Discussion
Our report describes temporospatial clustering of SSNHL in an audiology department in South East England.
The temporospatial clustering of these cases suggests they are caused by a common environmental agent. Clinical experience is that chemical or other environmental compounds tend to cause bilateral hearing loss, whereas viruses may cause unilateral hearing loss. The parsimonious (but unproven) explanation for the clustering we see here is that of a transmissible virus spread between cases.
What makes a single aetiological agent even more likely is the near identical pathology in all three cases (Figures 1-3), of an upsloping low frequency sensorineural hearing loss. In general, low frequency SSNHL is associated with higher rates of recovery (4) compared to high frequency loss, and in some cases may be a prelude to the onset of Ménière’s disease. However, those who go on to develop Ménière’s disease in subsequent years tend to present with additional symptoms of tinnitus or vertigo (7), which were not found in these three cases.
Viral infection is thought to underlie other sudden inner ear disorders, including vestibular neuritis, labyrinthitis, and idiopathic (Bell’s) facial palsy. In Bell’s palsy occupational clustering has also been reported (8) (9), albeit without identification of an aetiological agent. The pathology underlying the hearing loss in the cases presented here can only be conjecture, however the low frequency loss of the audiogram suggests the virus (or other aetiological agent) is either causing cochlear hydrops or is preferentially affecting hair cells or neurons of the cochlear apex.
In 2019, Gilani and Shin hypothesised there may be temporospatial clustering of SSNHL, but their visual inspection of data on attendances for SSNHL at a single tertiary referral centre in the United States did not provide supporting evidence (6). However, retrospective analysis of aggregate data from a single institution will likely fail to identify clustering if those with SSNHL do not present at the same time or location. This could be for example because of a delay in onset between cases, because those affected do not present to medical services at the same time (or at all), or because they present to primary or emergency care, or to different providers of specialist care. It is also possible that clustering only happens in some subtypes of SSNHL (such as low frequency loss), which will again be difficult to identify from aggregate data. All the cases presented here were employed in the same audiology department, and so they presented for assessment at their place of employment, making recognition of clustering easy.
To the best of our knowledge, this is the first report of temporospatial clustering of idiopathic SSNHL. Our findings add to the evidence that a transmissible agent such as a virus underlies aetiology in at least a proportion of such cases.