Noise-induced tinnitus: A comparison between four clinical groups without apparent hearing loss

The number of people with normal hearing thresholds seeking medical help for tinnitus and other hearing problems is increasing. For diagnostic purposes, existence/nonexistence of lesions or combinations of lesions in the inner ear not reflected in the audiogram was evaluated with advanced hearing tests applied to tinnitus patients with certain backgrounds, including noise exposure. For forty-six patients with pronounced tinnitus, and other symptoms, tentative diagnoses were established, including judgments of the influence of four causative factors: (1) acoustic trauma, (2) music, (3) suspected hereditary, and (4) nonauditory, for example, stress or muscular tension. They were analyzed with a test battery sensitive to lesions involving the outer hair cells, damage from impulse noise, and dysfunction of the efferent system. There were significant differences in test results between groups with individuals with the same most likely causative factor. Most patients claiming acoustic trauma had a specific type of result, ′hyper-PMTF′ (psychoacoustical modulation transfer function), and abnormal test results of the efferent system. Everyone in the hereditary group had dysfunction of the efferent system. All patients working with music, except one, had some abnormality, but without specific pattern. The nonauditory group mostly had normal test results. The investigation shows that it is possible to diagnose minor cochlear lesions as well as dysfunction of the efferent system, which might be causing the tinnitus. Those abnormalities could not be detected with routine audiological tests. Malfunctioning caused by impulse noise is an obvious example of this. These findings facilitate choice of treatment, rehabilitation programs, and medicolegal decisions

The efficacy of N-acetylcysteine to protect the human cochlea from subclinical hearing loss caused by impulse noise: A controlled trial

In military outdoor shooting training, with safety measures enforced, the risk of a permanent, noise-induced hearing loss is very small. But urban warfare training performed indoors, with reflections from walls, might increase the risk. A question is whether antioxidants can reduce the negative effects of noise on human hearing as it does on research animals. Hearing tests were performed on a control group of 23 military officers before and after a shooting session in a bunker-like room. The experiments were repeated on another group of 11 officers with peroral adminstration of N-acetyl-cysteine (NAC), directly after the shooting. The measurements performed were tone thresholds; transient-evoked otoacoustic emissions, with and without contralateral noise; and psycho-acoustical modulation transfer function (PMTF), thresholds for brief tones in modulated noise. Effects from shooting on hearing thresholds were small, but threshold behavior supports use of NAC treatment. On the PMTF, shooting without NAC gave strong effects. Those effects were like those from continuous noise, which means that strict safety measures should be enforced. The most striking finding was that the non-linearity of the cochlea, that was strongly reduced in the group without NAC, as manifested by the PMTF-results, was practically unchanged in the NAC-group throughout the study. NAC treatment directly after shooting in a bunkerlike room seems to give some protection of the cochlea

Tinnitus and Other Auditory Problems – Occupational Noise Exposure below Risk Limits May Cause Inner Ear Dysfunction

<div><p>The aim of the investigation was to study if dysfunctions associated to the cochlea or its regulatory system can be found, and possibly explain hearing problems in subjects with normal or near-normal audiograms. The design was a prospective study of subjects recruited from the general population. The included subjects were persons with auditory problems who had normal, or near-normal, pure tone hearing thresholds, who could be included in one of three subgroups: teachers, <i>Education</i>; people working with music, <i>Music</i>; and people with moderate or negligible noise exposure, <i>Other</i>. A fourth group included people with poorer pure tone hearing thresholds and a history of severe occupational noise, <i>Industry</i>. N_total = 193. The following hearing tests were used:</p><p>− pure tone audiometry with Békésy technique,</p><p>− transient evoked otoacoustic emissions and distortion product otoacoustic emissions, without and with contralateral noise;</p><p>− psychoacoustical modulation transfer function,</p><p>− forward masking,</p><p>− speech recognition in noise,</p><p>− tinnitus matching.</p><p>A questionnaire about occupations, noise exposure, stress/anxiety, muscular problems, medication, and heredity, was addressed to the participants. Forward masking results were significantly worse for <i>Education</i> and <i>Industry</i> than for the other groups, possibly associated to the inner hair cell area. Forward masking results were significantly correlated to louder matched tinnitus. For many subjects speech recognition in noise, left ear, did not increase in a normal way when the listening level was increased. Subjects hypersensitive to loud sound had significantly better speech recognition in noise at the lower test level than subjects not hypersensitive. Self-reported stress/anxiety was similar for all groups. In conclusion, hearing dysfunctions were found in subjects with tinnitus and other auditory problems, combined with normal or near-normal pure tone thresholds. The teachers, mostly regarded as a group exposed to noise below risk levels, had dysfunctions almost identical to those of the more exposed <i>Industry</i> group.</p></div

Mean audiograms for subjects with different types of PMTF-curves, 4000 Hz, left ear.

<p>See legend of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0097377#pone-0097377-g002" target="_blank">Figure 2</a> about the PMTF classification. Numbers of subjects for P-types NM, 0, 1, 2, and 3 are 12, 40, 54, 13, and 2, respectively. An extra curve, black continuous line with smaller markers, black boxes, is added, which includes 4 more subjects with hyper-PMTF (P-type = 3), but outside the threshold criterion limiting to 193 subjects. Note that the two curves for hyper-PMTF are fairly flat. The data points are horizontally displaced for clarity.</p

Number, age, and gender of subjects in groups.

<p>Number, age, and gender of subjects in groups.</p

Individual audiograms for subjects with a forward masking threshold difference of less than 5 dB.

<p>Left ear, n = 6. The three worst audiograms come from the group <i>Industry</i> without restrictions on pure tone thresholds.</p

Mean audiograms for the profession groups.

<p>Group names as shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0097377#pone-0097377-t001" target="_blank">Table 1</a>. Please note that the group <i>Industry</i> had no threshold restrictions. The vertical bars represent 95% confidence intervals. The data points are horizontally displaced for clarity.</p

Characteristic PMTF-curves.

<p>The PMTF-curves are classified as four different P-types. 0 = normal PMTF-curves (2A) or typical sensorineural loss (2B) that is still measurable; 1 = mildly abnormal (2C), see text; 2 = suspected hyper-PMTF (not shown in the figure); 3 = hyper-PMTF (2D), indicates damage from impulse noise or other sudden, loud noise with a rapid onset; NM = not measurable (not shown in the figure). Triangles indicate peak thresholds. Round markers indicate valley thresholds.</p

Mean of the speech recognition thresholds in noise.

<p>S/N at threshold for the profession groups, two noise levels 70 and 85 dB SPL, left ear. The vertical bars show 95% confidence intervals. Numbers of subjects in the different groups are from left to right 23, 58, 12, and 12. The horizontal lines show values for a <i>middle-aged reference group</i> without hearing problems for the noise levels 70 (dashed) and 85 (dotted) dB SPL. (The means and the numbers of measurements were obtained from the ANOVA, which explains the discrepancy from the number of measured ears.)</p