Ear Drops: Is it Safe? 

In our country where most of the times one can buy prescription medicine from a pharmacy without producing a prescription, self medication and prolonged use of medication, especially ear drops or nasal drops is very common.  We generally encounter patients who have been using ear drops for a number of months or years which was prescribed a few years ago by their doctor.  How safe is it to use these ear drops?

Ear drops are generally prescribed by an ENT surgeon for an ear infection. The infection could be in the external ear or the middle ear.  In an external ear infection the ear drum is found to be intact.   In cases of middle ear infection, the discharge originates in the middle ear and flows out through a hole in the ear drum onto the outside.

In normal individuals who have an intact ear drum there is no communication between the external ear and the middle ear. The middle ear (ME) communicates with the cochlea or the inner ear through the round window.  A semipermiable membrane known as the round window membrane acts as a curtain between the cochlea and the ME. The cochlea houses the delicate hair cells which transduces sound to the hearing nerve. The inner ear is also filled with a fluid called perilymph and endolymph.

Ototoxicity refers to medication-caused auditory and/or vestibular system dysfunction resulting in hearing loss or dysequilibrium. The propensity of specific classes of drugs to cause ototoxicity has been well established. Ototoxicity typically is associated with bilateral high-frequency sensorineural hearing loss and tinnitus (ear noise). Hearing loss can be temporary, but it is usually irreversible with most agents. Generally, antibiotic-induced ototoxicity is bilaterally symmetrical, but it also can be asymmetrical. The usual time of onset is often unpredictable, and marked hearing loss can occur even after a single dose. Additionally, hearing loss may not manifest until several weeks or months after completion of antibiotic therapy.

Since their introduction in 1944, multiple aminoglycoside preparations have become available, including streptomycin, dihydrostreptomycin, kanamycin, gentamicin, neomycin, tobramycin, netilmicin, and amikacin.  It became known that when these aminoglycosides, were administered intravenously they were capable of producing profound, irreversible sensorineural hearing loss. Morphologic studies demonstrated that the inner row of outer hair cells (OHC) of the basal turn of the cochlea are affected first, followed by the other two rows of OHCs and the inner hair cells, resulting in sensori-neural deafness. Prospective studies demonstrated an incidence ranging from 4% to 24%.  Aminoglycosides are still used frequently because of their effectiveness and the low risk of allergic reactions compared to other antibiotics.

These aminoglycosides, mainly Gentamycin, Neomycin, and Tobramycin  are also manufactured as Otic preparations to be used topically in the ear for external and middle ear infections. Initially it was thought that the Ototoxic effects of the aminoglycoside preparations occurred only when administered intravenously.  There is evidence now that  these aminoglycoside otic preparations when instilled into the middle ear, diffuses across the round window membrane and gets mixed up in the perilymph and reaches the delicate hair cells of the inner ear and damages it.  The damage thus caused is irreversible.  In other words, the aminoglycoside class of antibiotics exhibits their ototoxic side effects both on intravenous as well as topical administration.  Gentamicin and streptomycin are toxic to the vestibular (balance) part of the inner ear.  Neomycin is very toxic to the hearing part of the inner ear.  Some of the solvents for the antibiotics have also been found to be ototoxic.  If otic preparations containing Neomycin is used for prolonged periods in those having a perforated ear drum risk ototoxicity. However, these preparations can safely be used by those suffering from an external ear infection, since ototoxicity is not observed in those with an intact ear drum.

In humans, toxicity does not occur as often as would be expected.  Entry into the inner ear is generally attributed to diffusion across the round window membrane.  In humans the round window membrane is covered by a "false" second membrane in 20% of cases. In an additional 11% there is a fatty or fibrous plug covering the round window. The membrane is also thicker in humans.  All these factors might reduce toxicity of ear drops.  It has also been found that the ototoxic effects of these antibiotics occur when these ear drops are used for longer than 2 weeks.

In conclusion, ear drops are not as safe as you think it to be. Self medicating oneself with ear drops which had been prescribed by your doctor for some other ear ailment earlier could be dangerous.  Certain classes of ear drops cause ototoxicity if used for a prolonged period of time, especially in those having a perforated ear drum.  Hence always consult your doctor before using ear drops.