A 62-year-old man with chronic kidney disease (eGFR 28 mL/min/1.73m²) is prescribed gentamicin for a suspected gram-negative sepsis. After 5 days of therapy, he develops high-frequency sensorineural hearing loss and vertigo. Which investigation is most appropriate to confirm aminoglycoside-induced ototoxicity?

## Aminoglycoside Ototoxicity: Diagnosis and Confirmation ### Clinical Context Aminoglycosides cause dose-dependent, irreversible ototoxicity via damage to cochlear and vestibular hair cells. Risk is amplified in renal impairment (reduced drug clearance) and prolonged therapy. ### Investigation of Choice: High-Frequency Audiometry **Key Point:** High-frequency audiometry (4–8 kHz range) is the gold standard for detecting aminoglycoside-induced cochlear damage because: 1. Aminoglycosides preferentially damage the basal turn of the cochlea (high-frequency region) 2. High-frequency hearing loss precedes clinically noticeable loss at speech frequencies 3. Early detection allows drug discontinuation before irreversible damage ### Why High-Frequency Audiometry? | Feature | High-Frequency Audiometry | Alternative Tests | |---------|--------------------------|-------------------| | **Sensitivity** | Detects early cochlear injury (4–8 kHz) | Serum levels miss tissue damage; MRI shows structural changes only late | | **Specificity** | Characteristic pattern of aminoglycoside ototoxicity | ENG tests vestibular function, not cochlear damage | | **Timing** | Can detect before patient awareness of hearing loss | Serum levels guide dosing but don't confirm ototoxicity | | **Reversibility** | Helps decide if drug must be stopped | MRI cannot reverse damage | **High-Yield:** Baseline audiometry before aminoglycoside therapy and serial testing during treatment (especially in high-risk patients) is standard practice. ### Clinical Pearl Patients may not notice high-frequency hearing loss clinically until it progresses to speech frequencies (500–3000 Hz). Serial high-frequency audiometry is therefore a sentinel test for early toxicity. **Mnemonic:** **OTOTOX** = **O**toacoustic emissions (OAE) and **T**onal audiometry (high-frequency) are **O**ptimal; **T**rough levels guide dosing; **O**tocyst damage is irreversible; **X** marks the spot at 4–8 kHz. ### Why Not the Alternatives? 1. **Serum gentamicin peak/trough levels** — Guides dosing and prevents toxicity but does NOT confirm established ototoxicity; levels can be therapeutic yet toxicity still occurs. 2. **Electronystagmography (ENG) with caloric testing** — Assesses vestibular function (vertigo, nystagmus) but is not the confirmatory test for cochlear damage; vestibular and cochlear toxicity can dissociate. 3. **MRI of internal auditory canal** — Structural imaging is not the first-line diagnostic test; it may show late changes but is expensive and not sensitive for early ototoxicity. [cite:KD Tripathi 8e Ch 50]