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 conversation in noisy environments. Audiometry reveals a characteristic "notch" at 4 kHz bilaterally, with air-bone gap of 5 dB. Speech discrimination is preserved at 88%. Tympanometry is normal. What is the most likely diagnosis?

Explanation

## Audiogram Interpretation: Noise-Induced Hearing Loss ### Clinical Presentation The patient presents with progressive bilateral high-frequency sensorineural hearing loss with a characteristic **4 kHz notch** — the pathognomonic finding in noise-induced hearing loss (NIHL). ### Key Audiometric Features | Feature | Finding | Significance | |---------|---------|---------------| | Pattern | 4 kHz notch | Pathognomonic for NIHL | | Frequency affected | High frequencies (4–6 kHz) | Cochlear base most vulnerable | | Air-bone gap | 5 dB | Essentially absent (normal ≤10 dB) | | Speech discrimination | 88% | Preserved, typical of cochlear loss | | Tympanometry | Normal | Rules out conductive pathology | **Key Point:** The **4 kHz notch** is the classic audiometric signature of noise-induced hearing loss. It occurs because the cochlear base (which processes high frequencies) is most susceptible to acoustic trauma. ### Pathophysiology 1. Acoustic trauma damages outer hair cells at the 4 kHz region (cochlear base) 2. Progressive loss extends to adjacent frequencies with continued exposure 3. Bilateral and symmetric pattern (occupational or environmental noise) 4. Sensorineural type (air-bone gap absent or minimal) ### Why Speech Discrimination Is Preserved In NIHL, the cochlear damage is primarily to outer hair cells in the high-frequency region. Speech intelligibility depends on mid-frequency consonants (1–3 kHz), which are relatively spared early in the disease, explaining the preserved discrimination score of 88%. **Clinical Pearl:** Asymmetric NIHL or unilateral presentation should raise suspicion for retrocochlear pathology (acoustic neuroma) and warrant MRI. **High-Yield:** The 4 kHz notch is so characteristic that its absence in a patient with suspected NIHL should prompt investigation for other etiologies (ototoxicity, Ménière's disease, sudden sensorineural hearing loss).