Our study shows that elderly patients are at much higher risk of developing ABM than younger adults. We also observed a change in the etiology of the disease, Neisseria meningitidis ranked first for younger patients, whereas it was surpassed by pneumococcal, listerial, and meningitis of unknown origin in the elderly. Another important, although expectable, change is the increased prevalence of co-morbid conditions among elderly people. This may have an impact, not only on the etiology of ABM, but also on the rate of complications and eventually on mortality. Other differing findings for elderly people include a higher chance of having received out-of-hospital antibiotic therapy, more frequently lacking fever, neck stiffness and skin rash, developing neurologic and extra-neurologic complications more frequently, and having an increased risk of death because of ABM. In summary, the linkage and interaction of higher co-morbidity, different etiologic spectrum, more pre-admission antibiotic therapy and subtler clinical manifestations may contribute to explain most of the differences between elderly and younger patients.
However, our work has inherent limitations. First, our study is based on a single hospital and this may imply that the results may only be applicable to places with a population structure and local ecology similar to ours. However, changes in the spectrum of ABM similar to those observed in our study have been described, especially those referring to the epidemiology of meningococcal disease [2
]. Moreover, the pattern of population change documented in Spain, has also been reported in other Western, mainly European countries [27
]. Therefore, the population scenario may resemble that described in the present work. Our study spans over a long period of time; during 30
years it is obvious that the treatment and management of critically ill patients and of ABM have changed a lot. Although the case record forms were standardized and remained the same throughout the study, they may reflect differing sensitivities among treating physicians and in the end are a reflection of the “real world”. An important part of this real world is meningitis of unknown origin which is the most diagnosis-challenging ABM. We had strict criteria for this diagnosis, and when reasonable doubts arose, the case was not included in our series.
Changes in population structure contribute to changes in the spectrum of ABM. During the study period, Barcelona population has ranged from 1,754,900 inhabitants in 1982 to 1,619,337 in 2010, with a minimum of 1,508,805 in 1996 [28
]. However, the elderly population increased from 252,415 inhabitants in 1986 to 419,988 in 2006, whereas the population aged 15–24 years decreased from 266,791 in 1986 to 157,737 in 2006 [28
]. Therefore, a decrease in the population more prone to meningococcal meningitis was observed together with an increase in the population more susceptible to having pneumococcal and listerial meningitis. Surprisingly, meningitis of unknown origin also significantly increased in group II, which may be explained by a higher prevalence of out-of-hospital antibiotic therapy among elderly people. The negative impact of pre-admission antibiotic therapy on the positivity of microbiological tests is well known [12
is not a common cause of ABM in elderly patients, our epidemiological environment has for many years been one of high endemicity of meningococcal disease [24
]. Moreover, the incidence of disease due to serogroup C meningococci increased in Spain from the mid-nineties [32
]. This change of pattern led to the carrying out of a mass vaccination campaign against serogroup C in Spain and to inclusion of the meningococcal C conjugate vaccine in the routine vaccination schedule [33
]. Since group C meningococci have a predilection for causing disease in older adults [12
], the vaccination campaign may also have contributed to the decreased incidence of meningococcal meningitis among the elderly as our results show. This can be explained by the increase in herd immunity [34
With respect to clinical findings, although elderly patients were brought to the hospital not later than younger ones, the time before first antibiotic dose was significantly longer. There are a cluster of circumstances that may explain this finding; elderly patients had taken out-of- hospital antibiotic therapy more frequently, and this, together with being older, makes the symptoms and signs of ABM subtler, as exemplified by the less frequent presence of symptoms and/or signs of ABM and skin lesions [15
], which may eventually lead to a diagnostic delay. But, above all, elderly patients had a cerebral CT performed more frequently, and this delayed the spinal tap and consequently the start of antibiotic therapy. Delay in starting antibiotic therapy has been advocated as a main factor in ABM-driven mortality [36
]. Therefore, a high index of suspicion of ABM is needed in elderly patients with fever and CNS dysfunction to rule out meningeal infection eventually leading to a prompt spinal tap and early starting of empiric antibiotic therapy.
There were no differences between both groups in terms of CSF parameters except for a higher CSF protein content for group II patients, which is most likely due to the predominance of pneumococcal, listerial and gram-negative bacillary meningitis in this group. We know that these meningitis have a higher CSF protein content than meningococcal which predominates among group I patients (data not shown). Underlying immune suppression may cause changes in CSF parameters; and we found like Erdem et al. [37
] lower CSF counts in immunosuppressed patients but not a different CSF/blood glucose ratio. In addition, the diagnostic yield of CSF Gram-stained smear was inferior in elderly patients, but we must take into account that pre-admission antibiotic therapy had been taken by 37% of patients, and we know that this is the most frequent cause of meningitis with negative CSF Gram-stained smears [35
Both neurologic and extra-neurologic complications appeared more frequently in elderly patients. This may be explained by two facts; first, the physiological reserve and the ability of many organs and systems to respond to insults decrease with age, and second, in elderly patients, the baseline organ function may be compromised because of prior co-morbid conditions. Therefore, ABM and/or its associated systemic consequences may further damage an already dysfunctional organ, eventually leading to its failure. However, once present, they usually convey high mortality rates both in young and elder patients. The excess mortality of ABM in elderly people is a usual finding in related literature [37
]. In fact, this difference in mortality rate is mainly driven by an excess mortality in pneumococcal and meningitis of unknown origin.