Teach me Audiometry in detail from scratch for my third year pg exam of ENT. Also include various types of Audiometry in detail

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audiogram types conductive sensorineural mixed hearing loss

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🎧 Audiometry — Complete PG ENT Exam Guide

Sources: K J Lee's Essential Otolaryngology | Cummings Otolaryngology Head and Neck Surgery

1. INTRODUCTION & BASIC CONCEPTS

Audiometry is the science of measuring hearing sensitivity and diagnosing auditory disorders. It forms the cornerstone of any otological evaluation.

The Decibel (dB) Scale

dB SPL (Sound Pressure Level): Absolute physical measure of sound intensity; reference 0.0002 dynes/cm²
dB HL (Hearing Level): Clinically used; calibrated to average normal hearing thresholds at each frequency. "0 dB HL" = what a normal young adult barely hears at that frequency
dB SL (Sensation Level): Intensity above the patient's own threshold

Frequency Range

Human hearing: 20 Hz – 20,000 Hz
Speech range: 500–3000 Hz (most critical)
Standard audiometric test frequencies: 250, 500, 1000, 2000, 4000, 8000 Hz
Inter-octave frequencies (1500, 3000, 6000 Hz) tested when ≥25 dB difference exists between adjacent octaves

Classification of Hearing Loss by Severity (dB HL)

Degree	Threshold
Normal	≤25 dB HL (adults); ≤15 dB HL (children)
Mild	26–40 dB HL
Moderate	41–55 dB HL
Moderately severe	56–70 dB HL
Severe	71–90 dB HL
Profound	>90 dB HL

2. PURE-TONE AUDIOMETRY (PTA)

Principle

The foundation of audiometric testing. Measures hearing sensitivity to pure tones presented at specific frequencies.

Threshold = the lowest level at which a patient responds at least 50% of the time

Test Conditions

Sound-treated booth (quiet enough to measure 0 dB HL thresholds)
Calibrated equipment (complete calibration annually)
Trained audiologist
Otoscopic inspection prior to testing
Clear instructions, patient comfort

Methods of Threshold Determination

Modified Hughson-Westlake method (ascending): Standard clinical approach — decrease in 10 dB steps, increase in 5 dB steps; threshold = lowest level with ≥2/3 correct responses on ascent
Test sequence: Better ear first → AC → BC if needed

Air Conduction (AC)

Via earphones (supra-aural or insert)
Tests entire auditory pathway: outer ear → middle ear → cochlea → auditory nerve → brainstem
Symbols: O (right, unmasked), X (left, unmasked), △ (right, masked), □ (left, masked)

Bone Conduction (BC)

Via bone oscillator placed on mastoid or forehead
Bypasses outer and middle ear — tests only cochlea → auditory nerve → brainstem
Symbols: < (right, unmasked), > (left, unmasked), [ (right, masked), ] (left, masked)

Air-Bone Gap (ABG)

ABG = AC threshold − BC threshold
ABG ≥ 10 dB is clinically significant
Reflects conductive component of hearing loss (outer/middle ear pathology)

3. THE AUDIOGRAM — INTERPRETATION

Standard Symbols Table

Symbol	Meaning
O	Right ear, AC, unmasked
X	Left ear, AC, unmasked
△	Right ear, AC, masked
□	Left ear, AC, masked
<	Right ear, BC, unmasked
>	Left ear, BC, unmasked
[	Right ear, BC, masked
]	Left ear, BC, masked

Types of Audiograms

1. Normal Hearing

All AC thresholds ≤25 dB HL (adults)
No ABG; BC = AC

2. Conductive Hearing Loss (CHL)

AC thresholds elevated (>25 dB HL)
BC thresholds normal (≤25 dB HL)
Significant ABG (≥10 dB)
Causes: Otitis media with effusion, otosclerosis, tympanic membrane perforation, ossicular chain disruption

3. Sensorineural Hearing Loss (SNHL)

Both AC and BC thresholds equally elevated
No ABG (≤10 dB)
Causes: Noise-induced, presbycusis, Ménière's disease, ototoxicity, acoustic neuroma

4. Mixed Hearing Loss

Both AC and BC thresholds elevated
AC > BC → ABG present
Cause: Combination of conductive + sensorineural pathology

Special Audiometric Patterns

Pattern	Description	Condition
Carhart notch	BC dip at 2000 Hz	Otosclerosis (impedance mismatch)
4 kHz notch	AC dip at 4000 Hz	Noise-induced hearing loss (NIHL)
Flat	Equal loss across all frequencies	Otitis media with effusion
Cookie-bite	Mid-frequency loss (500–2000 Hz)	Genetic/hereditary SNHL
Low-frequency loss	Rising audiogram	Ménière's disease (early)
High-frequency loss	Falling audiogram	Presbycusis, ototoxicity

Pure-Tone Average (PTA)

PTA = (AC at 500 + 1000 + 2000 Hz) ÷ 3

Used to verify agreement with speech threshold (should be within ±10 dB)

4. MASKING

Why Mask?

When testing one ear, the sound can cross over through the skull and be heard by the non-test ear (NTE), giving a false response.

Interaural Attenuation (IA)

The reduction in sound as it travels from one side of the skull to the other:

Supra-aural earphones (AC): 40–65 dB depending on frequency
Insert earphones (AC): 70–90 dB (much less need for masking)
Bone conduction: 0–10 dB (essentially no attenuation → mask BC almost always)

Rules for When to Mask

Rule A – Air Conduction: Mask the NTE when:

AC stimulus level to test ear exceeds BC threshold of NTE by ≥40 dB (supra-aural) or ≥70 dB (insert earphones)

Rule B – Bone Conduction: Mask the NTE when:

There is an ABG >10 dB in the test ear

Type of Masker Used

For pure tones: Narrow-band noise (NBN) centered around the test frequency
For speech: Speech-spectrum noise

5. SPEECH AUDIOMETRY

Speech audiometry assesses how well a patient processes speech, providing functional information beyond what pure-tone testing shows.

Components

A. Speech Awareness/Detection Threshold (SAT/SDT)

Lowest level at which patient detects speech (not necessarily understands)
Should be within 10 dB of the best pure-tone threshold anywhere in 250–8000 Hz range
Used when SRT cannot be obtained (children, language barriers, disabilities)

B. Speech Recognition Threshold (SRT)

Lowest level at which patient can repeat spondaic words correctly 50% of the time
Spondee = two-syllable word with equal stress on both syllables (e.g., railroad, sidewalk, eardrum, icecream)
Key rule: SRT should be within ±10 dB of the PTA
Used to confirm pure-tone thresholds

C. Word Recognition Score (WRS) / Speech Discrimination Score (SDS)

Percentage of phonetically balanced (PB) words correctly repeated at suprathreshold level (25–35 dB SL above SRT)
Uses PB word lists (phonemes occur at same proportion as in normal English discourse)
Presented via recorded material (CDs/digital) — NOT monitored live voice (less reliable)

WRS Interpretation Table

Score	Interpretation
90–100%	Normal
76–88%	Slight difficulty
60–74%	Moderate difficulty
40–58%	Poor
≤40%	Very poor

Clinical Significance of WRS

Rollover: WRS decreases at higher intensities → suggests retrocochlear lesion (acoustic neuroma)
Disproportionately poor WRS relative to pure-tone loss → suggests retrocochlear pathology
In Ménière's disease: WRS may fluctuate and be surprisingly poor during attacks

6. IMMITTANCE (IMPEDANCE) AUDIOMETRY

Objective tests of middle ear function. Does NOT require a behavioral response from the patient. Based on the concept of acoustic impedance/admittance at the tympanic membrane.

Impedance = energy rejected; Admittance = energy accepted. "Immittance" encompasses both.

6A. TYMPANOMETRY

Principle: Measures mobility (compliance) of the middle ear system as a function of applied air pressure in the ear canal.

Probe tone: 226 Hz (standard); 1000 Hz in infants <6 months (more accurate in compliant neonatal canals)
Compliance expressed as acoustic admittance (mmho or cm³/mL)
Pressure expressed in daPa (dekapascals)

Tympanometric peak pressure (TPP): Pressure at which TM is most mobile = middle ear pressure Normal peak pressure: −100 to +100 daPa (adults)

Tympanogram Types (Jerger Classification)

Type	Shape	Compliance	Pressure	Clinical Association
A	Normal sharp peak	Normal	Normal (−100 to +100 daPa)	Normal middle ear
As (A-shallow)	Low/shallow peak	Reduced	Normal	Otosclerosis, tympanosclerosis, malleus fixation
Ad (A-deep)	Very high peak	Increased (hypercompliant)	Normal	Ossicular discontinuity, flaccid TM
B	Flat, no peak	Minimal	N/A	OME (fluid in middle ear), TM perforation (large ECV)
C	Peak shifted negative	Normal	< −100 daPa	Eustachian tube dysfunction, early OME

ECV (Equivalent Ear Canal Volume):

Normal: 0.6–1.5 cm³ (adults); 0.4–1.0 cm³ (children)

High ECV + Type B = TM perforation or patent PE tube

Low ECV = probe blocked against canal wall

6B. ACOUSTIC REFLEX TESTING

Principle: Loud sounds (≥70 dB SL) trigger contraction of the stapedius muscle (CN VII), stiffening the ossicular chain → measured as change in compliance.

Reflex arc: Sound → CN VIII → Superior olivary complex → CN VII nucleus → Stapedius muscle

Types of Reflexes:

Ipsilateral reflex: Stimulus and probe in same ear
Contralateral reflex: Stimulus in one ear, probe in other

Normal Acoustic Reflex Threshold (ART): 70–100 dB HL for pure tones

Acoustic Reflex Interpretation

Finding	Clinical Significance
Absent ipsilateral + contralateral	CHL (probe ear), severe SNHL, CN VII lesion
Absent contralateral only	Lesion in crossed CN VIII/brainstem pathway
Present reflex with CHL	Rules out significant middle ear pathology
Reflex threshold elevated	Moderate-severe SNHL
Diphasic (on-off) reflex	Early stapedial fixation (otosclerosis)
Absent reflex (with normal audiogram)	Retrocochlear lesion

Acoustic Reflex Decay

Sustained tone at 10 dB above ART for 10 seconds
Positive decay (>50% decrease in amplitude in ≤5 seconds) → CN VIII/retrocochlear lesion
Key test for acoustic neuroma screening

7. OTOACOUSTIC EMISSIONS (OAE)

Principle: The outer hair cells (OHC) of the cochlea not only receive sound but actively vibrate, generating sounds that travel backward through the middle ear to the ear canal, where they can be recorded.

Pathway: OHC → Basilar membrane → Cochlear fluids → Oval window → Ossicles → TM → External canal → Microphone

Key Facts:

Tests OHC function specifically — does not test inner hair cells, CN VIII, or central pathways
Objective, non-invasive, no behavioral response needed
Absent OAEs + normal ABR = central/neural lesion (Auditory Neuropathy)
Present OAEs = cochlear hearing ≤30–40 dB HL (if retrocochlear is intact)
Contraindicated/affected by: Middle ear pathology (fluid, perforation), ear canal debris

Types of OAE

A. Spontaneous OAE (SOAE)

Present without any stimulus
Found in ~35–60% of normally hearing individuals
Absence is non-diagnostic (many normal ears have none)
Rarely used clinically

B. Evoked OAE (EOAE)

i. Transient-Evoked OAE (TEOAE)

Stimulus: Click or brief tone burst
Response: Broad-frequency response (up to 4000 Hz)
Present = cochlear function ≤30–40 dB HL
Primary screening tool — used in Universal Neonatal Hearing Screening (UNHS)
Low stimulus level (<30 dB SPL); requires quiet environment

ii. Distortion Product OAE (DPOAE)

Stimulus: Two simultaneous pure tones (F1 and F2, with F2/F1 ratio ~1.2)
The healthy cochlea generates a distortion product at 2F1–F2 frequency
Stimulus levels: 55–65 dB SPL
More frequency-specific than TEOAE
Can sometimes be recorded even in moderate-to-severe hearing loss
Used to monitor ototoxic drug effects (can detect early cochlear damage before threshold shifts occur)

OAE Summary Table

Feature	TEOAE	DPOAE
Stimulus	Click	Two tones (F1, F2)
Main product	2F1–F2	2F1–F2
Frequency specificity	Low	High
Clinical use	Neonatal screening	Ototoxicity monitoring, frequency-specific cochlear assessment
Threshold correlate	>30–40 dB HL → absent	More sensitive

8. ELECTRICAL RESPONSE AUDIOMETRY (ERA) / AUDITORY EVOKED POTENTIALS (AEP)

ERA records electrical potentials from the auditory pathway in response to sound. Used when behavioral tests are unreliable (infants, difficult-to-test patients).

Classification by Latency

Category	Latency	Tests
Short-latency (<10 ms)	Cochlea + brainstem	ABR, ECoG
Middle-latency (10–50 ms)	Thalamo-cortical	MLR
Long-latency (>50 ms)	Cortical	CERA (N1-P2), P300, MMN

8A. AUDITORY BRAINSTEM RESPONSE (ABR) / BERA / BAER

The most clinically important ERA.

Principle: Click or tone-burst stimuli evoke 5 waves (I–V) within 10 ms, representing sequential activation of auditory neural generators.

Stimulus: Click (broad spectrum, 2–4 kHz region) or tone burst (frequency-specific) Recording: Surface electrodes (vertex positive, mastoid/earlobe negative)

ABR Wave Generators

Wave	Generator
I	Distal end of CN VIII (cochlear)
II	Proximal CN VIII
III	Caudal brainstem (trapezoid body, superior olivary complex)
IV	Superior olivary complex
V	Lateral lemniscus → inferior colliculus
VI, VII	Inferior colliculus

Most reliable waves: I, III, and V The I–V inter-peak latency is the key measure (~4 ms)

ABR — Clinical Applications

Application	Details
Threshold ABR	Estimates hearing threshold (wave V tracked to lowest level); used in infants, non-cooperative patients
Neurologic ABR	High intensity (80–95 dB nHL); detects retrocochlear lesions; acoustic neuroma screening (90% sensitivity, 80% specificity)
Intraoperative monitoring	During posterior fossa/acoustic neuroma surgery
Neonatal screening	Automated ABR (AABR) used in UNHS

ABR — Not Affected By:

Sedation, anesthesia, sleep state, most drugs → ideal for testing infants and uncooperative patients

ABR — Affected By:

Age, sex, body temperature, degree of hearing loss

ABR Abnormalities

Finding	Interpretation
Absent all waves	Profound HL or no auditory function
Wave I present, V absent/delayed	Retrocochlear lesion
Prolonged I–V IPL	Acoustic neuroma, demyelination
No wave, but CM present	Auditory neuropathy spectrum disorder

8B. ELECTROCOCHLEOGRAPHY (ECoG / ECochG)

Principle: Records electrical potentials from the cochlea and proximal CN VIII.

Three potentials recorded:

Summating Potential (SP): DC potential from outer hair cells (>50%) and inner hair cells; represents receptor potential during sound stimulation
Action Potential (AP / N1): From spiral ganglia and distal CN VIII afferents; equivalent to Wave I of ABR
Cochlear Microphonic (CM): AC potential from outer hair cells; mirrors the acoustic waveform exactly

Key Measurement: SP/AP Ratio

Normal: < 0.4 (or <40%)
Elevated SP/AP ratio (>0.4) → Ménière's disease (endolymphatic hydrops)
CM is used to differentiate auditory neuropathy

Electrode placement:

Transtympanic (needle on promontory) — most accurate
Extratympanic (ear canal) — less invasive

8C. AUDITORY STEADY-STATE RESPONSE (ASSR)

Uses modulated tones at specific frequencies (500, 1000, 2000, 4000 Hz)
Allows frequency-specific threshold estimation
Can test multiple frequencies simultaneously
Useful for fitting hearing aids in infants (predicts audiogram shape)
Less affected by stimulus artifact than ABR clicks

8D. MIDDLE LATENCY RESPONSE (MLR)

Latency: 10–50 ms
Waves: Po, Na, Pa, Nb, Pb
Generators: Thalamus (Na), Primary auditory cortex (Pa)
Less commonly used clinically
Useful in assessing auditory processing disorders and cortical hearing loss

8E. CORTICAL ELECTRIC RESPONSE AUDIOMETRY (CERA) — N1-P2

Long-latency response (>50 ms)
Generated in auditory cortex
Waves: P1, N1, P2, N2
Used for threshold estimation in cooperative patients, medicolegal assessment

8F. AUDITORY P300

Latency ~300 ms
Cognitive/event-related potential
Tests auditory discrimination and cognitive processing
Abnormal in dementia, auditory processing disorders
Generator: Hippocampus (medial temporal lobe)

9. BEHAVIORAL AUDIOMETRY (Pediatric)

Since young children cannot perform standard audiometry, age-appropriate behavioral tests are used.

Age-Based Approach

Age	Method
0–6 months	Behavioral Observation Audiometry (BOA)
6–30 months	Visual Reinforcement Audiometry (VRA) / COR
30 months – 5 years	Conditioned Play Audiometry (CPA)
5 years+	Standard pure-tone audiometry

A. Behavioral Observation Audiometry (BOA)

Age: ≤6 months (neurodevelopmental age)
No conditioning; observer watches for reflex responses (eye widening, startle, pause in activity, head turn)
Sound field via loudspeakers or earphones
Minimal Response Level (MRL) obtained — typically suprathreshold (not a true threshold)
Limitations: Response fatigues rapidly, high inter-subject variability, unreliable

B. Visual Reinforcement Audiometry (VRA)

Age: 6–30 months
Child is operantly conditioned to turn head toward sound source, rewarded by animated toy/video
Only requires head turn toward sound (does not need to localize direction)
Insert earphones preferred (avoids collapsing canal)
Conditioned Orienting Response (COR): Variant requiring correct direction localization (right sound → right head turn)

C. Conditioned Play Audiometry (CPA)

Age: 30 months and above
Game-based: Each tone → child performs play activity (drops block in bucket, places ring on peg)
The game itself is the reinforcer
By this age, most children tolerate earphones → ear-specific thresholds possible

10. SITE-OF-LESION TESTING

Tests to differentiate cochlear (sensory) vs. retrocochlear (neural) hearing loss.

Modern (Still Used)

Test	Principle	Cochlear finding	Retrocochlear finding
Acoustic reflex decay	Sustained tone causes reflex to decay	No decay	Decay >50% in ≤5 sec
ABR	Inter-peak latency	Normal or mildly elevated	Prolonged I–V IPL; absent waves
OAE + ABR combination	OHC vs neural function	OAE absent + ABR present (sensory)	OAE present + ABR abnormal (neural = ANSD)

Historical (No Longer Routinely Used)

Test	Principle	Cochlear	Neural
ABLB (Alternate Binaural Loudness Balance)	Balance loudness between ears	Recruitment present	No recruitment
SISI (Short Increment Sensitivity Index)	Detects 1-dB increments	Score >70% (cochlear)	Score <30% (neural)
Tone Decay Test	Auditory adaptation to continuous tone	<25 dB decay	>25 dB decay
Bekesy Audiometry	Pulsed vs. continuous tone tracings	Type II	Type III/IV

Bekesy Types

Type	Tracing	Meaning
I	Continuous ≈ Pulsed (overlap)	Normal
II	Continuous 10–20 dB below pulsed	Cochlear lesion
III	Continuous drops dramatically >20 dB	Neural lesion (severe fatigue)
IV	Continuous consistently >20 dB below pulsed	Neural lesion
V	Continuous better than pulsed	Functional/nonorganic hearing loss

11. AUDITORY NEUROPATHY SPECTRUM DISORDER (ANSD)

A distinct pattern:

OAEs: Present (OHC intact)
Cochlear Microphonic: Present
ABR: Absent or severely abnormal (neural dyssynchrony)
May or may not show threshold loss on PTA
Speech understanding disproportionately poor
Causes: Selective inner hair cell loss, synaptic dysfunction, CN VIII dyssynchrony
Management: Cochlear implant (benefits more than hearing aids)

12. DIAGNOSTIC FLOWCHART (Harrison's Approach)

13. QUICK REVISION — HIGH-YIELD POINTS FOR PG EXAM

Topic	Key Fact
Spondee	2-syllable, equal-stress word (railroad, eardrum)
SRT vs PTA	Should agree within ±10 dB
Carhart notch	BC dip at 2000 Hz → otosclerosis
4 kHz notch	Noise-induced hearing loss
Masking BC	Required when ABG >10 dB
Type B tympanogram	OME (normal ECV) vs. perforation (high ECV)
Type As	Otosclerosis, tympanosclerosis
Type Ad	Ossicular discontinuity
OAE absent + ABR normal	Not possible if cochlear; suggests neural
SP/AP ratio >0.4	Ménière's disease (ECoG)
ABR wave V generator	Lateral lemniscus / inferior colliculus
ABR not affected by	Sedation, anesthesia, sleep
Rollover on WRS	Retrocochlear (acoustic neuroma)
Diphasic acoustic reflex	Early otosclerosis
ANSD pattern	OAE present + ABR absent
VRA age	6–30 months
CPA age	30 months +
BOA limitation	Response extinguishes quickly, suprathreshold only
DPOAE clinical use	Ototoxicity monitoring, frequency-specific OHC testing
AABR	Used in neonatal universal hearing screening

14. SUMMARY OF AUDIOMETRY TYPES

Type	Objective/Subjective	What It Tests	Key Use
Pure-Tone Audiometry	Subjective	Threshold sensitivity, AC & BC	Foundation of hearing assessment
Speech Audiometry (SRT, WRS)	Subjective	Speech detection, recognition, discrimination	Functional hearing, retrocochlear
Tympanometry	Objective	Middle ear pressure & compliance	OME, otosclerosis, perforation
Acoustic Reflex	Objective	Stapedius reflex arc integrity	CN VII/VIII, retrocochlear
OAE (TEOAE, DPOAE)	Objective	OHC function	Neonatal screening, ototoxicity
ABR/BERA	Objective	Auditory nerve + brainstem	Threshold in infants, acoustic neuroma
ECoG	Objective	Cochlear potentials (SP, AP, CM)	Ménière's, ANSD
ASSR	Objective	Frequency-specific cochlear sensitivity	Infant hearing aid fitting
MLR	Objective	Thalamo-cortical pathways	Auditory processing
CERA / P300	Objective	Cortical processing	Medicolegal, cognitive assessment
BOA	Behavioral	Gross hearing in infants	<6 months
VRA / COR	Behavioral + conditioning	Threshold in toddlers	6–30 months
CPA	Behavioral + conditioning	Threshold in young children	30+ months
Bekesy Audiometry	Subjective (automated)	Auditory fatigue/adaptation	Site of lesion (historical)
SISI	Subjective	Recruitment	Site of lesion (historical)
Tone Decay	Subjective	Neural fatigue	Site of lesion (historical)

Compiled from: K J Lee's Essential Otolaryngology (9781260122237) and Cummings Otolaryngology Head and Neck Surgery (9780323612173)

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