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Ototoxicity and Hearing Loss
A number of medications are toxic to the cochlear and vestibular systems. Most ototoxic medications are used in the treatment of bacterial infection and cancer. Exposure to ototoxic medications can lead to hearing loss, tinnitus and dizziness, or any combination of these symptoms.

The typical pattern of hearing loss due to ototoxicity is a permanent, bilateral, high-frequency sensorineural hearing loss. Because the initial damage is usually in the very high-frequency range of hearing, the patient usually does not notice the early stages of hearing loss. Tinnitus may be a warning sign, but ototoxic hearing loss can also develop in the absence of tinnitus. The hearing loss can occur weeks or even months after treatment has ended.

Many factors contribute to the highly variable effects of ototoxic drugs on the auditory system. These factors include pre-existing hearing loss, noise exposure during and after exposure to ototoxic drugs, length of treatment, renal function, radiation, and interactive effects with other medications.

Aminoglycosides
The most widely used antibiotics in the world can also cause hearing and balance disorders. Usually used to treat bacterial infection, some aminoglycosides have a greater tendency to affect hearing (cochleotoxic), while others have a greater tendency to cause dizziness (vestibuleotoxic). Ototoxic aminoglycosides include streptomycin, kanamycin, and other medications in the -mycin family.

Aminoglycosides are especially popular in rural and developing countries because of their effectiveness and low cost. Several studies have suggested that more than half the cases of severe hearing loss in China are due to the widespread use of aminoglycosides.

Salicylates
Usually taken in the form of aspirin, salicylates may be the most widely used medications in the world. In high dosages (such as the treatment of arthritis), aspirin can cause sensorineural hearing loss, usually accompanied by tinnitus. Unlike most cases of hearing loss due to ototoxicity, hearing loss caused by salicylates is usually reversible within 48 to 72 hours once medication is terminated.

Ironically, animal studies conducted at the Kresge Hearing Research Institute suggest that low levels of salicylates may minimize the ototoxic effects of aminoglycosides. Guinea pigs receiving aminoglycosides and salicylates had minimal hearing loss, while guinea pigs receiving only gentamicin experienced about 70 dB hearing loss. The effectiveness of the antibiotic was not compromised.

Other ototoxic agents
Loop diuretics can be ototoxic, but drugs such as Lasix appear to have the greatest effects when used in conjunction with other ototoxic drugs and can cause permanent hearing loss. Carboplatin appears to be less ototoxic. Quinine, in sufficient dosage, can also cause hearing loss.

Audiologic monitoring
Although most patients on ototoxic medications do not receive audiologic monitoring, there are a number of reasons to monitor the hearing of these patients.

• Pre-existing hearing loss. Hearing loss is perhaps the most common unidentified health condition in adults. A baseline audiogram can separate pre-existing hearing loss from hearing loss due to ototoxicity.
  
• Early detection. Although change in treatment may not be an option, early detection of hearing changes allows other treatment options to be considered.
  
• Patient and family awareness. Even if treatment cannot be altered and hearing loss cannot be prevented for medical reasons, an early awareness of a developing hearing loss allows the individual and family to prepare and cope with increased communication problems.
  
• Enhanced communication. By identifying the hearing loss early, counseling and audiologic rehabilitation can improve the patient's ability to communicate with family, friends and healthcare providers.
  

Ototoxicity and audiologic monitoring can be difficult issues when treating patients for serious medical conditions. Nevertheless, audiologic monitoring can maximize short-term and long-term communication abilities and improve the quality of life for both patient and family.