Endolymphatic Hydrops Discovery Helps to Demystify Pathology of NIHL

In previous editions of BC Disease News, we have referred in passing to endolymphatic hydrops. We have done so in the context of a novel diagnosis of cochlear migraine (see here) and also when unpackaging the Court of Appeal’s ruling, in the historic acoustic shock case of Goldscheider v The Royal Opera House Covent Garden Foundation [2019] EWCA Civ 711 (see here).

Endolymphatic hydrops is a disorder of the vestibular system – the part of the body responsible for balance and spatial orientation. It is thought to result from abnormal fluctuations in endolymph fluid, which fills the hearing and balance structures of the inner ear. Ménière's disease (an inner ear disorder that causes vertigo, tinnitus and hearing loss) is routinely discussed in conjunction with endolymphatic hydrops, since all Ménière's patients have hydrops. It is, however, possible to be diagnosed with hydrops without having Ménière's.

The ‘primary link’ between inner ear fluid build-up and Ménière's disease is accepted by Dr. John Oghalai, an Otolaryngologist at the University of Southern California’s Keck School of Medicine.

However, he is the lead author of a new study whose findings have slightly modified his perception of the presupposed link, opening a ‘new window into understanding Meniere’s disease’.[i]

Exposure to loud noise can cause a loss of auditory nerve cells (responsible for sending acoustic information to the brain) in the inner ear, though the exact mechanism behind this form of hearing loss is not fully understood.

In 2018, a team of researchers, including Dr. Oghalai, exposed animal test subjects (mice) to blast pressure waves approximating a roadside bomb and discovered an association between nerve damage and fluid accumulation in the inner ear.[ii]

With the backing of the National Institute on Deafness and Other Communication Disorders, Dr. Oghalai has since developed his insight into this relationship by examining the effect of 80 dB to 100 dB noise exposure on the accrual of endolymph fluid in the cochlea.

Having subjected mice to common loud sound impulses, the University academics conducted optical coherence tomography, an imaging technique, to measure fluid levels.

According to the follow-up article, published in the Frontiers in Cell and Developmental Biology journal,[iii] the authors detected contemporaneous hydrops and inner ear nerve damage occurring with noise that one might encounter on a daily basis.

To be precise, inner ear fluid levels remained ‘normal’ at exposures of 95 dB or below, but within hours of exposure to 100 dB noise, a build-up amassed. 1-week post-exposure, it was uncovered that those same animals with fluid build-up had also suffered a loss of auditory nerve cells (cochlear synaptopathy).

Interestingly, where hypertonic saline (salt-based) solution was used to treat affected ears 1-hour post-exposure, both the immediate fluid accretion and the long-term nerve damage lessened, which led the authors to believe that hearing loss could be ‘at least partially’ prevented.

In this instance, the hearing loss referred to is that of ‘hidden hearing loss’ (HHL). This is a concept we have touched on in previous editions of BCDN, such as when we reported on the 2016 case of Ross v Lyjon (see here).[iv] Unlike conventional noise-induced sensorineural hearing loss (NIHL), damage caused by HHL cannot be demonstrated by standard pure tone audiometric testing methods. As Kujawa and Liberman (2015) described,[v] this is because the induction of temporary threshold shifts (TTS) does not induce hair cell death, but instead results in decreased responses to suprathreshold sounds that persist after thresholds recover. As such, HHL, or noise-induced synaptopathy, is expected to produce deficits in speech discrimination and intelligibility, especially in the presence of background noise.

The fact that the study conclusions indicated a correlation between cochlear synaptopathy and endolymphatic hydrops infers that both pathologic findings have a ‘common mechanistic basis’. They may provide clues to better comprehend how and when noise-induced damage to the ears happens and to also put forward new ways of detecting and diagnosing impending hearing loss, e.g. specialist scanning equipment. Validating the efficacy of a common nasal decongestant (hypertonic saline) as a possible ‘cure’ for HHL could also save significant time, money and resources.

Dr. Oghalai is hopeful that his latest study will prompt further research into the cause of endolymphatic hydrops and enhance the treatment options for Ménière's disease.

 

[i] ‘Study reveals common loud noises cause fluid buildup in the inner ear and suggests simple possible cure for noise-induced hearing loss’ (11 November 2021 Keck Medicine) <https://news.keckmedicine.org/study-reveals-common-loud-noises-cause-fluid-buildup-in-the-inner-ear-and-suggests-simple-possible-cure-for-noise-induced-hearing-loss/> accessed 12 November 2021.

[ii] Kim J et al., Osmotic stabilization prevents cochlear synaptopathy after blast trauma. Proc Natl Acad Sci U S A. 2018 May 22;115(21):E4853-E4860. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6003510/pdf/pnas.201720121.pdf> accessed 12 November 2021.

[iii] Badash I et al., Endolymphatic Hydrops is a Marker of Synaptopathy Following Traumatic Noise Exposure. Front. Cell Dev. Biol, 9:747870. (05 November 2021) <https://www.frontiersin.org/articles/10.3389/fcell.2021.747870/pdf> accessed 12 November 2021.

[iv] (23rd September 2016 Liverpool County Court).

[v] Kujawa SG and Liberman MC, Synaptopathy in the noise-exposed and aging cochlea: Primary neural degeneration in acquired sensorineural hearing loss. Hear Res. 2015 Dec; 330(0 0): 191–199. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4567542/pdf/nihms671500.pdf> accessed 12 November 2021.