Gentamicin Toxicity - Hearing Loss

Gentamicin is one of a group of antibiotics known as amino glycosides that also include streptomycin and neomycin. It is most commonly used in the treatment of serious Gram negative bacterial infections where other antibiotics have proved ineffective.

Since it was first discovered in the 1940's it has been known to have serious toxic effects in some individuals, particularly relating to the kidney and inner ear, but it is only relatively recently that the reasons for this have been fully understood.

Gentamicin is not toxic until it combines with the iron in the blood stream when it produces destructive chemical agents known as free radicals. The tiny hair cells in the inner ear are particularly vulnerable to these and once they become damaged are no longer able to transmit impulses to the auditory nerve and hence to the brain, causing deafness. Symptoms of vestibular damage, such as imbalance and transient vision blurring, can also occur. It is also highly nephrotoxic (harmful to the kidney)

There is also a genetic component involved in gentamicin toxicity with certain individuals having a normally harmless mutation in their DNA making them particularly susceptible to the side effects of the drug.

Gentamicin is not absorbed by the gut so has to be given as either an intramuscular or intravenous injection. The dose is estimated according to body weight and blood levels should be undertaken at frequent intervals to avoid a toxic build-up of the drug. A course of treatment is generally limited to 7 days although can be for longer in cases of severe infection.

The incidence of gentamicin toxicity is about 2-3%, and the general view is that this high figure is acceptable only if the drug is restricted to those patients who would die or suffer severe morbidity without is. It is most common in the elderly.

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