Abstract
To the Editor,
In a recent publication, Ambrosini et al. (1) investigated the diagnostic value of visual and intensity dependent auditory evoked potentials (VEP and IDAP) in migraine. They concluded that these tests had insufficient diagnostic efficacy alone, but a good efficacy when combined. This conclusion is based on a positive predictive value (PPV) of 94.1% and diagnostic accuracy of 81.1%. However, these values are misleading, because the authors have not taken pretest probability into account.
The pretest probability in the group tested with both VEP and IDAP was 86.5%, and a positive test increased the probability of disease by only 7.6 percentage points. A pretest probability (or migraine prevalence) of 15%, relevant for screening, reduces PPV and diagnostic accuracy to 30.7% and 69.2% respectively. The authors are in general too optimistic when interpreting their results. For instance, they describe a diagnostic accuracy of 54.3% for IDAP as “moderate to fair”. This is only 4.3 percentage points better than the 50% that is expected from chance alone. Also, unpublished accuracy estimates from our own database of blinded VEP-studies show values close to 50%. Hence, we agree with Ambrosini et. al. (1) on this point; VEP habituation is not useful in differentiating migraineurs from headache-free controls.
There are also other methodological limitations in the present multicentre study: (1) 87 % of the examinations were performed in one centre, the centres did not use the same VEP protocol, IDAP was only recorded in one centre, age and sex distributions were different in migraine and control groups, blinding during data recording was not applied and exclusions were not reported. These limitations may have an impact on results, as we have discussed previously (2). In addition, the study is retrospective, and the authors provide little information about how the subjects were recruited.
The authors argue that VEP and IDAP could be useful in uncertain headache cases, based on the usual comparison of controls who obviously do not have the disease with migraineurs who obviously do. However, sensitivity and specificity will likely be even lower within a clinical context characterised by diagnostic uncertainty (3). Lijmer et al. (4) found that diagnostic performance of a test will be severely overestimated if a diseased population is compared to healthy controls, probably because cases that are difficult to diagnose are omitted from case-control studies. In contrast, we recommend performing blinded cohort studies, preferably a multicentre study, within a clinically-relevant study population, e.g. comparing migraine with tension-type headache.
Although the concept of habituation is still of considerable scientific interest for several sensory and motor modalities in migraine, we have to conclude that the data presented by Ambrosini et. al. (1) suggests that the real diagnostic value of VEP and IDAP is very low. We therefore believe that uncertain headache cases should not be referred to VEP and IDAP. The proposed practice will waste time and resources, and it may also result in an incorrect diagnosis.
Footnotes
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.