Study finds higher cardiovascular fitness associated with greater hearing acuity

Abstract

A decline in hearing acuity is considered an inevitable consequence of advancing age.1-4 Significant hearing loss is present in approximately 33% of men and women 65 years and older.1 Initial signs of auditory aging start long before age 65. Johnson and Hawkins reported evidence of nerve degeneration in the cochlea secondary to hair cell degeneration by early adolescence.5 Numerous studies have reported deterioration of hearing levels with middle age and then a notable acceleration above 70 years.2,6–9 However, aging is only one of many factors that contribute to a decline in hearing sensitivity. Hearing ability is also commonly compromised by otologic disease and chronic exposure to noise.2 Reduction in blood circulation through the inner ear can also cause reduced hearing acuity over time. Metabolism and blood flow are directly related to the vascular pattern of the cochlea. Variations in cochlear blood flow may affect the availability of oxygen and glucose, which is more rapidly metabolized during sound stimulation.3 If a person's circulation is compromised, blood flow through the cochlea may also be reduced. Two studies reported a direct relationship between arteriosclerosis and hearing loss,10,11 while other investigators found no association between cholesterol and hearing deterioration.12-14 One study reported a significant correlation between degree of hearing loss and lipid levels.15 Brant et al. found a significant relation between high blood pressure and hearing loss.3 Significant correlations between hearing ability and cardiovascular disease have also been found in an older cohort of men and women.2 There is evidence that regular exercise may play a role in hearing conservation. Ismail et al. reported that subjects who completed a 20-week-long physical fitness program improved their cardiovascular health as measured by peak oxygen consumption (VO2 peak) as well as their baseline hearing thresholds.16 Cristell, Alessio, and Hutchinson reported improved pure-tone and temporary threshold shifts in healthy but low-average fitness level young adults who improved their VO2 peak following 8 weeks of twice-weekly aerobic exercise.17 The purpose of the study reported here was to determine if hearing acuity was influenced by cardiovascular health in subjects aged 12–82 years. The cross-sectional study collected information from participants who had been screened for otologic disorders and evidence of noise-induced hearing loss prior to assessment, thus reducing bias due to underlying hearing disorders. RESEARCH DESIGN AND METHODS Subjects were 154 volunteers: 106 females and 48 males. Table 1 illustrates participants by decade age group and gender. The largest number of subjects were in their 20s, reflecting the population of the university laboratory where testing and recruiting were conducted.Table 1: Sample size and gender distribution of 154 subjects.All subjects reported good general health and hearing ability. None reported smoking. All subjects were screened for middle ear disease. Participants with a unilateral hearing loss or any evidence of noise-induced hearing loss were excluded. For analysis purposes, one ear was randomly selected from each subject. We determined baseline, pure-tone, sweep-frequency thresholds and used a method of adjustment to track each subject's auditory threshold. Each ear of the subject was tested individually. We obtained thresholds using an automatic presentation of the pure tones. Test sweep-frequency procedures were as follows: Starting at 500 Hz, the program produced a continuous frequency sweep at 62.5 Hz per 4-second rate between 1000 Hz and 6000 Hz. Averages of the peaks and valleys of tracing excursions were calculated for 2000 Hz, 3000 Hz, and 4000 Hz from a printout of the monitor recording. Presbycusic effects typically show a decline above 1000 Hz. Baseline pure-tone thresholds were calculated. We determined VO2 peak using either a maximal or a submaximal graded exercise test on a Monark bicycle ergometer. Methods for assessing oxygen uptake have been described elsewhere.18 Heart rate and blood pressure were measured during both graded exercise tests. We analyzed the data using one- and two-factor ANOVA models as well as one-factor ANCOVA models. These methods are described in detail by Neter et al.19 To establish within-subject reliability and variability measurements for pure-tone thresholds, we retested each subject in two sessions separated by from 1 to 6 weeks. Results point to good repeatability, and statistics have been reported elsewhere.20 RESULTS Regression analysis of cardiovascular fitness and age from the cross-sectional data is shown in Figure 1. As expected, the older age group had lower VO2 peak (F1,154=50.44, p=0.0001, R= 0.50, and R2=0.25). Pure-tone hearing at 2000 Hz (F1,154=79.06), 3000 Hz (F1,154=50.44), and 4000 Hz (F1,154=95.53) also was lower in the older age groups (p=0.0001) (see Figures 2a–c). These data support presbycusis or age-related hearing loss. However, further inspection of the data revealed variability related to the interaction of cardiovascular fitness and age in pure-tone hearing.Figure 1: Regression analysis of cardiovascular fitness and age from the cross-sectional data, y=−0.27x+42.83, F1,154=50.44, p=0.0001.Figure 2a: Regression analysis of pure-tone threshold at 2000 Hz and age from the cross-sectional data, y=0.22x + −2.56, F1,154=79.06, p=0.0001.Figure 2b: Regression analysis of pure-tone threshold at 3000 Hz and age from the cross-sectional data, y=0.31x + −3.21, F1,154=79.47, p=0.0001.Figure 2c: Regression analysis of pure-tone threshold at 4000 Hz and age from the cross-sectional data, y=0.34x + −3.8, F1,154=95.53, p=0.0001.Considering age and fitness level as categorical variables (see Figures 3a–c), a two-factor ANOVA model tested for the influence of cardiovascular fitness, i.e., VO2 peak, when pure-tone thresholds were measured at 2000 Hz, 3000 Hz, and 4000 Hz. The categorical classifications of low, medium, and high cardiovascular fitness were then removed and replaced solely by the subjects' VO2 peak measurement. With these new parameters, the ANCOVA model (Hearing = [Hearing Mean] + Decade + VO2 peak + Decade* VO2 peak) was used to fit the data points. At 2000 Hz, 3000 Hz, and 4000 Hz, pure-tone hearing was influenced by cardiovascular fitness. The variable, Decade* VO2 peak, which relates to the effect of one's age and health on overall hearing, was significant (p <0.01), suggesting a difference in hearing for differing levels of VO2 peak in relation to age.Figure 3a: A two-factor ANOVA model with subjects' ages and fitness levels as categorical variables at 2000 Hz.Figure 3b: A two-factor ANOVA model with subjects' ages and fitness levels as categorical variables at 3000 Hz.Figure 3c: A two-factor ANOVA model with subjects' ages and fitness levels as categorical variables at 4000 Hz.In Figures 4a–c, residuals from LOESS fits to the data were used to generate graphs for 2000 Hz, 3000 Hz, and 4000 Hz, using age as a continuous variable for comparisons among the three different cardiovascular fitness groups. The data show differences in pure-tone hearing of the high cardiovascular fitness group compared with low and medium fitness levels at the older ages (i.e., >50 years). Age 50 appeared to be a separation point, after which fitness level and age were related in a statistically significant way, with high fitness being positively related to better hearing sensitivity.Figure 4a: –c. LOESS fits generating graphs for 2000 Hz, 3000 Hz, and 4000 Hz versus age as a continuous variable for comparisons among the three different cardiovascular fitness groups.DISCUSSION Numerous genetic, environmental, and lifestyle interactions affect hearing sensitivity. Over time, these factors tend to intensify a decline in hearing sensitivity. Delineating the effect of any individual etiologic influence is difficult, as single causes are rare and hard to identify. The relation of health risk factors that, over time, may exacerbate hearing loss has not been systematically studied. Therefore, the finding of a significant association of hearing loss and cardiovascular fitness level in small cohort groups is important. This finding may indicate that low cardiovascular fitness and presbycusis are related or have common pathogenic origins. One possibility is that a healthy cardiovascular system attenuates the effects of age on hearing processes, thus preventing presbycusis and maintaining hearing sensitivity. Results of the present study support the relevance of cardiovascular fitness in preserving hearing sensitivity over time. In our cross-sectional designed study, mean pure-tone hearing was most sensitive in the teenage group. Table 2 and Figures 3a–c illustrate better hearing levels in the younger groups (compared to the oldest group, aged 70s and 80s, in which mean pure-tone hearing was least sensitive: 2000 Hz=16.2 dB; 3000 Hz=21.3 dB; 4000 Hz=23.3 dB). The 350%, 769%, and 900% declines in pure-tone hearing at 2000 Hz, 3000 Hz, and 4000 Hz, respectively, between the teen years and age 60–80 show a dramatic difference in hearing sensitivity between young and old.Table 2: Pure-tone hearing levels in dB by decade age groups and frequency for low, medium, and high cardiovascular fitness levels.Nevertheless, our cross-sectional “snap shots” (Figures 1–3) indicate that pure-tone hearing sensitivity is similar in teens, 20-, 30-, and 40-year-olds. A significant drop-off in pure-tone hearing sensitivity was observed in the 50-year-old group and continued in the 60- and 70–80-year-old groups. Compared to published levels, the oldest high-fitness groups' hearing levels are better than the mean levels of 50-, 60-, and 70-year-olds (Figure 5).Figure 5: Presbycusic curves for thresholds by age (based on data from Kavanaugh30). Mean data for both women and men are combined in this illustration.Within the three older groups, persons in the high cardiovascular and medium fitness category demonstrated better hearing than those in the low cardiovascular fitness category in the same age groups. Although hearing decline occurred in persons in all cardiovascular fitness categories of all age groups, those with low cardiovascular fitness in virtually all age groups had the worst pure-tone hearing. The presbycusic curves follow general trends of published average levels of hearing loss (Figure 5). We have consistently reported a direct relationship between cardiovascular fitness and hearing sensitivity.17,18,20,21 Other laboratories have reported similar results using criteria such as VO2 max, blood pressure, and percent body fat, to distinguish persons with low and high cardiovascular health.16,22 Ismail et al. reported a 25% improvement of VO2 max and concomitant improvement in hearing sensitivity following a 20-week cardiovascular exercise program.16 Kolkhorst et al. reported better hearing in persons with higher cardiovascular fitness levels and lower body fat.23 Ismail et al. speculated that circulation of blood in persons with moderate or high VO2 max might be enhanced.16 Less resistance to blood flow, higher oxygenation of blood, or greater sensitivity and ability of tissues to receive blood may improve circulation of the blood. Gates et al. postulated that microvascular disease affects the capillaries and arterioles of the stria vascularis, a relatively high vascular area in the cochlea.24 The stria is the metabolic energy source of the cochlear DC potential, which is the electrochemical pump that powers cochlear function. Acute changes in blood pressure, heart rate, and core temperature have been implicated as possible mechanisms for reducing blood flow in the cochlea and causing hearing loss.25 Results from our laboratory have not supported an acute relation between blood pressure or heart rate and hearing.18 Yet, moderate and high cardiovascular fitness levels have protected against temporary hearing loss caused by noise.20,21 Current concepts in auditory physiology include an active mechanism that serves to counteract the effects of trauma and stress.26,27 The observations that hair cells contain specific proteins that undergo changes in expression with slight edema suggest that active elements exist to protect tissue from damage. Such proteins may also play key roles in protecting the hair cell from metabolic and aging changes, as has been suggested as the function of stress-induced proteins.22 Other studies have also raised the possibility that stress proteins could protect the auditory periphery from damage due to noise, ototoxic drugs, or trauma.28 Using computer-aided morphometric techniques, Pauler reported a significant relation between the extent of strial atrophy and hearing loss.29 The atrophy begins its slowly progressive course in the 3rd to 6th decade of life. Perhaps the well-known exercise training adaptations that occur in skeletal muscle carry over in some degree to the stria. Although hearing sensitivity and age are negatively related, considerable variability exists among hearing levels in older adults. In our study, persons in the high cardiovascular fitness category in all age groups had the greatest variability throughout most age groups, until age 50. In the 50-, 60-, and 70–80-year-old groups, persons with moderate cardiovascular fitness displayed better hearing at most frequencies than persons of the same age with lower cardiovascular health. High cardiovascular fitness was associated with the best hearing in the oldest age group. These findings must be considered in light of the small number of participants in the oldest age categories. CONCLUSIONS Several conclusions can be drawn from the present cross-sectional study: Hearing decline occurs over time, but is not discernible from peak (teenage) hearing levels until age 50. The relatively poor pure-tone hearing sensitivity observed in 50-year-olds is worse in persons with low cardiovascular fitness. Within the oldest age groups (50s-80s), persons with low cardiovascular fitness had worse hearing than persons with moderate and high cardiovascular fitness. ACKNOWLEDGMENTS The authors acknowledge support from Miami University's Undergraduate Research and Summer Scholars Program. The authors thank Neal Hubert, Gretchen Johnson, Katie Rebernak, and Matt Wheeler for their assistance with statistical analysis and data display.