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 with the greatest loss at 4 kHz, with an air-bone gap of less than 5 dB. Speech discrimination is relatively preserved at 70%. What is the most likely diagnosis?

Explanation

## Diagnosis: Noise-Induced Hearing Loss (NIHL) **Key Point:** Noise-induced hearing loss is characterized by a **notch at 4 kHz** (or 3–6 kHz range) on pure tone audiometry, which is pathognomonic for occupational noise exposure. ### Audiogram Pattern Recognition | Feature | NIHL | Presbycusis | SSHL | Otosclerosis | |---------|------|------------|------|---------------| | **Frequency affected** | 4 kHz notch (3–6 kHz) | High frequencies (>2 kHz) | Variable, often sudden | Low frequencies initially | | **Symmetry** | Bilateral symmetrical | Bilateral symmetrical | Usually unilateral | Often unilateral | | **Air-bone gap** | <5 dB (sensorineural) | <5 dB (sensorineural) | <5 dB (sensorineural) | >20 dB (conductive) | | **Speech discrimination** | Relatively preserved | Disproportionately poor | Variable | Relatively preserved | | **Onset** | Gradual (years) | Gradual (decades) | Sudden (days) | Gradual (months–years) | | **Occupational risk** | Yes (loud noise) | No | No | No | **High-Yield:** The **4 kHz notch** is the hallmark of NIHL and is tested frequently in NEET PG. This occurs because the cochlear region at 4 kHz is most vulnerable to mechanical trauma from loud noise. **Clinical Pearl:** NIHL is bilateral and symmetrical because both ears experience the same noise exposure. The relative preservation of speech discrimination (70%) is typical of NIHL because the notch is at a specific frequency; lower frequencies (crucial for speech) remain relatively intact. **Mnemonic: "NIHL = 4 kHz Notch"** — Remember the 4 kHz notch as the signature finding of noise-induced hearing loss. ### Pathophysiology Noise exposure causes mechanical damage to outer hair cells in the cochlea, with maximum vulnerability at the 4 kHz region due to the resonance characteristics of the cochlear partition. Chronic exposure leads to permanent hair cell loss and sensorineural hearing loss. ### Why This Patient? - **15-year occupational exposure** (factory worker) → classic NIHL timeline - **Bilateral and symmetrical** → both ears equally exposed - **Sensorineural pattern** (air-bone gap <5 dB) → cochlear origin - **4 kHz notch** → pathognomonic for noise exposure - **Preserved speech discrimination** → high-frequency notch does not severely impair speech perception [cite:Hazarika ENT Ch 5]