Effect of simulated acute bilateral severe conductive hearing loss on static balance function in healthy subjects: a prospective observational pilot study

The continuous maintenance of static balance in upright standing is a complex process. The position of the human body relative to gravity and spatial surroundings is mainly sensed by central integration of visual, vestibular and somatosensory inputs [1]. The role of hearing and spatial acoustic information to maintain static postural balance is not completely understood. It seems to be obvious that auditory and vestibular systems function together, but their exact mechanism of interaction is not clear. Chronic hearing loss but also aging is associated with postural instability [2]. This might be the age-dependent result of the inner ear cell damage effecting both the hearing and vestibular organ [3]. The redundancy of the normal multisensory static balance system allows for a certain degree of compensation for loss of function of one of the involved systems (visual, vestibular and somatosensory, central integration) [4]. Postural dysfunction first becomes clinically visible if more than one of the components is highly damaged. Interestingly, the situation can be improved if one system is rehabilitated: hearing aids can improve static balance function in elderly with moderate to severe hearing loss [5]. The same can be seen in deaf patients receiving a cochlear implant. These patients often have postural dysfunction not obvious in daily life but only during forced postural perturbations. The reason is that these patients have an inner ear damage of both the cochlea and the vestibular system [6]. Hence, it is unproven, if chronic hearing loss could affect the postural control in a completely normal vestibular system.

In this regard, it is of interest to see if sudden acute hearing loss in younger patients affects postural stability. In a recent study by Horowitz et al., all but one static balance tests remained unchanged in young normal-hearing persons before and after the ears were plugged with earplugs [7]. Surprisingly, based on the significant deterioration in one subtest (when the somatosensory input was disrupted), they broadly concluded that acute conductive hearing loss has a negative effect on balance. Actually, the deterioration was only seen when another component, the somatosensory input was completely blocked. Therefore, we repeated and extended the experiment, a) using two acute hearing loss settings (earplugs and then headphones with white noise), b) randomizing the sequence of testing without and with acute hearing loss to avoid a learning effect, and c) by a standardized static balance test battery consisting of the Sensory Organization Test (SOT), the Motor Control Test (MCT) and the Adaptation Test (ADT) protocols, all in normal hearing healthy probands.

Studies on the associations between acute or chronic hearing acuity and postural balance as well as between hearing acuity and falls are scarce and the results have been contradictory [11]. Some studies found only a minor or no association between chronic hearing loss and postural balance or falls [12, 13]. Some other studies found an association between hearing loss and postural control or falls [14]. A recent Finnish twin study revealed that people with chronic hearing loss have a higher risk for falls, which is partially explained by their poorer postural control [11]. The easiest explanation is that there may be a concomitant age-dependent dysfunction of both the cochlear and vestibular sense organs given their shared location within the inner ear. To our knowledge, this has not been shown yet, but decreased hearing might result in decreased auditory spatial information leading to balance disturbances even in older people with normal vestibular function. It might be that hearing aid use might improve static balance function [15]. A third explanation is that chronic hearing loss may also limit access to auditory cues that are needed for environmental awareness. Attentional resources are critical for maintaining postural control [16], and decrements in attentional and cognitive resources imposed by hearing loss may impair the maintenance of postural balance [14, 17].

To analyze the role of normal hearing on static balance in people with normal vestibular function, experiments like the present study simulating acute hearing loss not allowing any adaptation to the acute impairment are important. Such studies are even scarcer than studies with people with chronic hearing loss. Horowitz et al. used also earplugs (like in experiment 1 in the present study) to induce acute bilateral hearing loss in 20 normal hearing young adults [7]. They also used the SOT and the MCT as static balance tests. They saw only a significant deterioration in condition 4 of the SOT. The other SOT conditions and all MCT subtest showed no difference (It is irritating that Horowitz et al. nevertheless interpreted the results in a way that acute hearing loss affects postural stability). Hence, the present study mainly confirmed the results of Horowitz et al. and showed this in addition for the ADT: acute moderate hearing loss had no impact on static balance function. Furthermore, we showed that even severe acute hearing loss (experiment 2 with headphones and white noise) had no influence on static balance function. The only other study analyzing the effect of acute disturbed auditory acuity on static balance function is from [18]. The used a Nintendo Wii™ gaming console (i.e., no reference values for clinical setting are available) for static balance testing of postural sway. The tests were performed first without simulated hearing loss and then with ear defenders. Unfortunately, any information on these ear defenders is missing. Hence, the effect on hearing in the study by Kanegaonkar et al. remains unknown. Wearing these defenders in a normal room or a soundproof room showed no clear effect on the test results: only in the setting with a normal and with open eyes on foam (p = 0.0495), in a soundproof room with open eyes on foam (p = 0.0164), or with closed eyes on foam (p = 0.0495), wearing ear defenders resulted on more postural sway. The p values are even marginal in two of these three setting. In all other four conditions, the defenders had no significant impact (all p > 0.05). Hence, also the work of Kanegaonkar et al. is more confirming than contradicting the present results. From a practical point of view, it can be concluded that acute hearing loss (for instance by a noise trauma at work) should not affect static balance. This could be important for occupations with high demands on static postural stability.

The present study was limited to normal hearing 19–31-year-old adults. The sample size was small. Hence, the results should be interpreted with caution. As a next step, we want to confirm the results in a larger cohort of young adults. Second, we want to repeat the study with older adults allowing the normal spectrum of hearing loss in such an older cohort. Furthermore, our focus is on static balance. That does not simply mean that acute bilateral moderate or severe hearing loss also has no influence on dynamic balance. When older adults (65–86 years) with no diagnosed hearing loss walk on a treadmill, ear plugging results in longer step length and step time [18]. However, ear plugging or white noise had no influence on the control of center of body mass. Overall, the authors interpret this in a way, that during a steady-state walking task, healthy older adults can maintain walking control without auditory feedback. However, it might be that temporal auditory cues provide locomotor feedback that becomes increasingly valuable as balance deteriorates with age. We interpret these results in context to the present finding as follows: auditory information or feedback signals, if at all, plays only a subordinate role for postural control in healthy younger adult, but might be relevant if one of the important information channels (visual, vestibular and somatosensory inputs) is altered, for instance, in older people.

A large test battery consisting of the SOT, MCT, and ADT has shown that normal hearing adults between 19 and 31 years of age principally show an unaltered normal static balance during a simulated acute conductive moderate or severe bilateral hearing loss, at least in this first small pilot study. This suggest that hearing or spatial information does not play an important role to support static balance even during postural challenges in young adults.