Tinnitus and normal hearing

April 28, 2016 by Mathieu Telefoglou
accouphene
Tinnitus is the perception of sound when there is no exterior sound stimulation. There is general scientific agreement that tinnitus is a direct consequence of irreversible and permanent cochlear damage, and the perception of tinnitus is generally associated with deafness.

Still, some people with normal hearing also report hearing sound without stimuli. How is this possible? We can assume that having normal hearing does not necessarily mean that the cochlea is entirely unaffected. As mentioned above, tinnitus is a response to cochlear damage, permanent or not, but is not associated with deafness. It is possible that cochlear damage could cause permanent tinnitus.

What causes tinnitus?

Most tinnitus is idiopathic, or of unknown nature. In most cases, tinnitus is defined as a consequence of cochlear damage leading to a decrease in auditory acuity. Thus, it is not uncommon to observe both deafness and tinnitus in workers exposed to noise or in the elderly. In fact, these groups are often affected by irreversible and permanent cochlear damage.

First the ear, then the cochlea

To be able to hear a sound, it first has to go through the three parts of the ear (outer ear, middle ear, and inner ear). It then reaches the cochlea, where ciliated cells transform it into nerve impulses that travel to the brain via the auditory nerve. Cochlear deafness results from the death of ciliated cells of the cochlea, leading to fewer neurons delivering their message to the auditory nerve.

Studies show that people with normal hearing and permanent tinnitus have similar profiles to people with cochlear deafness.(1)(2) In fact, in tinnitus sufferers with normal hearing, fewer neurons deliver impulses to the auditory nerve. While this is generally the result of cochlear damage, there is no clear indication of deafness. The same phenomenon has been observed in rats exposed to excessive noise over a short term (110–120 dBHL for 60 minutes).(3)

Tinnitus due to noise overexposure

In humans, a subjective decrease in hearing after going to a club or concert, for example, associated with the perception of a whistling-type tinnitus, is not unusual. This is due to the fact that the ciliated cells have been overstimulated by the excessive noise and neuronal discharge has been altered during exposure, creating a change in the cochlea, most often temporary. After few hours, this perception tends to fade. However, certain people continue to perceive tinnitus after their hearing returns to normal.

Consult an audiologist to learn more.

References :
“Tinnitus and neural plasticity.” Presentation by Tonndorf during the XI International Tinnitus Seminar in Berlin, Germany, in 2014.
TUNKEL D. E. et al. (2014). “Clinical Practice Guideline: Tinnitus.” Otolaryngology–Head and Neck Surgery. Vol. 151(2S). S1-S40.
WANG, H., T. J. Brozoski, D. M. Caspary (2011). “Inhibitory neurotransmission in animal models of tinnitus: Maladaptive plasticity.” Hear Res, 279, pp. 111–117.
SCHAETTE, R., D. McAlpine (2011). “Tinnitus with a normal audiogram: Physiological evidence for hidden hearing loss and computational model.” J Neurosci, 31, pp. 13452–13457.