Audiometry

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Now I have comprehensive material. Here is a complete guide to interpreting audiograms.

Audiogram Interpretation

The Audiogram — Basic Layout

The audiogram plots frequency (Hz) on the horizontal axis (low pitch on the left → high pitch on the right: 125 to 8000 Hz) against hearing threshold level (dB HL) on the vertical axis — but crucially, worse hearing is downward (0 dB HL at top = normal; 120 dB HL at bottom = profound loss).

Step 1 — Assess Severity

Thresholds are classified per the ANSI standard:

Threshold (dB HL)	Grade
−10 to 25	Normal
26–40	Mild
41–55	Moderate
56–70	Moderately severe
71–90	Severe
> 90	Profound

The pure-tone average (PTA) summarises severity: average of thresholds at 500, 1000, and 2000 Hz.

Step 2 — Read the Symbols

Symbol	Meaning
O (circle, right)	Right ear, air conduction (unmasked)
X (cross, left)	Left ear, air conduction (unmasked)
△ (triangle)	Air conduction, masked
[ ] (bracket pointing right)	Right ear, bone conduction (unmasked)
] [ (bracket pointing left)	Left ear, bone conduction (unmasked)
< > (angle brackets)	Bone conduction, masked

Step 3 — Determine Type of Loss

Air-Bone Gap (ABG)

The key concept:

Air conduction (AC) tests the entire auditory pathway (outer ear → middle ear → cochlea → auditory nerve → brain)
Bone conduction (BC) bypasses the outer and middle ears and stimulates the cochlea directly

ABG = AC threshold − BC threshold at a given frequency. An ABG > 10 dB is significant.

Conductive Hearing Loss

BC is normal; AC is elevated → air-bone gap is present

Bone-conduction thresholds (brackets) sit at 0 dB — normal cochlear function
Air-conduction thresholds (circles/triangles) are depressed by 30–40 dB
The vertical gap between the two lines is the air-bone gap
Implies outer or middle ear pathology (e.g., cerumen impaction, otitis media, otosclerosis, ossicular discontinuity)
Speech discrimination scores are typically preserved once stimuli are loud enough

Sensorineural Hearing Loss (SNHL)

Both BC and AC are equally elevated → no air-bone gap

Air-conduction and bone-conduction thresholds track each other (both circles and angle brackets overlap or are very close)
No gap because the cochlea itself (or the auditory nerve) is impaired
Implies cochlear or retrocochlear pathology (e.g., noise-induced, presbycusis, ototoxicity, acoustic neuroma, Menière's disease)
Speech discrimination scores are often reduced, especially in retrocochlear lesions

Mixed Hearing Loss

Both BC and AC are elevated, but an air-bone gap is still present

BC is elevated (cochlear component)
AC is elevated further (additional conductive component)
The gap between AC and BC represents the conductive overlay on top of SNHL
Example: patient with presbycusis who also develops chronic otitis media

Step 4 — Recognise Audiogram Patterns

Common shapes point to specific diagnoses:

Pattern	Appearance	Typical Cause
High-frequency notch at 4 kHz	Dip at 4000 Hz, recovery at 8000 Hz	Noise-induced hearing loss (NIHL)
High-frequency sloping loss	Progressive drop from 2–8 kHz	Presbycusis (age-related)
Low-frequency loss	Elevated thresholds at 250–1000 Hz	Menière's disease (early stage)
Flat loss	Equal elevation across all frequencies	Otosclerosis, hereditary SNHL
Cookie-bite / U-shaped	Mid-frequency notch (1–2 kHz)	Hereditary/genetic loss
Rising loss (low-freq worst)	Worst at low, better at high	Endolymphatic hydrops

Step 5 — Speech Tests

Three tests complement the pure-tone audiogram:

Test	What it measures	Key facts
Speech Detection Threshold (SDT)	Lowest level to detect presence of speech	~equals PTA
Speech Reception Threshold (SRT)	Level at which 50% of words are correctly repeated	~8–9 dB above SDT
Speech Discrimination / Word Recognition	% of monosyllabic words correctly recognised	Presented 50 dB above SRT

Interpretation rule:

Conductive loss → discrimination near normal once volume is sufficient
Cochlear SNHL → reduced discrimination
Retrocochlear (CN VIII, auditory cortex) → disproportionately poor discrimination even with near-normal thresholds ("rollover" phenomenon)

Step 6 — Masking

Masking is noise delivered to the non-test ear to prevent it from responding to signals intended for the test ear (called crossover).

When to mask:

Air conduction: mask when AC stimulus to test ear exceeds the BC threshold of the non-test ear by ≥40 dB (supra-aural headphones) or ≥70 dB (insert earphones)
Bone conduction: mask whenever the test ear has an air-bone gap > 10 dB (because BC interaural attenuation is essentially 0 dB — the oscillator stimulates both cochleas simultaneously)

Summary Table

Feature	Conductive	Sensorineural	Mixed
AC threshold	Elevated	Elevated	Elevated
BC threshold	Normal	Elevated	Elevated
Air-bone gap	Present (>10 dB)	Absent	Present
Site of lesion	Outer/middle ear	Cochlea/CN VIII	Both
Speech discrimination	Good	Reduced	Variable
Typical examples	Otitis media, otosclerosis	Presbycusis, NIHL, Menière's	Otosclerosis + presbycusis

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

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