A 52-year-old man presents with progressive bilateral hearing loss over 6 months. Pure tone audiometry shows air-bone gap of 35 dB bilaterally. All of the following are possible causes EXCEPT:

Explanation

## Air-Bone Gap: Marker of Conductive Pathology ### Understanding Air-Bone Gap **Key Point:** An air-bone gap (difference between air conduction and bone conduction thresholds) is **pathognomonic for conductive hearing loss**. A gap of 35 dB indicates significant middle or external ear dysfunction. **High-Yield:** Air-bone gap = AC threshold − BC threshold. A gap >10 dB is abnormal and suggests conductive pathology. ### Why Sudden SNHL Does NOT Cause Air-Bone Gap **Clinical Pearl:** Sudden sensorineural hearing loss (SSNHL) affects the cochlea or auditory nerve directly. Both air conduction and bone conduction are equally impaired because the problem is in the sensory end-organ, not the conduction pathway. Therefore, **no air-bone gap** develops. **Mnemonic:** **SNHL = Both AC and BC down equally** (parallel drop, no gap). **Conductive HL = AC down, BC normal** (gap widens). ### Conductive Causes of Air-Bone Gap | Cause | Mechanism | Air-Bone Gap | |-------|-----------|---------------| | **Otosclerosis** | Stapes fixation → impaired ossicular motion | Present (typically 40–50 dB) | | **Chronic OM with ossicular erosion** | Erosion of malleus/incus → conduction loss | Present (varies with extent) | | **Cerumen impaction** | Blockage of external auditory canal | Present (typically 20–40 dB) | | **Sudden SNHL** | Cochlear dysfunction → equal AC & BC loss | **Absent** | **Warning:** Do not confuse SNHL with conductive loss. SNHL presents with elevated thresholds for BOTH AC and BC, with BC remaining better than AC (no gap). ### Clinical Context The patient has a **35 dB air-bone gap bilaterally**, which is diagnostic of conductive loss. Sudden SNHL, while it can present with bilateral hearing loss, does NOT produce an air-bone gap because the cochlea is equally affected for both conduction routes. --- ## Decision Tree: Interpreting Audiometry ```mermaid flowchart TD A[Hearing Loss on Audiometry]:::outcome --> B{Air-Bone Gap Present?}:::decision B -->|Yes, gap >10 dB| C[Conductive Hearing Loss]:::outcome B -->|No gap, AC=BC| D[Sensorineural Hearing Loss]:::outcome C --> E[Causes: Otosclerosis, OM, Cerumen, TM perforation]:::action D --> F[Causes: SNHL, Presbycusis, Noise-induced, Ototoxicity]:::action ```