A 58-year-old male factory worker presents with progressive bilateral hearing loss over 15 years. He reports difficulty hearing high-pitched sounds and conversations in noisy environments. Otoscopy is normal. Pure tone audiometry shows bilateral symmetrical sensorineural hearing loss, worse at 3000–6000 Hz with a characteristic 'notch' at 4000 Hz. Speech discrimination is relatively preserved. What is the most likely diagnosis?

Explanation

## Diagnosis: Noise-Induced Hearing Loss (NIHL) **Key Point:** The characteristic **4 kHz notch** (dip) on audiogram is pathognomonic for noise-induced hearing loss. This is the most sensitive early sign of cochlear damage from chronic noise exposure. **High-Yield:** NIHL audiometric features: - Bilateral and symmetrical sensorineural hearing loss - **4 kHz notch** — the classic finding (can also affect 3000–6000 Hz range) - Worse at high frequencies - Speech discrimination relatively preserved (unlike presbycusis) - Gradual onset over years of exposure **Clinical Pearl:** The 4 kHz notch occurs because the cochlear region most sensitive to mechanical damage from noise vibrations corresponds to the 4 kHz frequency. This is why factory workers, construction workers, and military personnel are at high risk. **Mnemonic:** **NIHL = Notch at 4 kHz** — the 4 kHz notch is virtually diagnostic of occupational or recreational noise exposure. ## Why This Patient Fits NIHL: 1. **Occupational exposure** — 15-year history as factory worker (chronic noise) 2. **Bilateral symmetrical pattern** — consistent with environmental exposure 3. **High-frequency loss worse at 4 kHz** — classic notch pattern 4. **Normal otoscopy** — sensorineural, not conductive 5. **Preserved speech discrimination** — cochlear outer hair cell damage, not central auditory pathology ## Differential Considerations: | Feature | NIHL | Presbycusis | Ototoxicity | SSNHL | | --- | --- | --- | --- | --- | | **Onset** | Gradual (years) | Gradual (decades) | Variable | Sudden (days) | | **Pattern** | 4 kHz notch | High-freq sloping | High-freq sloping | Variable | | **Bilaterality** | Bilateral symmetrical | Bilateral symmetrical | Often bilateral | Unilateral | | **Speech discrimination** | Preserved | Poor | Poor | Variable | | **History** | Noise exposure | Age >60 | Ototoxic drugs | Viral/vascular | [cite:Hazarika ENT 5e Ch 12]