A 58-year-old male factory worker presents with progressive bilateral hearing loss over 15 years. He reports difficulty hearing high-pitched sounds and tinnitus. Otoscopy is normal. Pure tone audiometry shows bilateral symmetrical sensorineural hearing loss with a characteristic dip at 4 kHz, sparing 8 kHz. Speech discrimination is relatively preserved. What is the most likely diagnosis?

## Diagnosis: Noise-Induced Hearing Loss (NIHL) **Key Point:** The characteristic 4 kHz dip (notch) on audiogram is pathognomonic for noise-induced hearing loss, even when 8 kHz is relatively spared. This is called the **Carhart notch** or **4 kHz notch**. **High-Yield:** Noise-induced hearing loss has several distinctive audiometric features: - Bilateral and symmetrical sensorineural hearing loss - Maximum loss at 3–6 kHz (classically 4 kHz) - Relatively preserved speech discrimination early in disease - Progressive over years of exposure - Occupational history (factory, construction, military) is a key clue **Clinical Pearl:** The 4 kHz notch occurs because the cochlea's resonance frequency and the mechanics of sound transmission make the 4 kHz region maximally vulnerable to acoustic trauma. As exposure continues, the notch widens and deepens, eventually affecting adjacent frequencies. **Mnemonic:** **NIHL = 4 kHz Notch** — when you see this dip on an audiogram in a worker with noise exposure, NIHL is the answer. ### Why This Patient Has NIHL - 15-year occupational exposure (factory worker) - Bilateral symmetrical sensorineural loss (hallmark of cochlear damage) - Characteristic 4 kHz dip - Preserved speech discrimination (indicates outer/middle ear intact, cochlear loss) - Normal otoscopy (rules out conductive pathology) ### Audiogram Pattern in NIHL | Frequency (Hz) | 500 | 1000 | 2000 | 4000 | 8000 | | --- | --- | --- | --- | --- | --- | | Typical NIHL | Normal | Mild loss | Moderate loss | **Maximum loss (notch)** | Moderate loss | | Presbycusis | Mild | Mild | Moderate | Moderate | Severe | | Ototoxicity | Normal | Normal | Moderate | Moderate | Severe |