Streptomycin was the forebear of the aminoglycoside family of antibiotics, and gentamicin (Garamycin) is one of its illustrious descendents, preferred because of its effectiveness and low cost. Injected for the treatment of very serious infections—most commonly caused by Gram-negative bacteria—the medical uses and unusual adverse effects of aminoglycosides are similar. There is a relatively thin margin between doses that effectively kill bacteria and those that cause major toxicity, so the dose must be carefully calibrated.
In addition to causing kidney toxicity, aminoglycosides can cause inner ear damage. The inner ear plays a critical role in hearing and in maintaining balance. The aminoglycosides cause ototoxicity by killing inner ear hair cells that play a critical role in these sensory functions. Of the six aminoglycosides, some preferentially affect hearing, while others disrupt balance.
Hearing involves the transduction or conversion of sound waves into electrical signals that travel to the brain. This auditory conversion takes place at sensory hair cells. Dead hair cells are incapable of regenerating, and hearing loss caused by aminoglycosides is permanent. The extent of this loss depends upon the number of hair cells killed and may affect up to 25 percent of patients taking these drugs. Aminoglycosides initially impair high-frequency sounds, progressing to the lower frequencies, but profound deafness can occur after a single injected dose.
Aspirin and cisplatin also commonly cause hearing deficits. High doses of aspirin can cause temporary hearing impairment and tinnitus, a ringing in the ears, but problems disappear when the dose is reduced. By contrast, the anticancer drug cisplatin can cause permanent hearing loss in between 30 and 100 percent of patients receiving high doses.
The vestibular organ hair cells of the inner ear maintain balance by detecting the rotations and movements of the head. Unlike auditory hair cells, the vestibular variety can partially recover after aminoglycoside damage.
SEE ALSO Aspirin (1899), Streptomycin (1944), Ampicillin (1961).
This anatomical drawing of the inner ear is from Bourgery & Jacob’s Traité de l’Anatomie Humaine (1862). Gentamicin’s effects on the inner ear can cause problems in hearing and balance.