A 38-year-old woman with a 2-year history of progressive hearing loss undergoes audiometry showing air-bone gap of 35 dB and normal bone conduction thresholds. Which feature most reliably distinguishes this conductive hearing loss from sensorineural hearing loss?

Explanation

## Air-Bone Gap: The Gold Standard Discriminator ### Definition and Significance **Key Point:** The **air-bone gap (ABG)** is the difference in decibels between air conduction (AC) and bone conduction (BC) thresholds at the same frequency. An ABG ≥20 dB is pathognomonic for conductive hearing loss. ### Audiometric Interpretation | Parameter | Conductive Loss | Sensorineural Loss | |-----------|-----------------|--------------------| | **Air conduction threshold** | Elevated (worse) | Elevated (worse) | | **Bone conduction threshold** | Normal (good) | Elevated (worse) | | **Air-bone gap** | **≥20 dB (present)** | **Absent (0 dB)** | | **Speech discrimination** | Normal or near-normal | Often reduced | | **Otoacoustic emissions** | Normal | Abnormal/absent | **High-Yield:** The air-bone gap is the *single most specific* audiometric finding for conductive hearing loss. It directly reflects the mechanical impedance mismatch in the middle ear. ### Mechanism In conductive loss: 1. Sound via air conduction must traverse the obstructed middle ear → threshold rises 2. Sound via bone conduction bypasses the middle ear → threshold remains normal 3. The difference = air-bone gap In sensorineural loss: - Both pathways (air and bone) depend on cochlear function - If the cochlea is damaged, *both* AC and BC thresholds rise equally - No gap develops **Clinical Pearl:** A normal bone conduction threshold with elevated air conduction is virtually diagnostic of conductive loss. This patient's 35 dB ABG with normal BC is classic for conductive pathology (e.g., otosclerosis, ossicular discontinuity, chronic otitis media). **Mnemonic:** **ABG = Air Bone Gap = Always Indicates conductive pathology** (when ≥20 dB).