Lasting damage from meningitis

Wednesday October 17 2007

Having meningitis can have subsequent effects on your mental ability, reported Reuters. Even though many make a recovery from meningitis, cognitive impairment is common and one third of adult survivors may suffer from mild impairment.

This report is based on a study that pooled the data from three previous meningitis studies to look at the mental ability of 155 people who had recovered from meningitis. The study found that about a third of these people had some degree of mental impairment between 4.5 and six years after recovery.

The study’s results do suggest that people who have had bacterial meningitis may be at risk for having some subsequent impairment of mental ability, such as deficits in attention, thought and movement function. The study also found that there was no difference in mood, intelligence or memory. Larger prospective studies are needed to confirm this finding.

Where did the story come from?

Martine Hoogman and colleagues from the University of Amsterdam, carried out this research. Funding was provided by the UK Meningitis Research Foundation, and individual studies by Roche Pharmaceuticals Organon NV, Netherlands Organisation for Health Research and Development and the Dr Jan Meerwaldt Foundation. The study was published in the peer-reviewed medical journal_ Journal of Neurology, Neurosurgery, and Psychiatry_ .

What kind of scientific study was this?

This is a cross-sectional analysis of the combined data from three longitudinal studies that examined the neurological outcomes in meningitis survivors.

The researchers pooled the data from the three studies as each study had only a small number of meningitis cases, and the researchers wanted a greater number of survivors to analyse.

The combined studies included the results from the European Dexamethasone Study (EDS) and two studies of people in the Dutch Meningitis Cohort, giving a total of 155 patients who had meningitis (79 cases were caused by pneumococcal bacteria and 76 by meningococcal bacteria). They were compared to 72 healthy control subjects, 50 of whom came from the  EDS and 25 from the Dutch Meningitis Cohort (three people were enrolled in both studies). All of the controls were spouses, relatives or friends of the meningitis sufferers.

Patients from the EDS had been involved in a randomised controlled trial (conducted between 1993 and 2001); a smaller number of this group had their longer term neurological outcomes monitored.

The Dutch Meningitis Cohort monitored people who acquired bacterial meningitis between 1998 and 2002, some of whom were followed up for a longer period to look at their neurological outcome.

The meningitis patients had varying levels of recovery as determined by the Glasgow Outcome Scale, a recognized scale for judging recovery in meningitis survivors.

Neurological testing was carried out 55 months (on average) after meningitis in the pneumococcal patients and 69 months after meningitis in the meningococcal patients. The patients received a variety of neurological tests covering aspects of memory, intelligence, psychomotor function, attention/executive function, and mood. Age, education, and mental ability prior to illness were considered in all of the patients. All the meningitis patients followed in the studies had no previous psychiatric disorders or serious illness prior to acquiring meningitis.

What were the results of the study?

Researchers found that patients who had had meningitis scored worse than controls in certain tests of attention/executive function (cognitive abilities that are necessary for goal-directed behaviour), and of psychomotor function (involving the coordination of sensory or thought processes and movement). Cognitive speed was slower in people who had had meningitis than controls.

There was no difference between meningitis survivors and control subjects in mood, intelligence, or memory.

Overall, cognitive impairment (defined as having impairments in three or more test results) occurred in 37% of people who had had pneumococcal meningitis, 28% of people who had had meningococcal meningitis and only 6% of control subjects.

What interpretations did the researchers draw from these results?

The authors conclude that their “results showed that approximately one third of adult survivors of bacterial meningitis experience subtle cognitive impairment which consists mainly of slight mental slowness”.

What does the NHS Knowledge Service make of this study?

This analysis presents a preliminary analysis of the cognitive outcomes following meningitis and suggests that survivors of these bacterial infections may be at risk of some impairment following recovery. The limitations that should be considered when interpreting this study mostly relate to the possibility of selection bias (the way the patients were recruited into the studies), and include:

  • Researchers were able to present the findings of only a small number of cases of meningitis. Although they combined the three smaller studies, there was still a small number of cases to include in the analysis. This increases the likelyhood that any observed differences may have arisen by chance.
  • As the EDS study was a randomised controlled trial, patients would have had to fulfil specific selection criteria to be included. If these criteria were quite restrictive, many patients might not meet these criteria, and the people in the trial might not be representative of all patients with meningitis. However, the authors of the study report that characteristics of people in the EDS study were similar to those in the population-based Dutch Meningitis Cohort, which the authors suggest makes it more likely that the results of their study are representative.
  • The researchers also had to exclude patients who could not undertake all of the neurological tests that were used in the study, such as those with severe disability or poor outcome following meningitis. This has meant that the outcomes of a considerable number of meningitis patients were not available.
  • One major concern with this type of study is that the patients and the control groups may not have similar characteristics, and therefore any comparisons between them may reflect inherent differences between them rather than the effects of the illness. Studies usually try to avoid this problem by carefully selecting control participants who are as similar as possible to the patient group for characteristics that may affect the outcome. It is not clear whether controls were matched to the patients in this way in the original studies included here. However, the authors do report that the meningitis and control groups were similar in terms of age, education, and premorbid intelligence, which does give some reassurance that the groups are similar. As with all observational studies, it is not possible to ensure that all medical, genetic or other factors that may affect cognitive ability have been taken into account.
  • Although the study reports that the IQ of the meningitis patients before they became ill, was similar to the control subjects IQ, it is unclear how this information was obtained, especially as people would usually already have to be ill to be enrolled in a randomised controlled trial.

We are not able to speculate from these results what the longer term effects of cognitive outcome would be, how the patient would adapt and (as the authors mention) whether they would have an influence on the development of dementia.

Further larger studies using carefully selected controls will be needed before firm conclusions are drawn.