Hyperacusis is characterized by decreased sound tolerance or auditory hypersensitivity to ordinary environmental sounds [1]. A person with hyperacusis may have an exaggerated response or discomfort to sounds that are normally non-aversive to a typical person [2,3]. Hyperacusis has been described using several terms such as phonophobia, misophonia, and recruitment. These terms, however, have substantial differences. Phonophobia is the fear of episodic sound intolerance, which is brought about by excessive stimulation of the autonomic and limbic systems. Misophonia is a developed aversion to sounds such as chewing or breathing.

The diagnosis of hyperacusis remains obscure since there is no gold-standard objective test that can accurately determine the presence of hyperacusis in a person [4]. Most studies have described diagnosing hyperacusis based on a self-report questionnaire called the Hyperacusis Questionnaire formulated by Khalfa et al. [5] This hyperacusis questionnaire was found to be beneficial for the quantification and characterization of the clinical phenomenon of hyperacusis over three major dimensions: attentional, social, and emotional [6].

Several methods have been used in attempts to measure hyperacusis objectively. The assessment of hyperacusis usually involves pure tone audiometry (PTA), acoustic reflex test (ART), tone decay test, otoacoustic emissions (OAE), auditory brainstem response (ABR), and loudness discomfort levels (LDLs). Since hyperacusis is primarily characterized by an individual’s abnormal loudness perception, it is logical that an individual’s loudness discomfort perception is measured. Hence, this study focused on the measurement of LDLs.

LDLs predict the level at which tones are detected by a person to be uncomfortably loud. LDLs have been commonly used to quantify hyperacusis, but their reliability as a diagnostic tool has not yet actually been proven. The average LDL of normal-hearing individuals is approximately 100 dB HL [7]. The average LDL of hyperacusis patients varies across studies because the criteria for a strict diagnosis of hyperacusis are not generally agreed upon. The study of Goldstein and Shulman [8] suggested that an LDL less than 95 dB HL is an indication of hyperacusis, while it was 70 dB HL according to Anari et al. [9], and 80, 85, 80, and 75 dB HL at 500, 1,000, 2,000, and 4,000 Hz, respectively, according to Sherlock and Formby [7]. This variability of LDL values across studies is, therefore, a major limitation of LDL measurements [10].

Hence, the first part of this study aimed to investigate the test-retest reliability of LDL measurements in the diagnosis of hyperacusis. After establishing the reliability of the test, we can determine whether it is actually a good measure of hyperacusis. We also aimed to suggest cut-off values for diagnosing hyperacusis. In the second part of the study, we aimed to evaluate the clinical value of LDL measurements by monitoring LDL changes in patients with hyperacusis who were prescribed sound generators. The use of sound generators in the treatment of hyperacusis has been promising [11-13]. It would therefore be interesting to evaluate whether the efficacy of sound generators for patients with hyperacusis is reflected in their LDLs.

Hyperacusis is a condition wherein a person has abnormally strong reactions after exposure to sounds that are low to moderate in intensity, which leads to decreased tolerance to suprathreshold sounds [14,15]. Hyperacusis is said to be a precursor of tinnitus, and about 40% of patients with tinnitus also experience hyperacusis [16,17].

It is important to differentiate hyperacusis from the development of the loudness recruitment phenomenon. A person who experiences the loudness recruitment phenomenon most likely has cochlear hearing loss and dysfunction of the outer hair cells of the organ of Corti. The individual’s perceived loudness increases faster than normal upon exposure to an increasing intensity of sound in loudness recruitment. There is also narrowing of the dynamic range caused by the higher detection threshold, and the discomfort level is unaffected [18]. This is in contrast with hyperacusis, where an individual’s perceived loudness increases even in response to sounds of low to moderate intensity. There is usually no associated hearing loss, as was demonstrated in studies 1 and 2, since the patients enrolled in both studies had normal hearing thresholds (Tables 1 and 3).

The diagnosis of hyperacusis usually relies on the functional handicap or effect on life produced by this symptom. The handicap associated with hyperacusis can be quantified using the instrument published by Khalfa et al. [5], which is a self-report questionnaire with 14 items focusing on three factors (attentional, social, and emotional). The Multiple Activity Scale for Hyperacusis is another questionnaire used to evaluate the effect of hyperacusis on individuals’ daily living [19].

These are, nonetheless, only subjective measures of hyperacusis. There is still no uniform standardized diagnostic procedure for hyperacusis. Some researchers have attempted to measure hyperacusis objectively by using PTA, ART, the tone decay test, OAE, ABR, and LDLs. These tests, however, measure different areas in the auditory pathway, which would then perpetuate the question of how hyperacusis develops in the first place. It is therefore important to determine the pathogenesis of hyperacusis to find out whether it is quantifiable by a single diagnostic test.

Several potential mechanisms have been proposed in attempts to explain hyperacusis. One mechanism is rooted in the observation that both hyperacusis and tinnitus worsen during episodes of anxiety, stress, or fatigue. Endogenous dynorphins are said to be released during times of stress. These dynorphins are released into the synaptic regions of hair cells, and cause the release of glutamate, which is known to influence the intensity tuning of auditory processing. This, in turn, causes sound stimuli to be perceived as louder than they should be [20]. Auditory efferent dysfunction is another possible mechanism of hyperacusis. This mechanism is based on the problem of central gain modulation, which causes sensitivity to sound [21,22].

Another well-referenced mechanism is the neurophysiological model suggested by Jastreboff et al. [23]. This model postulates that the limbic system, autonomic nervous system, and auditory systems are all activated when a person has hyperacusis and tinnitus. There is central hyperexcitability in the central auditory system [20,23]. This central hyperexcitability is a key point supporting the possibility that a diagnostic test measuring the central auditory system, such as LDL measurements, would be a more appropriate objective measure of hyperacusis than other diagnostic tests. In this study, we focused on evaluating LDL measurements in patients with hyperacusis.

For study 1, inter-hour and inter-day testing of LDLs using two different types of stimuli were done. Two different types of stimuli were used because patients with hyperacusis usually complain of discomfort when exposed to certain environmental sounds. Pure tone stimuli only have a narrow frequency spectrum; hence, the additional use of white-band noise can increase the spectrum to which patients must respond. The retest reliability of LDL measurements using pure tone stimuli and white-band noise was generally excellent. The reliability values were higher than 0.9 using white-band noise and pure tone stimuli. The high retest reliability of LDL indicates that the patients’ LDLs were consistent even across different days and different hours in the day. The good test-retest reliability of LDL measurement in this study may be attributed to the type of patients included. In particular, group 3 comprised patients who solely complained of hyperacusis, unlike previous studies focusing on patients who had both hyperacusis and tinnitus [10]. Another factor that may have contributed to the high test-retest reliability of LDLs in our study seems to be proper instruction and the fact that the pure tone test signal was given to the study subjects by a single specialized audiologist, which minimized the measurement error caused by the tester.

Initially, cut-off values were determined in study 1 to predict the exact level at which hyperacusis can be diagnosed. Based on the mean hyperacusis response values in study 1, the cut-off values derived were 90 and 62 dB for pure tone and white-band noise stimuli, respectively. These cut-off values showed high specificity, but low sensitivity. A screening test with good specificity has the ability to correctly identify those without the disease. Test sensitivity, in contrast, is the ability to correctly identify those with the disease. The high specificity and low sensitivity of our derived cut-off values imply that LDL measurement may not be effective for the initial screening of patients with hyperacusis. LDL measurement may instead be more beneficial in ruling out those without hyperacusis and in monitoring patients’ progress/improvement when given a particular intervention. Hence, LDL use is more beneficial when measuring the effectiveness of a particular treatment. For the initial diagnosis, it is advisable to use LDLs in conjunction with other validated questionnaires to accurately determine whether a patient actually has hyperacusis. It is also important to mention that the cut-off values suggested are particularly useful for patients complaining of hyperacusis alone. For patients complaining of both hyperacusis and tinnitus, a higher cut-off value may be recommended in future studies.

Since the results of the first study showed that LDL measurements are suitable for monitoring hyperacusis during treatment, we incorporated our initial findings and used LDL measurements to monitor the progress of patients with hyperacusis and tinnitus upon prescription of sound generators. Sound generators, when given in addition to counseling, have been shown to be effective for the treatment of patients with hyperacusis [1,12,13]. Desensitization is the main concept of sound therapy [3].

Using this concept, study 2 involved prescribing sound generators to patients who had distressing hyperacusis or did not respond to environmental sound therapy. An increase in the LDL value after sound generator use would mean that there was an increase in the threshold of discomfort experienced by the patient. The results of study 2 showed that there was a significant increase in LDL values in patients who used sound generators. This objective improvement in LDL values was seen to be associated with the subjective improvement of the patients’ hyperacusic symptoms (Fig. 4). The improvement was also reflected in patients’ responses to questionnaires: the THI and VAS scores for loudness, awareness, annoyance, and effect on life. As their LDL values increased, their scores on the questionnaires decreased, showing a strong and significant inverse correlation. There was a greater improvement in their THI scores as their LDL thresholds increased (Fig. 5).

Conflicting reports have been published regarding the relationship between LDLs and patients’ self-reported hyperacusis handicap [24]. Some studies reported a weak negative correlation between across-frequency average LDLs and self-reported sound tolerance problems [25]. Some studies did not find a statistically significant relationship [19], while others reported a moderate and highly significant negative correlation between self-reports of hyperacusis and LDLs [26]. In our study, there was a strong inverse correlation between LDLs and patients’ scores on the questionnaires, meaning that measurements of LDLs indeed reflected the hyperacusic component of the subjective complaints of tinnitus.

The patients who complained of hyperacusis in study 2 showed a mean LDL value of 97 dB, which was slightly higher than the suggested cut-off value in study 1, which was 90 dB. This difference might be explained by the presence of comorbid tinnitus in the patients in study 2. In study 1, the patients in group 3 complained of hyperacusis only, while in study 2, the patients in group B complained of tinnitus and comorbid hyperacusis. Therefore, the patients with only hyperacusis in study 1 seemed to have lower LDLs than the hyperacusis patients with tinnitus in study 2. This may indicate that the good sensitivity, specificity, and test-retest reliability of LDL measurements are particularly useful for patients who complain of hyperacusis only.

Based on the results of study 1 and study 2, LDL measurements are important in the stepwise approach to hyperacusis. The results of study 1 show that LDL measurements are a reliable tool for monitoring hyperacusis. The good test-retest reliability signifies the internal validity of the test and that the measurements obtained are both representative and stable over time. The cut-off values for diagnosis, 90 dB for pure tone stimuli and 62 dB for white band noise, had a relatively low sensitivity but high specificity, which indicates the clinical value of LDL measurements as a diagnostic test for determining patients’ progress during an intervention rather than as a screening test. After being diagnosed with hyperacusis, patients can be monitored for improvement in response to an intervention. The objective improvement in patients’ LDL can also have a positive effect as they can objectively see that they are increasing their threshold for uncomfortable sounds, as was clearly demonstrated in study 2.

In conclusion, LDL measurements have high inter-day and inter-hour reliability regardless of the type of stimulus used (pure tone or white-band noise), indicating that this test has good internal validity. LDL measurements are particularly reliable in patients who complain of hyperacusis alone but are also helpful in those who have comorbid tinnitus. Significantly increased LDLs and decreased hyperacusis symptoms were observed in tinnitus patients with hyperacusis after 6 months of sound generator use. A study analyzing the correlation of scores on hyperacusis questionnaires and LDL scores can be done in the future to further strengthen the results of this study. LDL measurement is a reliable and valuable diagnostic tool to reflect the condition of hyperacusis during the course of an intervention.