A patient with sudden sensorineural hearing loss shows a characteristic notch on the audiogram at 4 kHz with better thresholds at 8 kHz. What is this finding called?

Explanation

## Acoustic Notch in Audiometry **Key Point:** The characteristic notch at 4 kHz on an audiogram — with better (improved) thresholds at 8 kHz — is called the **acoustic notch** (also known as the "4 kHz notch" or "noise notch"). It is the hallmark audiometric finding in noise-induced hearing loss (NIHL) and can also be seen in sudden sensorineural hearing loss. ### Definition and Characteristics The acoustic notch represents: - A **selective depression (worsening) of hearing threshold at 4 kHz** - Relative recovery (better thresholds) at 8 kHz - Air conduction and bone conduction thresholds both affected equally (sensorineural pattern) - Typically bilateral and symmetric in chronic NIHL; may be unilateral in acute/sudden SNHL ### Why 4 kHz? | Feature | Details | |---------|----------| | **Anatomical basis** | The outer hair cells of the basal cochlear turn (tuned to ~4 kHz) are most vulnerable to acoustic trauma due to resonance characteristics of the ear canal and cochlear mechanics | | **Classic cause** | Noise-induced hearing loss (occupational or recreational noise exposure) | | **Also seen in** | Sudden sensorineural hearing loss, ototoxicity (early stages) | | **Recovery at 8 kHz** | Distinguishes acoustic notch from presbycusis (which shows progressive high-frequency loss without recovery) | ### Why NOT Carhart's Notch? **Carhart's notch** is a dip in **bone conduction** thresholds at **2 kHz** (classically) seen in **otosclerosis** (stapes fixation). It is a mechanical artifact of ossicular chain dysfunction — not a true sensorineural loss — and typically recovers after stapedectomy. The stem describes a **sensorineural** pattern at **4 kHz**, which is inconsistent with Carhart's notch. **High-Yield:** Acoustic notch at 4 kHz = noise-induced or sudden SNHL. Carhart's notch at 2 kHz = otosclerosis (mixed hearing loss, bone conduction dip). **Clinical Pearl:** According to Scott-Brown's Otorhinolaryngology and Cummings Otolaryngology, the 4 kHz acoustic notch is pathognomonic of cochlear damage from acoustic trauma. The notch at 4 kHz (not 2 kHz) with recovery at 8 kHz is the defining feature of the acoustic notch. **Mnemonic:** **4 kHz Notch = Noise** (acoustic trauma → acoustic notch).