How gentamicin destroys the inner ear

A popular, low-cost antibiotic developed in the 1960s, gentamicin has proved remarkably effective against a particular group of bacteria. But if not administered with caution it can also destroy the inner ear, the body's balance mechanism.

Popular super-bug fighting antibiotic can have a destructive side effect

Dr. John Rutka, a neurotologist at Toronto's University Health Network, is one of the country's leading experts on vertigo and inner ear problems. (Meredith Levine / CBC)

Gentamicin, the drug that destroyed Janice Mackay's entire vestibular system, was discovered at the Schering pharmaceutical lab in Bloomfield, New Jersey, in 1963.

It proved remarkably effective against a particular class of bacteria (called Gram-negative), and quickly became a popular treatment for a range of infections, especially those found in the blood and kidneys.

But by 1964, within the first year of the discovery of gentamicin, medical journals were publishing studies showing that some gentamicin patients developed substantial bilateral vestibular loss in the inner ears where the body's balance mechanism is found.

Since then, there have been scores of studies connecting gentamicin to this bilateral loss.

But despite these warnings, gentamicin is still being incorrectly administered and monitored in hospitals, neurotologists and other experts say, though it is hard to get exact figures on how frequently.

In Canada, the use of gentamicin is down significantly from 15 years ago, according to Rob Ariano, an associate professor of pharmacology at the University of Manitoba and an internationally respected researcher on gentamicin poisoning.

But it is stilled used here to treat life-threatening infections, such as those that cause an enlarged heart, according to Ariano and others.

The vestibular system, shown here in an oversized model, is part of the delicate labyrinth of the inner ear that governs the body's sense of motion and balance. (CBC)

In the U.S., with its profit-driven medical system, the situation is somewhat different.

Dr. Tim Hain, a neurotologist and professor of medicine at Northwestern University in Chicago, says that gentamicin and related micin antibiotics are still a preferred choice over newer antibiotics in some health-care institutions, mostly because of costs.

"There is maybe a hundred-to-one [price] difference between these new super-antibiotics, which are very, very expensive, and gentamicin, which is pretty well known and very cheap," says Hain.

He also notes that gentamicin is effective on the so-called super bug  —​ MRSA or methicillin-resistant Staphylococcus aureus — infections where so many others aren't.

"It's a gigantically used drug," he says.

Calculating safe dosage

Gentamicin is also in wide use internationally, particularly in developing countries, with cost again being a factor.

More disconcerting is what appears to be a vibrant black market in the drug. Enter the phrase "order gentamicin online without prescription" on Google and you get 836,000 results.

Gentamicin can be administered safely if given at a proper dosage and usually not for longer than seven days.

But, judging by the lawsuits that have arisen over the proper administration of gentamicin in the U.S. and Canada, the problem seems to be that not enough medical professionals are aware of the Food and Drug Administration warnings about safe durations and dosing.

Another factor here is that the drug requires a different way of calculating and monitoring medication than is typical.

With most drugs, you ensure safety by monitoring how much of the drug ends up in the bloodstream at a given time. But gentamicin is different, in that it accumulates and doesn't get eliminated quickly through urine like most medications.

In fact, it can hang around the inner ears for up to a year before it is eliminated.

"The longer you are on the drug the greater your risk," explains Ariano.

That's why gentamicin poisoning — and the chronic vertigo that comes with it — comes on slowly, killing off the inner ear little by little.

It can be months after a patient has stopped taking the drug that vestibular symptoms set in.

By then it's too late to stop the progression of damage and, in some cases, the drug will end up destroying everything.

Not reading the label

What seems to be better known about gentamicin's side effects is the drug's connection to potential hearing loss or kidney failure.

Doctors will closely monitor kidneys and hearing in gentamicin patients often without ever asking patients if they are feeling dizzy, or giving balance tests.

Keith Douglass is a medical malpractice lawyer in Spokane, Wash., who has taken on about 150 separate gentamicin malpractice cases in the 15 or so years, the majority of them successful.

"I've probably had 30 cases," he says, "where patients have told me: 'I reported I was dizzy, that I was nauseated, I was having difficulty walking,' and the doctor said, or the nurse said, 'Don't worry'."

Douglass also says that in many physician depositions that he has taken in gentamicin litigation it becomes quite clear that the doctors "don't read the package insert. They don't read the most basic information about the about the drug they're prescribing."

And it's not just physicians. Douglass has encountered pharmacists with doctorates, who are supposed to be the top medical experts on drug safety, who have recommended unsafe protocols of gentamicin because "they do not have a clue about gentamicin ototoxicity [inner ear poisoning], what caused it."

Just how large a population of patients end up with gentamicin poisoning? The total is only around two per cent of all patients given the drug.

Rob Ariano is an associate professor of pharmacology at the University of Manitoba and an internationally respected researcher on gentamicin poisoning. (Courtesy Rob Ariano / CBC)

But as Rob Ariano points out, that total population includes a significant portion of patients who are on the drug for less than a week.

According to Ariano, no one has yet calculated what percentage of patients who are on the drug for longer than one week end up with destroyed vestibular systems. But he says confidently, "we know it's going to be much higher than two per cent."

At St. Boniface Hospital in Winnipeg, where Ariano is on staff, there is a policy that after a patient has been on gentamicin for seven days, pharmacists need to start hunting for an alternative.

But with the rise in antibiotic resistant bacteria and few new antibiotics coming on the market to replace them, Ariano says finding a good replacement can be tough "when now you're faced with few choices."

Pilot project

Last year, at Toronto's University Health Network, Dr. Rutka and Wanda Dillon, a nurse on his team, launched a pilot project to help reduce the number of gentamicin poisoning cases at their hospital.

Doctors at UHN can refer gentamicin patients for vestibular testing to Rutka's unit, where Dillon conducts a baseline assessment before the drug is administered, then does follow-up during and after treatment.

To date, 14 patients have been followed. Four showed signs of vestibular problems while on the medication, and three of them were immediately switched to another drug, averting permanent damage.

The fourth was in critical condition due to an infection in his heart; tests showed gentamicin was the only drug that could treat the infection.

He remained on the medication and survived, but his vestibular system was destroyed by the drug.


Meredith Levine

Health research

Meredith Levine is an award-winning, multimedia journalist and full-time faculty member with the graduate journalism program at Western University. She became interested in reporting on chronic vertigo after living with it for a year and a half and then, luckily, recovering. This project is supported, in part, by the Canadian Institutes of Health Research Tier 1 Journalism Award and by the faculty of information and media studies at Western University.