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.