Two patients undergo audiometric testing. Patient A, a 28-year-old with sudden sensorineural hearing loss and a positive Romberg test, shows a bone conduction threshold of 45 dB at 2 kHz and air conduction of 50 dB at the same frequency. Patient B, a 56-year-old with progressive bilateral hearing loss, shows bone conduction of 35 dB and air conduction of 55 dB at 2 kHz. Which finding best distinguishes conductive hearing loss from sensorineural hearing loss in these audiograms?

Explanation

## Air-Bone Gap: The Gold Standard Discriminator ### Definition and Calculation The **air-bone gap (ABG)** is the difference between air conduction (AC) and bone conduction (BC) thresholds at the same frequency: $$\text{Air-Bone Gap} = \text{AC threshold (dB)} - \text{BC threshold (dB)}$$ ### Interpretation | Air-Bone Gap | Interpretation | Pathology | |--------------|----------------|----------| | **≤5 dB** | Normal or sensorineural | Inner ear or retrocochlear | | **6–10 dB** | Borderline (may be normal variation) | Mild conductive or mixed | | **≥15 dB** | Significant conductive loss | Middle ear pathology | | **>30 dB** | Severe conductive loss | Ossicular fixation, TM perforation, stapes fixation | ### Application to the Cases **Patient A (Sudden SNHL):** - AC = 50 dB, BC = 45 dB - ABG = 50 − 45 = **5 dB** → **Sensorineural loss** (BC affected equally) **Patient B (Progressive bilateral loss):** - AC = 55 dB, BC = 35 dB - ABG = 55 − 35 = **20 dB** → **Conductive or mixed loss** (AC worse than BC) **Key Point:** The air-bone gap is the **single most reliable audiometric discriminator** between conductive and sensorineural hearing loss. A gap ≥15 dB indicates conductive pathology; ≤5 dB indicates sensorineural pathology. **High-Yield:** - **Conductive loss** = AC ↓↓, BC ↑ (normal) → **ABG widens** - **Sensorineural loss** = AC ↓ and BC ↓ equally → **ABG ≤5 dB** - **Mixed loss** = Both AC and BC ↓, but AC worse → **ABG 6–15 dB** **Clinical Pearl:** In conductive hearing loss, bone conduction remains at or near normal limits because the inner ear is intact. The problem is in sound transmission (middle ear). In sensorineural loss, both AC and BC are affected equally because the cochlea itself is damaged. ### Why This Matters Clinically - **ABG ≥15 dB** → Refer for otoscopy, tympanometry, and imaging (may need surgery) - **ABG ≤5 dB** → Investigate cochlear/retrocochlear causes (MRI, imaging for acoustic neuroma, metabolic workup) [cite:Schuknecht's Pathology of the Ear 3e Ch 3; Ramsden RT (2002) Ménière's disease. BMJ 324:1133–1134]