A 42-year-old woman with cystic fibrosis is hospitalized for acute exacerbation due to Pseudomonas aeruginosa respiratory infection. She is started on intravenous tobramycin 5 mg/kg once daily. On day 5 of therapy, she develops sudden-onset hearing loss and tinnitus. Serum creatinine is 0.9 mg/dL (baseline 0.8 mg/dL). What is the most appropriate next step?

Explanation

## Clinical Presentation: Aminoglycoside Ototoxicity This patient has developed **ototoxicity**, a serious and largely irreversible adverse effect of aminoglycosides. Sudden-onset hearing loss and tinnitus on day 5 of tobramycin therapy are classic signs. Renal function is essentially unchanged (creatinine 0.9 vs. baseline 0.8 mg/dL), confirming this is **ototoxicity, not nephrotoxicity**. ## Why Immediate Discontinuation Is the Correct Next Step **Key Point:** Aminoglycoside ototoxicity results from direct, cumulative destruction of cochlear (and vestibular) hair cells. Unlike nephrotoxicity, which is often reversible with dose reduction and supportive care, **cochlear hair cell damage is permanent and irreversible** (KD Tripathi, *Essentials of Medical Pharmacology*, 8th ed.; Harrison's Principles of Internal Medicine, 21st ed.). Once symptomatic ototoxicity (hearing loss, tinnitus) is clinically evident, the **only action that can prevent further irreversible damage is immediate discontinuation** of the offending aminoglycoside. Continuing the drug—even at a reduced dose—risks progressive, permanent hearing loss. | Option | Why It Is Incorrect | |---|---| | **A – Continue + audiometry + trough gentamicin level** | Continuing tobramycin while awaiting results exposes the patient to further irreversible cochlear damage; additionally, referencing "trough gentamicin level" is erroneous since the patient is on tobramycin | | **B – Obtain peak level only; continue if in range** | Peak level monitoring is used for efficacy, not ototoxicity management; continuing therapy ignores an active, serious adverse effect | | **D – Reduce dose to 3 mg/kg and continue** | Dose reduction does not reverse existing hair-cell damage and may allow further injury; not appropriate once symptomatic ototoxicity is present | ## Management After Discontinuation **Clinical Pearl:** After stopping tobramycin, the clinical team should: 1. **Switch to a non-aminoglycoside anti-pseudomonal agent** — options include ciprofloxacin/levofloxacin (fluoroquinolones), ceftazidime, cefepime, piperacillin-tazobactam, or aztreonam, guided by susceptibility testing. 2. **Obtain formal audiometry** — documents baseline hearing loss for follow-up and medicolegal purposes, but this should not delay discontinuation. 3. **Refer to audiology/ENT** — for ongoing monitoring and rehabilitation if hearing loss persists. ## Risk Factors for Aminoglycoside Ototoxicity **High-Yield:** Cystic fibrosis patients are at heightened risk because they often require repeated, prolonged aminoglycoside courses. Additional risk factors include: - Prolonged therapy (>7–10 days) - Elevated trough levels (>4–5 μg/mL for tobramycin/gentamicin) - Concurrent ototoxic drugs (loop diuretics, cisplatin) - Renal impairment (drug accumulation) - Genetic predisposition (mitochondrial 12S rRNA mutations, e.g., m.1555A>G) **Key Point (Harrison's):** The cochleotoxic potential of aminoglycosides follows the order: neomycin > gentamicin ≈ tobramycin > amikacin > streptomycin. High-frequency hearing loss typically precedes clinically noticeable loss, underscoring the importance of baseline and serial audiometry in patients on prolonged courses. Once symptomatic ototoxicity is identified, immediate discontinuation is the standard of care.