A 58-year-old male factory worker presents with progressive bilateral hearing loss over 15 years. He reports tinnitus and difficulty hearing high-pitched sounds. Pure tone audiometry shows bone conduction thresholds within normal limits (0–15 dB HL), while air conduction thresholds are elevated at 500 Hz (35 dB HL), 1 kHz (40 dB HL), 2 kHz (45 dB HL), 4 kHz (50 dB HL), and 8 kHz (48 dB HL). Speech discrimination is preserved at 88%. Otoscopy is normal. What is the most likely diagnosis?

Explanation

## Audiogram Pattern Analysis **Key Point:** The audiogram shows a characteristic **air-bone gap** (difference between air and bone conduction thresholds), confirming sensorineural hearing loss. The pattern of maximum loss at 4 kHz with relative preservation at 8 kHz is pathognomonic for noise-induced hearing loss. ### Audiometric Features of Noise-Induced Hearing Loss | Feature | Noise-Induced HL | Presbycusis | Conductive HL | |---------|------------------|-------------|---------------| | **Air-bone gap** | Present (sensorineural) | Present (sensorineural) | Present (air > bone) | | **Peak loss frequency** | 3–6 kHz (classically 4 kHz) | Gradual slope 500–8 kHz | Flat or low-frequency | | **Tinnitus** | Common, high-pitched | Less prominent | Absent | | **Speech discrimination** | Preserved early | Disproportionately reduced | Normal | | **Bilateral symmetry** | Yes | Yes | Often asymmetric | | **Bone conduction** | Normal | Normal | Elevated | **High-Yield:** The **4 kHz notch** (dip at 4 kHz with relative sparing at 8 kHz) is the hallmark of occupational/noise-induced hearing loss. This occurs because the cochlea's resonance frequency at the 4 kHz region is maximally vulnerable to acoustic trauma. ### Clinical Pearl The patient's 15-year occupational exposure in a factory, combined with the classic 4 kHz notch and preserved speech discrimination, makes noise-induced hearing loss the most likely diagnosis. The normal bone conduction rules out conductive pathology. ### Differential Reasoning - **Presbycusis** typically shows a gradual high-frequency slope (worse at 8 kHz than 4 kHz) and disproportionate speech discrimination loss. - **Conductive hearing loss** would show bone conduction thresholds within normal limits but air conduction elevated across all frequencies equally; no 4 kHz notch. - **Mixed hearing loss** would require both an air-bone gap AND abnormal bone conduction, which this patient does not have. [cite:Dhingra 7e Ch 8]