Effects of Infrasound Exposure on Measures of Endolymphatic Hydrops

Infrasound is generated within the human body by processes such as respiration and myocardial contraction. External sources include those produced naturally, such as wind and earthquakes, and those that are human-made, such as automobile engines and heavy machinery. Wind turbines are known to emit infrasound with a fundamental frequency of 1 Hz with intensities approaching 100 decibels (dB), depending on wind speed. Over 75,000 wind turbines have been deployed between 2003 and 2015 in the U.S. alone. As environmental infrasound exposure has increased in prevalence and intensity with the advent of technologies such as large-scale wind turbines, renewed attention has been directed to the effects of infrasound on exposed individuals.

As it falls below audible thresholds, conventional wisdom would dictate that infrasound does not affect humans. However, some individuals living in proximity to wind turbines experience increased levels of annoyance and sleep disturbance in a dose-response fashion. Other reported symptoms from infrasound exposure include aural fullness, tinnitus, dizziness, and vertigo. Some researchers hypothesize that these otologic symptoms are related to the infrasonic component of wind turbine noise affecting inner ear function. However, since the mechanism or causal role have yet to be established, others attribute such symptoms to a psychosomatic or "nocebo" effect (i.e. worsening symptoms produced by negative expectations). As wind farms and other infrasound-generating sources become widespread, there is now a critical need to determine the effects of infrasound on inner ear function.

Studies conducted in humans have confirmed that infrasound has measurable effects within the cochlea. Hensel et al presented infrasound tones of 6 and 12 Hz at 130 dB sound pressure level (SPL) while simultaneously measuring distortion product otoacoustic emissions (DPOAEs). They observed considerable increases in DPOAE amplitudes in the presence of infrasound compared to when these tones were absent. The authors attributed this effect to the displacement of the cochlear partition during infrasound exposure. Further, Dommes et al demonstrated activity in the primary auditory cortex on functional magnetic resonance imaging during infrasound exposure, providing evidence that perception of infrasound occurs through known auditory pathways.

Reversible hydropic changes of the endolymphatic space have been observed during short-term exposure to infrasound and low frequency sound in several guinea pig models. Flock and Flock utilized an explanted guinea pig temporal bone model to visualize expansion of the endolymphatic space on confocal microscopy while applying tone bursts of 140 Hz between 88-112 dB. Shortly after this work, Salt detected changes indicative of endolymphatic hydrops in vivo using volume and flow markers iontophoresed into the endolymphatic space of guinea pigs during 3 minutes of exposure to 200 Hz tone bursts at 115 dB SPL. The observed changes in flow and volume in the endolymphatic space were reversible. The recovery half time in this study was 3.2 minutes. Subsequent work by Salt et al demonstrated that infrasound at 5 Hz generated larger endolymphatic potentials in the third cochlear turn than did frequencies in the audible range from 50-500 Hz. This was despite a presentation level expected to be below the hearing threshold of the guinea pigs. These studies demonstrate that infrasound and low-frequency tones have measurable effects on inner ear physiology, even at sub-threshold hearing levels.

While there is evidence that the human cochlea is stimulated by infrasound, it is not known if infrasound induces endolymphatic hydrops in humans. The proposed work will test the central hypothesis that short-term infrasound exposure induces reversible endolymphatic hydrops in the human inner ear. This hypothesis is based on the observations in the presented animal studies and the observed combination of auditory and vestibular symptoms reported to be associated with infrasound exposure.

In order to test the hypothesis in living humans, the proposed study will utilize electrophysiologic tests that are currently employed as clinical tests for endolymphatic hydrops. By using a combination of tests, evidence of hydrops will be sought in both the cochlea and the vestibular system.

1. Electrocochleography (ECoG). ECoG is an electrophysiologic test of cochlear function. Conditions such as Ménière's disease, which are characterized by endolymphatic hydrops, demonstrate an elevated summating potential to action potential (SP/AP) ratio on electrocochleography (ECoG). An increase in the SP relative to the AP is thought to be due to a deflection of the basilar membrane position toward the scala tympani. Accordingly, abnormal ECoG has been correlated with the finding of cochlear hydrops (in the basal turn) on gadolinium-enhanced MRI.
2. Vestibular evoked myogenic potentials (VEMPs). VEMPs arise from sound-induced activation of otolith organs and their associated vestibular neurons. The cervical VEMP (cVEMP) and ocular VEMP (oVEMP) are theorized to originate from the saccule and utricle, respectively. Thresholds, defined as the lowest stimulus intensity at which a response is seen, can be obtained at multiple test stimulus frequencies (250, 500, 750, 1000 Hz) and threshold response curves can be constructed. The lowest threshold for eliciting a response is typically seen at 500 Hz for both oVEMP and cVEMP. In hydropic conditions such as Ménière's disease, VEMP thresholds can be elevated or absent at all tested frequencies. Additionally, VEMP tuning curves can be shifted such that the lowest threshold is observed at a different tested frequency (e.g. 750 or 1000 Hz). A shift in resonance frequency of the otolithic organs due to pressure changes in the endolymphatic space is hypothesized to cause these changes.

Successful completion of the aims of this study will afford better understanding of the potential effects of infrasound on inner ear function. The findings of this work will fuel additional investigation of risks of infrasound exposure and may spur efforts to reduce individual and environmental exposure. A newly described mechanism would provide researchers, regulators and advocacy groups with a previously absent and crucial understanding of the effects of infrasound on inner ear function when crafting policy, designing new technologies, and ensuring the safety of exposed individuals