A 68-year-old man with chronic kidney disease (eGFR 35 mL/min/1.73 m²) is prescribed gentamicin for a gram-negative urinary tract infection. After 5 days of therapy, he develops sudden-onset high-frequency hearing loss and tinnitus. Which investigation is most appropriate to confirm aminoglycoside-induced ototoxicity?

## Aminoglycoside-Induced Ototoxicity: Investigation of Choice ### Clinical Context Aminoglycosides cause irreversible, dose-dependent ototoxicity via damage to cochlear hair cells. The toxicity is **cumulative** and **irreversible**, making early detection critical. ### Why Audiometry is the Investigation of Choice **Key Point:** High-frequency audiometry (4–8 kHz range) is the **gold standard** for detecting aminoglycoside-induced ototoxicity because: 1. Aminoglycoside damage begins at **high frequencies (4–8 kHz)** before affecting speech frequencies 2. Early detection at high frequencies allows **timely drug discontinuation** before irreversible hearing loss progresses to clinically significant frequencies 3. Serial audiometry can track progression and guide therapeutic decisions ### Mechanism of Ototoxicity **High-Yield:** Aminoglycosides accumulate in the **endolymph** of the inner ear and cause: - Irreversible destruction of outer hair cells (cochlea) - Damage to vestibular hair cells (less common but more severe) - Risk increases with: **prolonged therapy, renal impairment, high cumulative dose, concurrent ototoxic drugs** (loop diuretics, NSAIDs, cisplatin) ### Why Other Investigations Are Inadequate | Investigation | Why It's Not the Answer | |---|---| | **Serum gentamicin levels** | Confirms drug exposure but does NOT detect ototoxicity; levels correlate poorly with ototoxic risk in individual patients | | **24-hour urine creatinine clearance** | Assesses renal function (a risk factor) but does NOT diagnose ototoxicity itself | | **Temporal bone CT** | Structural imaging is normal in aminoglycoside ototoxicity; no anatomical lesion visible on imaging | **Clinical Pearl:** Ototoxicity is a **clinical diagnosis** made by audiometry, not by imaging or drug levels. Once detected, it is **irreversible**—prevention through dose adjustment and renal monitoring is the key strategy. ### Prevention Strategy in This Patient **Mnemonic: RENAL DOSE** — Aminoglycosides require aggressive renal dose adjustment: - **R**enal function assessment (eGFR, creatinine clearance) - **E**xtended-interval dosing (e.g., 7 mg/kg gentamicin once daily) preferred over traditional dosing in renal impairment - **N**ephrotoxicity and ototoxicity monitoring - **A**voiding concurrent ototoxic agents - **L**evel monitoring (peak/trough) if traditional dosing used [cite:KD Tripathi 8e Ch 49]