Discussion
In this retrospective study, which comprised data from an entire Swedish
county during 18 years, post-meningitis hearing loss was associated with
age, pneumococcal infection and concurrent AOM. Despite recommendations
in the national guidelines, more
than a third of patients had not done a hearing test after recovery.
The main outcome in this study was post-meningitis hearing loss. Even
when we assumed that patients lacking audiometry had normal hearing, as
many as 43% had hearing loss. This is a higher prevalence than the 30%
wich has been described previously(3). This might, in part, be explained
by different definitions of hearing loss, and by some patients in the
present study having had previously undiagnosed hearing loss. The
conservative assumption that patients lacking audiometries had normal
hearing should at least not lead to an over-estimation of the prevalence
of hearing loss, and it seems reasonable to suppose that patients who
complained about hearing loss in the recovery period should have been
tested.
An obvious finding in this study was the increased risk for adults and
elderly of developing hearing loss. Age >70 years has
previously been associated with an “unfavourable outcome”(8), Since
most patients in this study had not tested their hearing prior to their
meningitis, the finding might, at least partly, be explained by the fact
that the prevalence of hearing loss increases with age. Some patients
might therefore have had previously undiagnosed presbyacusis. This risk
should partly be alleviated by our assumption that all patients in whom
hearing tests were missing had normal hearing. Missing audiometries were
not less common among the older age group (data not shown).
The correlation between AOM and hearing loss seems to have been partly
confounded by pneumococcal infection, since it decreased in the
multivariate analysis, however, the odds of hearing loss was still twice
as high among patients with AOM in the multivariate analysis. A Dutch
study also found that the odds of hearing loss increased by 2.6 in
patients with concurrent AOM (9).
S. pneumoniae – a common otopathogen - increased the odds of
hearing loss almost four-fold after controlling for other risk factors.
That patients with pneumococcal meningitis are more likely to develop
long-term hearing loss has been noticed in a previous meta-analysis as
well as in a retrospective review on children(3, 10). The negative
association between meningococcal infection and hearing loss in the
univariate analysis was not present in the multivariate analysis,
indicating the former results were confounded by age, meningococcal
infections being almost exclusively found among children and teenagers
(data not shown).
This study has several limitations, one of which is the retrospective
design, meaning that many patients did not undergo hearing tests. In
addition, most patients had not done hearing tests before their
meningitis, so there was no way of knowing for certain that the hearing
loss was caused by the meningitis. The design also means that
microbiological PCR tests and serotyping were not performed.
A strength of the study is that it encompasses a whole county of Sweden
during a period of 18 years, implying that the results should be
generalisable.
Better knowledge of risk factors for post-meningitis hearing loss can
hopefully result in better compliance with existing guidelines, leading
to more patients undergoing otoscopy when admitted to hospital with
bacterial meningitis, and more patients being followed up audiologically
after recovery. As discussed above, early diagnosis of concurrent AOM,
and subsequent myringotomy might decrease the risk of developing hearing
loss.
Conclusion
This study showed that the incidence of hearing loss after bacterial
meningitis was strongly associated with age, but also with concurrent
acute otitis media and S. pneumoniae infection. In an on-going
prospective study on the same population, we hope to be able to confirm
the findings more robustly.