A 62-year-old man presents with progressive bilateral hearing loss over 10 years. Audiometry shows bone conduction thresholds within normal limits but air conduction thresholds elevated across all frequencies with an air-bone gap of 35 dB. A 58-year-old woman with similar age of onset shows bilateral high-frequency sensorineural hearing loss (4–8 kHz) with air and bone conduction equally affected and no air-bone gap. Which audiometric finding best distinguishes conductive from sensorineural hearing loss in these two patients?

Explanation

## Distinguishing Conductive from Sensorineural Hearing Loss ### Audiometric Hallmark **Key Point:** The air-bone gap (ABG) is the single most reliable discriminator between conductive and sensorineural hearing loss (SNHL). An ABG ≥20 dB indicates conductive pathology; an ABG ≤10 dB indicates SNHL. ### Mechanism **High-Yield:** In conductive hearing loss, the middle ear ossicular chain fails to transmit sound energy efficiently to the cochlea. Bone conduction bypasses the middle ear entirely—vibrations travel directly through bone to the inner ear—so bone conduction thresholds remain normal. Air conduction (which depends on middle ear transmission) is elevated, creating the gap. In SNHL, the cochlea or retrocochlear pathway is damaged. Both air and bone conduction are equally affected because both ultimately rely on cochlear function. No gap exists. ### Comparison Table | Feature | Conductive Loss | Sensorineural Loss | |---------|-----------------|--------------------| | **Air-bone gap** | ≥20 dB (pathognomonic) | ≤10 dB (absent) | | **Bone conduction** | Normal (≤20 dB HL) | Elevated (>20 dB HL) | | **Air conduction** | Elevated | Elevated equally | | **Pattern** | Flat or low-frequency | High-frequency or flat | | **Common causes** | Otitis media, otosclerosis, ossicular fixation | Presbycusis, noise-induced, SNHL | **Clinical Pearl:** In the stem, the first patient (man, 35 dB ABG, normal bone conduction) has conductive loss (likely otosclerosis given age and progression). The second patient (woman, no ABG, bilateral high-frequency loss) has SNHL (likely presbycusis). ### Why ABG is the Best Discriminator **Mnemonic:** **ABG = Air minus Bone Gap** - If ABG ≥20 dB → Conductive - If ABG ≤10 dB → Sensorineural - If ABG 10–20 dB → Mixed (both components present) This single measurement answers the question: "Is the problem in sound transmission (middle ear) or sound perception (cochlea)?" [cite:Dhingra 7e Ch 4] --- ## Why Other Options Fail **Option 1 (Elevation of bone conduction thresholds):** This indicates SNHL, not the discriminator between the two. Both conductive and SNHL can be bilateral and progressive—these are not discriminators. A normal bone conduction in the presence of elevated air conduction IS the air-bone gap, so this is correct but less direct than stating the gap itself. **Option 2 (Bilateral symmetric pattern):** Both conductive and SNHL can be bilateral and symmetric. Otosclerosis is typically bilateral; presbycusis is also bilateral. This feature does not distinguish them. **Option 3 (Progressive nature over years):** Both conditions progress. Otosclerosis progresses over decades; presbycusis progresses with age. Progression alone is not discriminatory.