By Ronna Hertzano, M.D., Ph.D.
Tinnitus is the perception of sound in the absence of an external sound source. About 10 percent of the population has tinnitus, with a small portion of that population whose tinnitus is bothersome enough to affect sleep, concentration, and/or mood.
Tinnitus itself is not a disease but rather a symptom of an underlying condition that can be classified into subtypes based on its many causes. In our article in the April 30, 2020, volume of the journal Otolaryngologic Clinics of North America, we focus primarily on noise-induced tinnitus, whether acute or chronic, which is most likely the most common type.
Scientists believe that tinnitus is always associated with one or more forms of hearing loss, often noise-induced. As a result, although a treatment of most forms of tinnitus will likely emerge in the years to come, curing tinnitus will first require curing hearing loss.
Mounting evidence implicates tinnitus as an indicator of underlying auditory deficits, however mild these deficits might be, and including “hidden hearing loss” that isn’t captured via the standard audiogram. This takes us from the former concept of “some form of hearing loss is associated with tinnitus” to a picture in which “tinnitus is a symptom of a form of hearing loss.”
An early theory considers tinnitus to be a result of nerve hyperactivity that is overcompensating for the lack of neural input from the ear to the brain that would be occurring with a healthy inner ear. Even normal hearing volunteers who wore silicone earplugs for seven days to simulate a temporary hearing loss developed tinnitus in that short timescale, although the tinnitus resolved after removing the earplug.
In other words, a sudden reduction in cochlear output seems to cause increased spontaneous neural activity in reaction to the lack of signals. This leads to the perception of a continuous sound and may also underlie the hyperacusis, or a lowered threshold to discomfort from sound, often associated with chronic noise-induced tinnitus.
In conclusion, all indications are that tinnitus, when not caused directly by a central nervous system issue (e.g., stroke), is always associated with one or more forms of hearing loss. As a result, although a treatment of most forms of tinnitus will likely emerge in the years to come, curing tinnitus will first require curing hearing loss. It also points to tinnitus potentially being an early symptom of an underlying auditory injury before measurable audiometric changes.
That said, the relationship between the characteristics of the hearing loss (for instance the shape of the audiogram) and the tinnitus perception (its pitch if any and its perceived spectrum and localization) does not follow any clear rules. Also, although tinnitus is intimately linked with peripheral cochlear dysfunction, cochlear damage does not account for the sustained and sometimes distressing nature of tinnitus, including the reaction of the patient to the tinnitus.
A lot of progress has been made in understanding the events that lead to hearing loss and its associated tinnitus from noise exposure, whether sudden and traumatic or resulting from a lifelong exposure to noise, but much more has yet to be observed, learned, and understood before we can hope to have a cure for hearing loss in all its forms and the associated tinnitus.
Hearing Restoration Project member Ronna Hertzano, M.D., Ph.D., is an associate professor of otorhinolaryngology–head and neck surgery at the University of Maryland School of Medicine. For more, see hhf.org/hrp.
