Neurological syndromes associated with medication use

Dear Editor,

We have read with great interest the recently published study by Álvarez Soria et al. describing neurological adverse events associated with medication use from a patient safety perspective.¹

In recent years, numerous studies have shown that prevalence of adverse drug reactions is high and that many such reactions are potentially preventable.^2,3 However, most of the available data are gathered from hospital settings and the general population. From our point of view, conducting this type of studies in specific populations or for specific adverse reactions, as in the publication by Álvarez Soria et al., provides more concrete information about adverse drug effects and their causes in specific populations. This serves to make healthcare professionals aware of the importance of detection and prevention, as well as promoting the development of initiatives aimed at improving drug safety.

Published studies on neurological adverse events associated with medication use are scarce. Some years ago, we completed a study⁴ whose aim was to analyse and determine the annual incidence of neurological adverse reactions to medication which required a neurological consultation. We concluded that there is a need for more studies on neurological syndromes caused by medication. We are delighted that this second study by Álvarez Soria et al. has been published, and furthermore, we feel it is appropriate to make readers interested in this topic aware of our own study, since it provides useful additional information, especially regarding possible causes and prevention measures. Additionally, we wished to comment on those findings which seem to be the most divergent between the two studies.

Firstly, it should be noted that the prevalence of adverse reactions is quite low (0.586%), a finding which Álvarez Soria et al. attribute to the study design and its possible biases. Although the rates are not comparable due to differences in methodology, the annual incidence of neurological adverse reactions was 8.7% in our study; we included only those reactions that were directly responsible for an initial neurological consultation. The low percentage of preventable cases in this new study (21.9%) compared to the percentage reported by our study (91.6%) is also noteworthy. Such a difference in the percentage of preventability could be due to the methodological differences between the studies and to the fact that labelling an adverse reaction as preventable or non-preventable is largely a subjective judgement call.

Regarding the types of neurological syndromes observed, drug-induced parkinsonism was a frequent adverse reaction in both studies. However, the most frequent adverse reaction in our study was medication overuse headache (4.5%), whereas Álvarez Soria et al. found no cases. Since medication overuse headache is common, with a prevalence of about 1% in the general population,⁵ the absence of this neurological syndrome in all of the 17 896 patients assessed by this new study is striking.

Finally, in the study conducted by Álvarez Soria et al., adverse reactions were analysed by drug group instead of by separate active ingredients. Given the study's aim of characterising a particular type of adverse reaction (neurological reactions in this case), we believe the drugs involved should have been analysed separately. If not, researchers may fall victim to the fallacy of assuming that all drugs in the same class have a similar safety profile.⁶ If only one of the drugs is responsible for most of the adverse reactions, it will remain masked by the drug group and the trigger of these adverse reactions will be wrongly attributed to all drugs in the group. Furthermore, and more importantly from our point of view, the study does not clearly express the importance of selecting a specific drug from a group according to its safety profile.

We would like to use this opportunity to stress the need for similar studies, conducted in neurological patients, which will help raise awareness about the important healthcare consequences of drug-induced neurological disorders and promote the implementation of prevention measures.