Unfortunately, these symptoms may not be detected until after the acute phase of severe infection and diagnosis is thus delayed. Symptoms of cochleotoxicity include hearing loss and/or tinnitus, while those of vestibulotoxicity consist of disequilibrium and dizziness. Different regimens of AG administration and different definitions of ototoxic damage may have contributed to the variation of incidence. Vestibulotoxicity occurs in up to 15% of patients after AG administration, whereas cochleotoxicity in 2% to 25% of patients. Ototoxic side effects occur within days or weeks after systemic application and are often bilateral in presentation.
Gentamicin and tobramycin are predominantly vestibulotoxic, whereas neomycin, kanamycin, and amikacin are mainly cochleotoxic. Generally, each AG is capable of irreversibly damaging both the auditory and vestibular organs, but “typically affects one more than the other”. Modification of streptomycin to dihydrostreptomycin, however, resulted in a shift of ototoxic damage from the vestibular organ to the cochlea. Within the inner ear, streptomycin preferably damages the vestibular organ. This nephro- and ototoxicity was initially discovered in the first clinical trials of streptomycin. While damage inflicted by AG on the kidney is usually reversible, damage to the inner ear is permanent. In addition to their potent antimicrobial efficacy, all AGs can cause toxic side effects to the kidneys and inner ear. Currently, nine AGs (streptomycin, neomycin, tobramycin, kanamycin, paromomycin, spectinomycin, gentamicin, netilmicin, and amikacin) are approved by the Food and Drug Administration (FDA). In contrast to these organic derivatives of soil-dwelling bacteria, synthetic AGs such as amikacin could be developed in vitro. With the isolation of gentamicin from Micromonospora purpurea, the ending “-micin” was implemented to specify the bacterial origin of the individual AG. In subsequent years, other AGs were isolated from Streptomyces spp., commonly integrating the ending “-mycin” in their nomenclature. The initial isolation of streptomycin from Streptomyces griseus provided the long-sought treatment for tuberculosis and an effective antibiotic against gram-negative bacteria. Aminoglycosides (AGs) are a well-known and successful class of antibiotics.
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