ → conductive or mixed.
• Cautions:
  • Frequency range limited $250\text{–}6000\,\text{Hz}$.
  • Max outputs lower (≈ $45\text{–}70\,\text{dB HL}$).
  • Tactile sensation possible at low freq high level (esp. $250\,\text{Hz}$).
  • Cross-hearing risk → masking may be necessary.

Audiogram Fundamentals

• Graph axes:
  • X-axis = Frequency (Hz) low→high.
  • Y-axis = Hearing Level in $\text{dB HL}$ (−10 top, 120 bottom).
• Symbol key:
  • $O$ = Right-ear AC unmasked.
  • $X$ = Left-ear AC unmasked.
  • $\triangle$ or $\Delta$ = Right masked AC.
  • $[\ ]$ = BC unmasked (left / right variations).
  • Arrows down-left or down-right = no response.
  • $S$ = sound-field.
• Degrees of loss (based on Pure-Tone Average):
  • None $-10\text{–}15\,\text{dB}$.
  • Slight $16\text{–}25$.
  • Mild $26\text{–}40$.
  • Moderate $41\text{–}55$.
  • Moderately-severe $56\text{–}70$.
  • Severe $71\text{–}90$.
  • Profound $\ge 91$.

Pure-Tone Averages (PTA) & Speech Prediction

• Three-frequency PTA: mean of $500, 1000, 2000\,\text{Hz}$.
• Two-frequency PTA: choose two lowest of those three → better predictor when slope steep.
• Variable PTA: mean of poorest three among $500, 1000, 2000, 4000\,\text{Hz}$; correlates with real-world communication impact.

Air-Bone Relationships Summary

• Normal: AC = BC within normal limits.
• Conductive: Elevated AC, normal BC → ABG $\ge 15\,\text{dB}$.
• Sensorineural: Elevated AC and BC equally → no ABG.
• Mixed: Elevated both, plus ABG $\ge 15\,\text{dB}$.

Reduced-Time & Automated Audiometry

• Automatic or computer-controlled audiometers can run the 10-down/5-up algorithm independently.
• Clinician still needed for:
  • Instructions & consent.
  • Proper transducer placement.
  • Data interpretation & counseling.
• Populations requiring full manual involvement: paediatrics, cognitively impaired, unco-operative, many elderly.

Illustrative Audiograms (Visual Recognition)

• Bilateral Conductive HL: Normal BC line near $0\text{–}20\,\text{dB}$, AC elevated creating parallel ABG across frequencies.
• Bilateral Sensorineural HL: AC & BC overlapped, sloping pattern; no ABG.
• Bilateral Mixed HL: Both AC & BC elevated, AC worse → visible ABG.

Ethical & Practical Considerations

• Copyright notice: material for instructor & student educational use only; redistribution prohibited.
• Clinical note-taking must protect patient confidentiality.
• Malingering detection vital for medico-legal & compensation cases; corroborate behavioural tests with objective measures (otoacoustic emissions, ABR).

Key Equations & Numeric References

• Threshold definition: response at $\ge 50\%$ detection probability.
• Ascending technique rule: $\Delta I<em>{down}=10\,\text{dB},\; \Delta I</em>{up}=5\,\text{dB}$.
• ABG criterion for pathology: $\text{ABG} \ge 15\,\text{dB}$.
• PTA categories (recap):
  $\text{None} = -10\text{–}15\,\text{dB}$; $\text{Slight} = 16\text{–}25$; $\text{Mild} = 26\text{–}40$; $\text{Moderate} = 41\text{–}55$; $\text{Mod-Severe} = 56\text{–}70$; $\text{Severe} = 71\text{–}90$; $\text{Profound} \ge 91$.

Connections & Real-World Relevance

• Pure-tone protocols underpin newborn screening follow-ups, occupational HL monitoring, ototoxic drug tracking, and hearing-aid candidacy evaluations.
• Tuning-fork tests remain valuable in ER & primary-care settings where audiometer unavailable.
• Understanding ABG guides surgical decisions (e.g., ossiculoplasty vs cochlear implant).
• Controlling ambient noise is analogous to signal-to-noise optimisation in all diagnostic imaging & neurophysiologic testing.
• Ethical detection of non-organic HL prevents insurance fraud and ensures genuine patients receive resources.