碩士 / 國立臺灣大學 / 臨床醫學研究所 / 95 / Background
Tinnitus (from Latin tinnire, which means to ring or to tinkle) describes an auditory perception experienced in the absence of any evident external stimulus.
Overall, 10-14% of adults have prolonged spontaneous tinnitus or have tinnitus ‘always’ or ‘often’. In 0.5-1%, tinnitus has a severe effect on quality of life or on ability to lead a normal life. Hearing impairment is commonly associated with tinnitus, but it is not exceptional for tinnitus to occur in patients with normal hearing. In a Swedish population study, 19% of subjects with subjective normal hearing said they suffered from tinnitus "always", and the British national study found that 7% of adults were annoyed by prolonged spontaneous tinnitus but did not have difficulty hearing. (Axelsson-1989; Vesterager-1997)
The conditions in which tinnitus occurs are legion, but the pathophysiology of tinnitus remains obscure. The source of ‘tinnitogenic’ activity could be any level of the auditory system. Purely subjective tinnitus can be evaluated by psychoacoustic methods. Most of these psychoacoustic features have a high test-retest reliability, which proves that subjective tinnitus is based on some constant pathophysiological process (Hoke-1989).
The complexity of the changes in the nervous system associated with tinnitus may explain why it is so resistant to treatment. In view of the great number of individuals
(0.5-1%) who are unable to lead a normal life and the ineffectiveness of current tinnitus therapy (Lockwood-2002) it is highly desirable that some method be devised to objectively assess the existence of tinnitus.
Møller et al had ever recorded the compound action potentials (CAPs) directly from the exposed intracranial portion of the eighth nerve in 19 patients undergoing microvascular decompression of the eighth nerve for intractable tinnitus. In the study, brainstem auditory evoked responses (BAEPs) in response to click sounds were also recorded during the operation. According the analyses of results, it may be assumed that the abnormalities in the latencies of peak V of BAEPs in the patients with tinnitus are a result of abnormalities in the superior olivary complex (SOC). (Møller-1992)
In pervious studies, auditory cortical reorganization was demonstrated in normal hearing tinnitus (Hoke-1989; Mühlnickel-1998; Andersson-2000). Similarities between the data and the previous demonstrations that phantom limb pain is highly correlated with somatosensory cortical reorganization suggest that tinnitus may be an auditory phantom phenomenon (Mühlnickel-1998).
The discovery of otoacoustic emissions, energy emitted by the outer hair cells (OHCs) in cochlea and recordable as acoustic vibrations in external auditory canal (Kemp-1978), was important for both theoretical and practical reasons. As otoacoustic emissions are invariably associated with functioning outer hair cells, their presence is a reliable indicator of cochlear structural integrity, and their absence may indicate a cochlear lesions. Besides being an expression of the cochlear structural status, otoacoustic emissions may also give an indication of functional integrity of the mechanisms which control the cochlea. The olivocochlear efferent system is a part of that regulatory complex, and its medial division, widely known as the medial olivocochlear (MOC) system, seems to be of particular importance in the modulation of cochlear activity.
It has been suggested that tinnitus may be related to cochlear mechanical activity (Kemp-1981; O-Uchi-1988; Plinkert-1990; Norton-1990; Mckee-1992; Chéry-Croze-1994). Less effectiveness of MOC system had been revealed in tinnitus also (Veuillet-1992; Chéry-Croze-1993; Attias-1996; Ceranic-1998).
Our proposed hypothesis is that dysfunction of MOC system in tinnitus alters the cochlear mechanical process, cochlear neural output and compound action potential of auditory nerve, and then the consequent cortical reorganization triggers the phantom perception of tinnitus.
A better comprehension of the mechanisms underlying a symptom will probably come from exhaustive exploration of simplified cases. A more precise evaluation of the medial olivocochlear efferent activity in tinnitus patients requires a homogenous population. Factors including age, gender, general health, hearing threshold, duration of tinnitus suffering, clearly must be controlled for.
Our study tried to explore the dysfunction level of the auditory system in the patients who suffer from annoying tinnitus with normal or nearly normal hearing by assessment with otoacoustic emissions. By this study, we will show the potential value of otoacoustic emissions in the assessment of tinnitus. This information may contribute to a better understanding of the origin and mechanisms underlying tinnitus.
Materials and methods
We collected patients complain of unilateral or bilateral tinnitus. All patients included in the study should have normal or nearly normal hearing. Normal subjects without auditory complaints were introduced to be the control group. Pitch matching test and loudness-balance test were carried out on tinnitus sufferers for objectifying tinnitus. All subjects underwent a protocol which includes an interview to obtain relevant information, otoscopy, standard pure tone audiometry, tympanometry, static compliance measurement, acoustic reflex test, auditory brainstem response survey, otoacoustic emissions examination (including SOAEs, TEOAEs, DPOAEs), and olivocochlear suppression test. Then we compared the results in tinnitus-disease ears, tinnitus-health ears and control-normal ears.
An effort was made to match the age and gender proportion of the tinnitus to those of control subjects. Informed consent was obtained from all subjects.
Results and Discussion
Although there were no significantly statistical differences in the thresholds of pure tone audiometry, prevalence of SOAEs and TEOAE responses between three ear groups, averaged DPOAE levels of tinnitus-disease and tinnitus-health ears were significantly decreased over limited frequency range in comparison with those of control-normal group. Furthermore, higher percentage in tinnitus-disease and tinnitus-health ears were noted to have significant decreases or increases on DPOAE amplitude over a limited frequency range. But, in the present study, the pitch-matched tinnitus frequency was out of the frequency range in numbers of the cases. It is suggested that changes of cochlear dynamic properties exist in tinnitus, but simplex cochlear mechanism can not explain the tinnitogenesis.
Significantly reduction of TEOAE responses under contralateral acoustic stimulation was observed in both tinnitus-health and control-normal groups, but not in tinnitus-disease ears. In comparison with control-normal group, the averaged central efferent suppression on DPOAEs of tinnitus-disease and tinnitus-health ears were significantly decreased over limited frequency range. Deducible connection between dysfunction of descending auditory pathway and tinnitus could be made logically.
At last, inspection of the records of ABR showed significant prolonged latencies of peak III and V in the tinnitus-disease group compared with the control-normal group, although the values of IPL III-V were not statistically different from each other. Caudal pontine lesion ipsilateral to tinnitus affected ear was implied and may be the source of tinnitogenic activity.
Conclusions
This study has shown the potential value of otoacoustic emissions in the assessment of tinnitus. The dysfunction of descending auditory pathway caudal to SOC (medial olivocochlear efferents, MOC system) maybe play the leading role in the tinnitogenesis.
| Identifer | oai:union.ndltd.org:TW/095NTU05521008 |
| Date | January 2006 |
| Creators | Su-Yi Hsu, 徐愫儀 |
| Contributors | 許權振 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 74 |
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