11Pharmacologic and Ototoxic Causes

FTinnitus as an early-warning sign

Several drug classes are ototoxic: they damage the inner ear, producing hearing loss, tinnitus, or imbalance. Crucially, tinnitus frequently appears before measurable hearing loss reaches the conversational frequencies, because the injury usually begins in the high-frequency base of the cochlea. A new ringing in a patient on a known ototoxic drug should therefore be treated not as a nuisance but as a signal that the cochlea is under threat [1999].

Ototoxic injury falls into two broad patterns. Some agents cause reversible tinnitus that disappears when the drug is stopped; others cause permanent damage by killing hair cells or the stria vascularis. Knowing which is which guides urgency: reversible toxicity tolerates a wait-and-stop approach, whereas permanent toxicity demands proactive monitoring so the drug can be modified before irretrievable loss occurs [1999].

TSalicylates and NSAIDs — the reversible model

High-dose aspirin is the classic reversible ototoxin. Salicylate produces a bilateral, high-pitched tinnitus and a mild, flat hearing loss that resolve within days of stopping the drug. The mechanism is not hair-cell death but a functional disruption of outer-hair-cell electromotility: salicylate interferes with the motor protein prestin and alters cochlear amplification, so the cochlea misbehaves while the cells remain alive [2000].

Because the effect is dose-dependent and concentration-linked, salicylate tinnitus has historically even been used as a rough clinical marker of toxic blood levels. Other NSAIDs can contribute to tinnitus, generally reversibly, and the salicylate model remains the textbook illustration that tinnitus can arise from a temporary, fully recoverable cochlear dysfunction [2000].

TAminoglycosides and loop diuretics

Aminoglycoside antibiotics (gentamicin, tobramycin, amikacin, streptomycin, neomycin) cause permanent cochlear damage. They enter hair cells through mechanotransduction channels, generate reactive oxygen species, and trigger apoptosis, killing outer hair cells preferentially in the high-frequency basal turn first. The result is high-frequency sensorineural loss and tinnitus that, once established, does not recover [2011].

Loop diuretics (furosemide, ethacrynic acid, bumetanide) act differently, disturbing the ionic gradient maintained by the stria vascularis and the endocochlear potential. Their ototoxicity is usually reversible when used alone, but their most dangerous feature is synergy: a loop diuretic given alongside an aminoglycoside markedly potentiates aminoglycoside ototoxicity, so the combination should be avoided or monitored closely [2011].

CPlatinum chemotherapy

Cisplatin is among the most ototoxic drugs in clinical use. It accumulates in the cochlea — notably the stria vascularis and outer hair cells — and produces dose-dependent, usually bilateral, high-frequency sensorineural hearing loss and tinnitus that are frequently permanent. In large survivor cohorts of cisplatin-treated patients, a substantial proportion report persistent tinnitus and measurable high-frequency hearing impairment years after treatment, and severity rises with cumulative dose [2016].

Mechanistically, platinum agents drive oxidative stress and apoptosis through pathways overlapping with the aminoglycosides, which is why antioxidant and otoprotective strategies are an active area of research. Carboplatin is generally less cochleotoxic than cisplatin at standard doses. Because the loss is irreversible and dose-related, oncology guidelines increasingly build baseline and serial audiometric monitoring into platinum regimens [2016][2007].

CQuinine, and the case for audiometric monitoring

Quinine and related antimalarials produce a reversible syndrome of tinnitus, hearing loss, and visual disturbance known as cinchonism, again largely recovering on withdrawal. Across all ototoxic drugs the clinical lesson converges on one principle: tinnitus is often the earliest audible sign of cochlear injury, and the high-frequency base is hit first [2000].

This is the rationale for ototoxicity monitoring. A sensitive, individualised high-frequency audiometric protocol can detect threshold shifts before they reach the speech frequencies, giving clinicians a window to adjust dose, switch agents, or counsel the patient while the loss is still limited — the strategy that most reliably mitigates permanent damage during aminoglycoside or chemotherapy treatment [1999].