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.