Keywords
hypertension - differential sensitivity - blood supply - temporal coding
Introduction
Hypertension (HTN), commonly referred to as increased blood pressure, is a condition in which the blood vessels have persistently raised pressure. Blood is supplied to all parts of the body through the vessels from the heart. Blood pressure is produced by the force of blood pushing against the walls of blood vessels (arteries) as it is pumped by the heart. If the blood pressure is high, it is made harder for the heart.[1] HTN is popularly called the “silent killer” because it has no obvious signs or symptoms as a warning, and many people are not even aware of their problem. Eventually, the increased pressure overload due to HTN causes accumulating damage that the circulatory system can handle, leading to profound health conditions.[2]
HTN is a major risk factor for cardiovascular disorders, especially in India.[3] The most important cause of premature death globally is HTN that is rated by the World Health Organization (WHO).[1] South Asia's diseases of burden high blood pressure rank HTN as the third most vulnerable disease.[4] HTN is directly responsible for 57% of all stroke deaths and 24% of all coronary heart disease (CHD) deaths in India.[2] The global burden of HTN worldwide analysis revealed 20.6% of men and 20.9% of women in India were suffering from HTN. The rates for HTN in 2025 may go up to 22.9 and 23.6% for men and women in India.[5] Studies on the Indian population have shown a prevalence of HTN to be 25% in urban and 10% in rural areas.[6] However, noncommunicable diseases country profiles[7] revealed an increase in the prevalence of HTN among Indians at a rate of 33.2% in men and 31.7% in women. The relation between damaged hearing and high blood pressure is not complicated to comprehend. High blood pressure results in damaged blood vessels not centered only to one area of the body; somewhat, the entire body is affected, including the ears. The hearing could be affected due to the deposition of fatty plaque in the blood vessels of the ear.[2]
The damage of the cochlea is due to the loss of sensory hair cells in the inner ear or the damage to the eighth cranial nerve. This occurs naturally or as a result of aging, degenerative disease, injury, etc. There are few research works that report cochlear damage leading to hearing loss in individuals with HTN. Agarwal et al[8] reported a significant relationship between HTN and hearing loss by estimating the hearing threshold on HTN group and compared it with non-HTN group. de Moraes Marchiori et al[9] conducted a study on 308 individuals in the age range of 45 to 64 years, out of which 154 were individuals with HTN and 154 were controls. They revealed a significant relationship between HTN and hearing loss by using a questionnaire on hearing loss and correlated it with blood pressure reading and a poorer pure tone threshold. Soares et al[10] examined the audiological profile of adults with and without HTN using transient-evoked otoacoustic emissions (TEOAEs) and reported a lower TEOAE amplitude in individual with HTN.
Goyel et al[11] studied the impact of severity of hypertension on auditory brainstem responses in 50 healthy age and sex-matched controls (group I) and 50 hypertensive patients (group II). The latter group was further subdivided into group IIa (grade 1 HTN), group IIb (grade 2 HTN), and group IIc (grade 3 hypertension), as per WHO guidelines. They reported that the absolute latency of wave I was observed to be significantly increased in group IIa and IIb hypertensives, while wave V absolute latency was significantly prolonged among Group IIb and IIc, as compared with that of the normal control group.
Thus, all the above studies suggest cochlear or neural damage in individuals with HTN. The cause for hearing impairment in HTN individuals is explained as the reduction in oxygen transport, which increases the blood viscosity that reduces blood flow in the capillaries. This causes a reduction in oxygen, that is, hypoxia, thus causing hearing loss in the individual with HTN.[2] High pressure in the vascular system causes the inner ear to get damaged. This can lead to progressive hearing loss.
It is well known that normal cochlear/neural functioning is important for differential sensitivity or limen, which plays a vital role in speech perception. Since there is cochlear or neural damage reported in individuals with HTN, it is important to study the differential sensitivity in them. Pitch, loudness, and duration play an important role in the perception of speech, language, and music. The talker’s emotion will be identified based on the pitch, loudness, and duration in the case of speech. Traditionally, psychophysical methods, which are done in humans, depend mainly on the auditory system. Pure tones are used as principal stimuli to judge the difference between the two sounds.[12] The auditory processing is a fundamental component of most auditory tasks, which is seen at different stages, starting from the onset of a stimulus process by neurons and response to speech, a higher-level function controlled by the cortex. Hence, when the cochlea is damaged, functioning abilities such as coding, differentiation, and temporal processing will be affected. Previous studies have also reported sensory and neural damage in individuals with HTN.[2] Therefore, there might be deficits in differential sensitivity in individuals with HTN. There is a need to study the effect of HTN on differential sensitivity as there is limited literature in this area.
Methods
The study was performed using a standard group comparison design. The participants were selected using a purposive sampling technique. Thirty participants in the age range of 25 to 45 years were included in the study and classified into two groups. Group I included individuals with HTN, and Group II included individuals with normal blood pressure. All the audiological tests were performed in an acoustically treated suite that met the specification of ANSI S3.1–1999 (R2013). Additionally, the test room had optimum temperature and lighting and was free of distraction. Approval was taken from the ethical approval committee of the institute, and the testing was done using noninvasive procedures. Informed consent was taken from all the participants regarding the procedures and the objectives of the study. The equipment used in the study is the audiometer, immittance meter, otoscope, and laptop, in which MATLAB software was installed.
Procedure for Participant Selection
The detailed case history was taken as a closed set interview to rule out the auditory and vestibular disorder symptoms, the sensation of tinnitus, exposure to hearing harmful agents, such as occupational or leisure noise, drugs that are ototoxic general health conditions. The otoscopic examination was performed. It is done to visually inspect the status of the external ear canal and tympanic membrane. This procedure preceded the audiological evaluation. Normal hearing was ensured using pure tone audiometry, and all the participants had hearing thresholds within 25dBHL at octave frequencies between 250 and 8,000 Hz. They had a type “A” tympanogram with the presence of acoustic reflexes indicating normal middle ear functioning.
Assessment of Difference Limen Abilities
All the tests were performed using the MLP toolbox, which implements a maximum likelihood procedure for threshold estimation in MATLAB.[13] In all of the psychophysical tests, stimuli were presented monoaurally at an intensity of 60 dB HL. Further, the psychometric function that gives the maximum likelihood was used to decide the stimulus to be presented in the upcoming trial. The MLP usually meets the quite stable approximation of the most probable psychometric function, which can be used to approximate thresholds[13]-[15]
Frequency discrimination: In this, the minimum frequency difference necessary to discriminate between two closely spaced frequencies was assessed. Frequency difference limen was measured at 500 Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz. The standard and variable stimuli were 250 milliseconds long pure tones with the onset and offsets of 10 milliseconds raised cosine ramp.[13],[16] On each trial of three blocks, two blocks had a standard frequency of a pure tone and another block of variable frequency of a pure tone, which is always higher than the standard frequency. The variable block was identified by the participant. The psychometric function of 79.4% was achieved by using the three down, one up rule to estimate the difference limen for frequency.
Intensity discrimination: In this, the minimum intensity difference necessary to discriminate between two otherwise same sounds was assessed. Intensity difference limen was measured at 500 Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz. Both the standard and variable stimuli were of 250 milliseconds long, pure tones with 10 milliseconds rise and fall time.[13],[16] The minimum and maximum intensity deviations used were at 0.99 and 10 dB. On each trial of three blocks, two blocks had a pure tone at a standard intensity. Another block selected at random contained a pure tone of variable intensity, which is always higher than the standard intensity. The participant identified the variable block. The psychometric function of 79.4% was achieved by using the three down, one up rule to estimate the difference limen for intensity.
Duration discrimination: In this, the minimum time difference necessary to discriminate between two otherwise same sounds was assessed. Time difference limen was measured at 500 Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz. Both the standard and variable stimuli were of 250 milliseconds long, pure tones with 10 milliseconds rise and fall time.[13],[16] On each trial of three blocks, two blocks had a pure tone at a standard duration. Another block selected at random contained a pure tone of variable duration, which is always different from the standard duration. The participant’s task was to identify the variable block which will be of 0 to 25 milliseconds difference in duration between the two stimuli. The psychometric function of 79.4% was achieved by using the three down, one up rule to estimate the difference limen for time.
Analyses
Statistical Package for the Social Sciences (SPSS) was used to analyze the data. Shapiro–Wilk test of normality was performed to determine whether the data were normally distributed or not. The test scores of the difference limen abilities for individuals with HTN and with normal blood pressure were statistically analyzed.
Compliance with Ethical Standards
In this study, all of the tests were conducted on humans using noninvasive techniques, in accordance with guidelines provided by the Ethics Approval Committee at the institute. All the procedures were explained to the participants, and they were asked for their informed consent prior to participation in the study. The authors declare no conflicts of interest.
Results
In the study, two groups of participants were included. Group I included individuals with hypertension, and group II included individuals with normal blood pressure. In each group, 15 participants (30 ears) were considered for the study. For both the groups, differential limen for frequency, intensity, and time were assessed using the “MLP” toolbox, which implements a maximum likelihood procedure in MATLAB.
Comparison of Frequency Discrimination in Individuals with Hypertension and with Normal Blood Pressure
The mean, median, and standard deviation (SD) of frequency discrimination across frequencies such as 500Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz in individuals with and with normal blood pressure are shown in [►Table 1]. The data showed the difference between both the groups, which indicated poorer mean scores for the HTN group compared with with normal blood pressure group.
Table 1
Mean, median, and standard deviation (SD) scores of frequency discrimination across frequencies such as 500Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz in individuals with hypertension and with normal blood pressure
|
Mean (Hz)
|
SD
|
Median (Hz)
|
|
500 Hz
|
Normal blood pressure
|
59.0
|
23.5
|
65.0
|
|
Hypertension
|
98.0
|
18.2
|
95.9
|
|
1,000 Hz
|
Normal blood pressure
|
73.5
|
23.5
|
77.5
|
|
Hypertension
|
99.4
|
27.9
|
99.4
|
|
2,000 Hz
|
Normal blood pressure
|
75.3
|
14.9
|
78.0
|
|
Hypertension
|
102.3
|
27.6
|
99.0
|
|
4,000 Hz
|
Normal blood pressure
|
75.4
|
31.5
|
78.0
|
|
Hypertension
|
117.7
|
30.5
|
110.5
|
Shapiro–Wilk test of normality was administered to check whether the data are normally distributed or not and was found to be not normally distributed (p < 0.05). Hence, nonparametric inferential statistics were done. Mann–Whitney U test was administered for between-group comparison, and the effect size was also calculated. The results of the Mann–Whitney U test are shown in [►Table 2] for frequency discrimination. The data indicates the scores are significantly poorer for the individuals with HTN when compared with the individuals with normal blood pressure, as shown in [►Fig. 1]. The effect size of this difference was determined using the formula r = Z/ formula_57968_1. The effect size is 0.95, 0.73, 0.84, and 0.98 for 500, 1,000, 2,000, and 4,000 Hz, indicating that the effect size is strong across all the frequencies.
Table 2
Results of Mann–Whitney U test and effect size for frequency discrimination test
|
U
|
Z
|
p-Value
|
Effect size
|
|
500 Hz
|
98.0
|
5.2
|
< 0.01
|
0.95
|
|
1,000 Hz
|
176.0
|
4.0
|
< 0.01
|
0.73
|
|
2,000 Hz
|
133.0
|
4.6
|
< 0.01
|
0.84
|
|
4,000 Hz
|
81.5
|
5.4
|
< 0.01
|
0.98
|
Fig. 1 Mean and standard deviation of frequency discrimination for 500 Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz frequencies in individuals with hypertension (HTN) and with normal blood pressure. DLF, difference limen for frequency; DLI, difference limen for intensity; DLT, difference limen for time.
Comparison of Intensity Discrimination in Individuals with Hypertension and with Normal Blood Pressure
The mean, median, and SD of intensity discrimination across frequencies 500 Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz in individuals with HTN and with normal blood pressure are shown in [►Table 3]. The data showed the difference between both the groups, which indicated poorer scores in the HTN group compared with with normal blood pressure group.
Table 3
Mean, median, and standard deviation (SD) scores of intensity discrimination across frequencies 500Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz in individuals with hypertension and with normal blood pressure
|
Mean (dB)
|
SD
|
Median (dB)
|
|
500 Hz
|
Normal blood pressure
|
3.3
|
1.4
|
2.9
|
|
Hypertension
|
6.4
|
2.9
|
6.9
|
|
1,000 Hz
|
Normal blood pressure
|
3.1
|
1.5
|
2.
|
|
Hypertension
|
6.9
|
2.6
|
7.8
|
|
2,000 Hz
|
Normal blood pressure
|
2.8
|
1.5
|
2.7
|
|
Hypertension
|
5.4
|
3.1
|
6.4
|
|
4,000 Hz
|
Normal blood pressure
|
3.0
|
1.4
|
2.9
|
|
Hypertension
|
6.6
|
2.9
|
7.5
|
Shapiro–Wilk test of normality was administered to check whether the data are normally distributed or not and was found to be not normally distributed (p < 0.05). Hence, nonparametric inferential statistics were done. Mann–Whitney U test was administered for between-group comparison, and the effect size was also calculated. The results of the Mann–Whitney U test are shown in [►Table 4]. The results showed that the scores were significantly poorer for individuals with HTN compared with the control group, as shown in [►Fig. 2]. The effect size of this difference was determined using the formula r = Z/ formula_57968_2 . The effect size is 0.71, 0.82, and 0.71 for 500, 1,000, and 4,000 Hz, respectively, and it indicates that the effect size is strong across all these frequencies and the effect size is 0.42 for 2,000 Hz, which indicates the effect size is weak.
Table 4
Results of Mann–Whitney U test and effect size for intensity discrimination test
|
U
|
Z
|
p-Value
|
Effect size
|
|
500 Hz
|
182.0
|
3.9
|
< 0.01
|
0.71
|
|
1,000 Hz
|
145.0
|
4.5
|
< 0.01
|
0.82
|
|
2,000 Hz
|
286.5
|
2.4
|
< 0.05
|
0.42
|
|
4,000 Hz
|
185.5
|
3.9
|
< 0.01
|
0.71
|
Fig. 2 Mean and standard deviation of intensity discrimination for 500 Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz frequencies in individuals with hypertension (HTN) and normal blood group.
Comparison of Duration Discrimination in Individuals with Hypertension and with Normal Blood Pressure
The mean, median, and standard deviation (SD) of intensity discrimination across frequencies such as 500Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz in individuals with and with normal blood pressure are shown in [►Table 5]. The data showed the difference between both the groups, which indicated poorer mean scores for the HTN group compared with with normal blood pressure group.
Table 5
Mean, median, and standard deviation (SD) scores of duration discrimination across frequencies 500 Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz in individuals with and with normal blood pressure
|
Mean (ms)
|
SD
|
Median (ms)
|
|
500 Hz
|
Normal blood pressure
|
43.5
|
12.8
|
40.2
|
|
Hypertension
|
75.4
|
29.4
|
70.0
|
|
1,000 Hz
|
Normal blood pressure
|
60.6
|
36.6
|
50.4
|
|
Hypertension
|
110.9
|
34.9
|
127.0
|
|
2,000 Hz
|
Normal blood pressure
|
63.0
|
23.6
|
59.4
|
|
Hypertension
|
114.2
|
36.4
|
128.0
|
|
4,000 Hz
|
Normal blood pressure
|
53.2
|
20.3
|
52.1
|
|
Hypertension
|
116.1
|
38.5
|
133.5
|
Shapiro–Wilk test of normality was administered to check whether the data are normally distributed or not and was found to be not normally distributed (p < 0.05). Hence, nonparametric inferential statistics were done. Mann–Whitney U test was administered for between-group comparison, and the effect size was also calculated. The results of the Mann–Whitney U test are shown in [►Table 6]. The results showed that the scores were significantly lower in individuals with HTN, as shown in [►Fig. 3]. The effect size of this difference was determined using the formula r = Z / formula_57968_3 . The effect size is 0.80, 0.82, 0.85, and 0.87 for 500, 1,000, 2,000, and 4,000 Hz, respectively, and it indicates that the effect size is strong across all the frequencies.
Table 6
Results of Mann–Whitney U test and effect size for duration discrimination test
|
U
|
Z
|
p-Value
|
Effect size
|
|
500 Hz
|
147.5
|
4.474
|
< 0.01
|
0.80
|
|
1,000 Hz
|
141.5
|
4.564
|
< 0.01
|
0.82
|
|
2,000 Hz
|
128.5
|
4.761
|
< 0.01
|
0.85
|
|
4,000 Hz
|
120.0
|
4.881
|
< 0.01
|
0.87
|
Fig. 3 Mean and standard deviation of duration discrimination for 500 Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz frequencies in individuals with hypertension (HTN) and normal blood group.
In all the three test conditions, such as frequency, intensity, and duration discrimination, the scores were significantly poorer in individuals with HTN compared with individuals of the normal at all the frequencies such as 500 Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz.
Discussion
Our human body depends on a proper blood supply for the normal functioning and survival of the living cells in the body. The most common vascular disease is HTN that may create various changes such as in the heart and vessels. This may cause hemorrhage to the inner ear, which is supplied by the anterior and inferior cerebellar artery.[17]–[19] Sudden and progressive hearing loss is also caused due to HTN.[20] HTN can damage the ear in numerous ways. One of the possible mechanisms could be related to the increase in blood viscosity, which reduces capillary blood flow and ends up reducing oxygen transport, causing tissue hypoxia, thus causing hearing complaints and hearing loss in patients.[21] In addition to this, arterial HTN may also cause ionic changes in the cochlea's cell potentials, which could lead to hearing impairment.[22]
Khullar et al[23] reported significant prolongation of absolute peak latencies of waves I, II, and V and interpeak latency III to V in individuals with HTN. They conclude that there was a significant correlation of a rise in systolic and diastolic blood pressure with absolute peak latencies of ABRs in hypertensive patients. However, no significant difference in nerve conduction velocity was seen. They suggested that neural damage was seen in individuals with HTN. Goyal et al[11] studied to correlate and to assess the degree of involvement of peripheral and central regions of brainstem auditory pathways with increasing severity of HTN among the patients of essential HTN. The results revealed that all the hypertensives, that is, Group IIa, IIb, and IIc patients, were found to have highly significantly prolonged III to V inter peak latency compared with normal healthy controls. Hence, there has been greater involvement of pontomesenchymal region with the increasing severity of the disease.
Thus, all the above studies suggest cochlear or neural damage in individuals with HTN. The reason for hearing impairment in individuals with HTN is explained as the increase in blood viscosity that reduces blood flow in the capillaries, thereby reducing oxygen transport. This causes tissue hypoxia (reduced oxygenation), thus causing hearing loss in patients.[2] Inner ear damage can occur due to high pressure in the vascular system. This can cause progressive hearing loss. Arterial HTN may cause ionic changes that can lead to hearing loss. HTN causes degeneration of the inner ear due to alteration in microcirculation, thus accelerating the aging process in the inner ears.[2]
It is well known that normal cochlear/neural functioning is important for differential sensitivity or limen, which plays a vital role in the perception of speech. Since cochlear or neural damage is reported in individuals with hypertension, this could have led to poor performance in differential sensitivity in the present study. Pitch, loudness, and duration play an important role in the perception of speech, language, and music. Traditionally, psychophysical methods, which are done in humans, depend mainly on the auditory system. The auditory processing is a fundamental component of most auditory tasks, which is seen at different stages, starting from the onset of a stimulus process by neurons and response to the speech, a higher-level function controlled by the cortex. Hence, when the cochlea is damaged, functioning abilities such as coding, differentiation, and temporal processing will be affected. Previous studies have also reported sensory and neural damage in individuals with HTN.[2] In this study, the results revealed significantly poorer scores in differential limen for frequency, intensity, and duration in individuals with hypertension. This might be because of reduced frequency selectivity, poor temporal coding, and difficulty responding to rapid change in the envelope of sound over time because of cochlear and neural damage in individuals with HTN. However, the effect of HTN on the functioning of the central auditory nervous system should be further studied.
Conclusion
The results revealed a significant difference between the two groups in all three tests. HTN causes degeneration of the inner ear due to alteration in microcirculation, thus accelerating the aging process in the inner ears. It is well known that normal cochlear/neural functioning is important for auditory processing, like sensitivity limen abilities. The differential limen is an essential component of most auditory processing capacity, which can be seen at several levels, ranging from neuronal sensitivity to the effects of stimulus onset time to cortical processing of auditory information such as speech stimuli. When the cochlea is damaged, functioning abilities such as coding, differentiation, and temporal processing will be affected. Previous studies have also reported sensory and neural damage in individuals with HTN. These could be the possible reasons for deficits in differential sensitivity among individuals with HTN.
