A 58-year-old man with a 20-year history of occupational noise exposure presents with bilateral hearing loss. His audiogram shows a characteristic U-shaped curve with a prominent dip at 4 kHz, normal low-frequency thresholds, and air-bone gap of 10 dB at 2 kHz. All of the following statements about his audiometric pattern are correct EXCEPT:

Explanation

## Noise-Induced Hearing Loss (NIHL) — Audiometric Interpretation ### Clinical Presentation of NIHL **Key Point:** The classic audiometric pattern of noise-induced hearing loss is a **U-shaped curve with a C5 notch (4 kHz dip)**, but this pattern is NOT pathognomonic — it can also be seen in ototoxicity (aminoglycosides, cisplatin), syphilis, and other cochlear insults. ### Characteristic Audiometric Features of NIHL | Feature | Significance | Differential Consideration | |---------|--------------|----------------------------| | **4 kHz notch (C5 dip)** | Resonance frequency of external auditory canal | Seen in noise, ototoxins, syphilis | | **U-shaped curve** | Preservation of low frequencies | Typical of cochlear damage | | **Bilateral symmetry** | Occupational exposure pattern | Asymmetry suggests retrocochlear lesion | | **High-frequency loss** | Basal turn cochlear damage | Progressive with continued exposure | | **No air-bone gap** | Pure sensorineural pattern | Excludes ossicular involvement | ### Why Option 0 Is Wrong **High-Yield:** The 4 kHz notch is **NOT pathognomonic for NIHL**. While it is highly characteristic, the same pattern can occur with: 1. **Ototoxic drugs** (aminoglycosides, loop diuretics, cisplatin) 2. **Congenital syphilis** (Henle's knife-edge notch) 3. **Sudden sensorineural hearing loss** (early phase) 4. **Presbycusis** (age-related, though usually more gradual) The notch is a sign of **cochlear damage**, not a specific etiology. Diagnosis of NIHL requires **history of noise exposure** + compatible audiometric pattern, not the notch alone. **Clinical Pearl:** A patient with a 4 kHz notch and no occupational noise history should prompt investigation for ototoxicity, syphilis, or other systemic causes before attributing it to noise. ### Why Options 1, 2, and 3 Are Correct **Option 1 (Normal low frequencies + high-frequency loss):** This is the expected pattern in early NIHL. The cochlear apex (low frequencies) is spared; basal turn damage (high frequencies) occurs first with noise exposure. **Option 2 (Small air-bone gap at 2 kHz):** A 10 dB gap is borderline and may represent: - Concurrent mild conductive pathology (cerumen, serous otitis) - Measurement artifact or transducer placement error - Not typical of pure NIHL, which should show no gap **Option 3 (Recruitment in cochlear SNHL):** Recruitment is the hallmark of cochlear damage and distinguishes it from retrocochlear (VIIIth nerve) pathology, where recruitment is absent or reduced. ### Mnemonic: NIHL vs. Ototoxicity **"NIHL = Noise History; Ototoxins = Drug History"** - Both produce 4 kHz notch - Both are bilateral and symmetric - **Distinction:** Clinical history (occupational vs. medication exposure) - Audiometric pattern alone CANNOT differentiate them