Chapter 3 – The Human Ear, Hearing Loss, and Pure-Tone Hearing Tests

Anatomy & Physiology of the Auditory System

• Anatomy answers “HOW IS IT BUILT?”, physiology answers “HOW DOES IT WORK?”
• Path of sound energy inside the auditory tract:
  • Acoustic (air-borne) waves ⟶ mechanical vibration (tympanic membrane & ossicles) ⟶ hydraulic (cochlear fluid waves) ⟶ chemo-electrical (hair-cell → VIII Nerve) ⟶ central electrical coding (brain-stem → cortex).
• Three traditional anatomical parts
  • Outer Ear – pinna + external auditory canal + tympanic membrane.
  • Middle Ear – air-filled cavity holding ossicles; stapes = smallest human bone.
  • Inner Ear (Cochlea) – fluid filled, sensory epithelia convert hydromechanical waves to neural impulses; connected to brain-stem through VIII nerve and multiple relay nuclei.
• Functional division used by clinicians
  • Conductive Mechanism = outer + middle ear – conveys sound.
  • Sensory/Neural Mechanism = cochlea + VIII N + central pathways – analyzes and transmits sound.

Classification of Hearing Loss

• Conductive (CHL)
  • Obstruction/dysfunction of conductive mechanism → attenuation of sound.
  • Air-conduction (AC) thresholds elevated; bone-conduction (BC) thresholds normal.
  • Results in an air-bone gap (ABG); usually treatable medically/surgically.
• Sensory/Neural (SNHL)
  • Damage to cochlea and/or VIII nerve.
  • Equal elevation of AC & BC thresholds (no ABG); seldom medically reversible; may cause loudness attenuation PLUS distortion.
  • Historical terms (“nerve loss”, “perceptive loss”, “sensorineural”) replaced with sensory/neural to stress possibility of separate sensory vs neural sites.
• Mixed Loss
  • Pathology simultaneously in conductive + sensory/neural portions.
  • BC elevated (sensory/neural component) AND AC elevated further (added conductive component).
  • \text{AC}=\text{BC}+\text{ABG}
• Central (APD) – normal peripheral thresholds but impaired cortical processing (discrimination, memory, sequencing).
• Non-Organic / Functional – behavioural thresholds poorer than true physiology; includes malingering & psychogenic factors.

Air vs Bone Conduction Pathways

• Air Conduction (AC) tests the entire auditory system (outer→cortex).
• Bone Conduction (BC) bypasses conductive mechanism; skull vibration directly excites both cochleae via:
  1. Distortional BC – cochlear shell deformation.
  2. Inertial BC – ossicular inertia drives stapes.
  3. Osseo-tympanic BC – ear-canal sound re-radiation.

Pure-Tone Audiometry: Purpose & Reliability Factors

• Aids diagnosis of site & extent of loss; directs (re)habilitation planning.
• Reliability hinges on:
  • ANSI-calibrated equipment.
  • Controlled ambient noise.
  • Adequate patient instructions & co-operation.
  • Clinician sophistication.
• “We do not measure hearing, we measure behavioural responses to acoustic stimuli.”

Test Environment & Noise Control

• ANSI maximum permissible ambient SPLs must be met (see Table 2.4 of text).
• Attenuation options
  • Supra-aural earphone enclosure cups (fluid cushion) – extra low-freq attenuation; influence calibration.
  • Insert earphones – deep foam expansion ↑ attenuation & infection control; minimise ear-canal collapse; reduce occlusion effect.
  • Active noise-cancellation headphones over inserts – permit booth-less testing in quiet rooms.
  • Sound-isolated chambers – prefabricated or custom; mass + insulation + dead air; issues: HVAC noise & vibration; “sound-treated” not “sound-proof”.

Patient Role & Response Modes

• Must understand: respond whenever a tone is just heard.
• Response options: raise hand/finger, pushbutton, vocal “yes/now”, or conditioned play.
• False responses
  • False Negative – hears but does not respond (exaggeration, confusion).
  • False Positive – responds when no tone (cueing, tinnitus).

Clinician Responsibilities

• Clear multilingual/written instructions; observe through window without offering visual cues.
• Position: patient at right-angles; examiner hidden.

Air-Conduction Procedure (Carhart & Jerger, 1959)

1. Test better ear first (if known); start at 1000\,\text{Hz}.
2. Present 30\,\text{dB HL}. If NR raise to 50\,\text{dB HL} then in 10\,\text{dB} steps to first response.
3. Threshold Search: descend 10\,\text{dB} until NR, then ascend in 5\,\text{dB} steps; threshold = lowest level with ≥2/3 positive responses.
4. Test order: 1000, 2000, 3000, 4000, 6000, 8000, \text{re-test }1000, 500, 250\,(125) Hz.
5. ASHA (2005): test inter-octaves (750, 1500) when adjacent octaves differ ≥20\,\text{dB}; always test 3000 & 6000 Hz.

Earphone Placement

• Supra-aural: diaphragm centered over canal; remove glasses/earrings; manage collapsing canals (foam wedge).
• Insert: compress foam tip, place deeply, allow expansion; colour coding Red = Right, Blue = Left.

Threshold Averages & Communication Impact

• Three-frequency PTA = 500 + 1000 + 2000 Hz ÷3.
• Two-frequency PTA = best 2 of 500-1000-2000 Hz.
• Variable PTA (VPTA) = mean of poorest 3 at 500-1000-2000-4000.
• Example communication scale (Clark, 1981):
  • 0{-}15\,\text{dB} none, 16{-}25 slight, 26{-}40 mild, 41{-}55 moderate, 56{-}70 mod-sev, 71{-}90 severe, >90 profound.

Percentage Hearing Impairment (historical AAOO 1979)

1. Pure-tone average at 500+1000+2000+3000 Hz.
2. \text{Impairment(ear)} = (\text{PTA}-25)\times1.5\%.
3. \text{Binaural}\% = \dfrac{5\times\text{better ear} + \text{poorer ear}}{6}.

Audiogram Conventions (ASHA 1990)

• Frequency (Hz) on x-axis; dB HL on y-axis (0 dB at TOP).
• Equal scaling: 1 octave horizontally = 20\,\text{dB} vertically.
• Symbols (unmasked): Right AC \bigcirc (red), Left AC \times (blue); BC-forehead \mathbf{V}; BC-mastoid Right \langle, Left \rangle.
• Masked symbols: Right AC \triangle, Left AC \square; Right BC [\,\text{red}], Left BC ] \text{ blue}.
• No response: symbol at maximum output with arrow ↓.

Bone-Conduction Testing

• Placement choices
  • Mastoid (traditional): louder, but variable & prone to OE and acoustic radiation.
  • Forehead (recommended): easier, less variability; requires +\approx10\,\text{dB} more output.
• Occlusion Effect (OE) – artificial improvement in BC thresholds when ear is covered; typical mean values with supra-aurals: \text{OE}{250}=20\,\text{dB},\;\text{OE}{500}=15\,\text{dB},\;\text{OE}_{1000}=5\,\text{dB}.
  • Deep-insert phones or active NC headphones greatly reduce OE.
• BC frequency range: 250–4000 Hz; max outputs ≈50\,\text{dB} at 250 Hz, 70{-}80\,\text{dB} above 500 Hz.

Tuning-Fork Tests

• Rinne: compare AC vs BC at same ear.
  • Positive Rinne (AC > BC) = normal or SNHL.
  • Negative Rinne (BC > AC) = CHL.
  • Beware false-negative Rinne in unilateral SNHL (better ear hears through skull).
• Weber: mid-skull BC tone, patient reports side.
  • Lateralises to poorer ear in CHL, to better ear in SNHL, midline if symmetric.
  • Relies on Stenger principle: only louder of 2 identical tones perceived.

Automatic & Computerised Audiometry

• Békésy self-tracking: continuous or pulsed tone sweeps; graphs threshold trace.
• Modern microprocessor systems can deliver AC, BC, masking, record thresholds, print audiogram; useful for adult screening, industrial & tele-audiology; do NOT replace clinician for paediatrics, difficult patients, counselling & rehabilitation.

Interpretation Rules & Formulas

• Fundamental relationship: \boxed{AC = BC + ABG}.
• Decision table
  • AC normal + BC normal = Normal.
  • BC normal, AC elevated (ABG >10 dB) = Conductive.
  • AC≈BC elevated (ABG ≤10 dB) = Sensory/Neural.
  • AC elevated, BC elevated, ABG >10 dB = Mixed.
• Beware tactile responses at high intensities (especially ≤1000 Hz); can mimic mixed losses; confirm by changing vibrator site (forehead↔mastoid) – auditory thresholds shift opposite to tactile.
• Cross-hearing risk: signal presented to test ear may reach non-test ear if it exceeds inter-aural attenuation (≈40\,\text{dB} supra-aural, 50{-}75\,\text{dB} insert). Requires masking (see Chapter 5).

Practical Examples

• Normal: 5 = 5 + 0 dB (no ABG).
• Bilateral CHL: 35 = 0 + 35 dB.
• Bilateral SNHL: 35 = 35 + 0 dB.
• Mixed: 60 = 35 + 25 dB.

Ethical, Clinical & Real-World Considerations

• Incorrect interpretation of audiogram (e.g., mis-labelling mixed vs tactile) can lead to unwarranted surgery.
• Infection control: disposable inserts, disinfected supra-aurals.
• Booth-less protocols (insert + NC) facilitate remote/industrial/community programmes.
• Descriptive reporting: state type, degree, configuration (flat, sloping, precipitous), symmetry.
• Counselling: use PTA, configuration & speech tests (Ch. 4) to explain impact; percentage impairment mostly insurance/legal, may mislead patients.