A 58-year-old man presents to the ENT clinic with progressive bilateral hearing loss over the past 3 years. He reports difficulty hearing high-pitched sounds and tinnitus. He works as a factory supervisor and has been exposed to loud machinery for 30 years. Otoscopy is normal. Pure tone audiometry shows bilateral symmetrical sensorineural hearing loss with a characteristic notch at 4 kHz, with air-bone gap absent. Speech discrimination is relatively preserved. What is the most likely diagnosis?

Explanation

## Diagnosis: Noise-Induced Hearing Loss (NIHL) ### Audiometric Pattern Recognition **Key Point:** The pathognomonic finding in noise-induced hearing loss is a **notch at 4 kHz** (sometimes 3–6 kHz range), which is the frequency most sensitive to acoustic trauma. **High-Yield:** The characteristic 4 kHz notch is virtually diagnostic of NIHL. This occurs because: 1. The cochlear resonance at 4 kHz amplifies sound energy 2. The stapes footplate resonance also peaks near this frequency 3. Outer hair cells at the 4 kHz location are maximally vulnerable to mechanical trauma ### Clinical Features Supporting NIHL | Feature | NIHL | Presbycusis | SSHL | Otosclerosis | |---------|------|-------------|------|---------------| | **Pattern** | Notch at 4 kHz | Gradual high-freq slope | Sudden, any freq | Low-freq or flat | | **Onset** | Gradual (years) | Gradual (decades) | Acute (days) | Progressive | | **Bilateral** | Yes, symmetrical | Yes | Usually unilateral | Often unilateral | | **Air-bone gap** | Absent | Absent | Absent | Present (early) | | **Speech discrimination** | Relatively preserved | Disproportionately poor | Variable | Preserved | | **Tinnitus** | Common | Common | Very common | Less common | **Clinical Pearl:** The 4 kHz notch may fill in with time (recovery phase), but permanent threshold shift occurs if exposure continues. This patient's 30-year occupational exposure and classic notch pattern make NIHL the diagnosis. **Mnemonic: NIHL Features — "4 kHz Notch, Noise, Bilateral, Preserved Speech"** - **4 kHz notch** = pathognomonic audiometric sign - **Noise exposure** = occupational or recreational history - **Bilateral symmetrical** = both ears affected equally - **Preserved speech discrimination** = cochlear damage spares auditory nerve ### Why This Case Is NIHL 1. **Occupational history:** 30 years of machinery exposure 2. **Bilateral symmetrical pattern:** Consistent with chronic noise exposure 3. **4 kHz notch:** Diagnostic finding 4. **Normal otoscopy:** Rules out conductive pathology 5. **Absent air-bone gap:** Confirms sensorineural origin 6. **Preserved speech discrimination:** Typical of cochlear (outer hair cell) damage rather than retrocochlear or conductive loss