Specific findings in distortion product otoacoustic emissions and growth functions with chronic tinnitus

Chronic tinnitus has a very high prevalence in industrialized countries. Latest studies found chronic tinnitus in 4% of the German population, with almost 2% suffering severely in their daily life. Because no curative therapeutic approach is available--neither pharmacological nor surgical--the main focus lies in treatments that enhance the habituation of tinnitus. Habituation occurs in almost 50% of affected patients as a normal process, leading to a complete compensation. This finding is based on the ability of the auditory perception to habituate random noise and focus on important acoustic information. According to our audiological data, 90% of tinnitus patients have deficits in inner-ear function as a generator of tinnitus, mainly in the outer hair cells. This occurrence can be verified by registration of distortion products of otoacoustic emissions. Thus, the main origin of tinnitus is peripheral, and most patients suffer from accompanying hearing loss, even though it is sometimes mild or subjectively not even noticed. In almost 50% of our patients, we find hyperfunction of outer hair cells, again recorded via distortion products of otoacoustic emissions and their growth functions. Normal efferent reduction of distortion products through contralateral acoustic stimulation does not take place in most tinnitus patients. This finding shows that central auditory functions are also disturbed in chronic tinnitus patients, leading to reduced efferent effects on the hair cells and thus impeding habituation. Tests to verify these more central pathological findings have yet to be developed. We have data on diminished ability to distinguish stimuli from random noise by bilateral sound processing: The so-called bilateral masking difference test results are pathological in almost 30% of patients suffering from chronic tinnitus. We concluded from our audiological data that chronic tinnitus is primarily a cochlear dysfunction, but habituation is impeded by accompanying or consecutive deficits of the central auditory pathway. Regarding therapeutic approaches, these central functions can be trained by hearing therapy, as we know from patients' rehabilitation.