When does chronic migraine strike?

To the Editor:

We have carefully read the paper: “Proposed new diagnostic criteria for chronic migraine” by Chalmer et al. (1) recently published online in Cephalalgia.

In this study, the hypothesis is that patients with eight or more migraine days per month but not 15 days with headache per month, are as disabled as patients with chronic migraine (CM) defined by ICHD-3.

We want to point out several issues, in regard to:

Methodology

This is a prospective study, which evaluates disability based on economic variables (prescription registry, retirement pension, sickness benefit, cash assistance). In the headache field there are many patient related outcome measures (PRO's) which can be used to evaluate disability and quality of life which have not been used. So, we believe that this study correlates migraine with costs, not disability.

In regard to the sample included, the Danish cohort has 310 patients (CM = 151, HFEM = 159) and the Russian cohort has 46 patients (CM = 17, HFEM = 29). We are not sure if both samples can be compared as the Russian sample is smaller and from a different economic background. Also, this study did not include low-frequency episodic migraine (LFEM) or controls in the analysis to compare.

In Chalmer et al., 8 days per month is arbitrarily established.

Discussion

Chalmer et al. clearly state that their study provides strong evidence that the diagnostic criteria for CM should include patients with HFEM. This line of thinking is in accordance with principles previously outlined by Schulte and May (2).

After reading this paper, we want to clearly comment that in Chalmer et al. there is no mention of the study published in Cephalalgia in 2017 by Torres-Ferrus et al. (3). Our study was the first one to compare patients with CM with HFEM, LFEM, and controls in regards to clinical characteristics and disability, according to the ICHD-3 beta criteria, with a larger sample which had higher comorbidities similar to other epidemiological studies.

In Torres-Ferrus et al. we explored the number of days that correlated with a statistically different increase in burden and disability, finding a cut-off point with logistic regression models. In order to evaluate disability and impact we used the following scales: MIDAS, HIT-6, STAI, BDI, SF-36. By doing this, we found two cut-off points:

≤5 days/month: This indicates the threshold of where disability starts.

≥10 days/month: This is where we found a comparable disability and burden of patients to chronic migraine patients.

So, in Torres-Ferrus et al., we clearly demonstrated that patients suffering from 10 or more headache days per month (HFEM) are closer to CM than to those who suffer from fewer than 10 headache days per month (LFEM). Our reviewers back then did not allow us to propose any changes in the ICHD.

Moreover, the relationship between headache frequency and headache-related disability and impact on daily life is not linear, so participants suffering from 10 or more headache days per month showed similar emotional and functional disabilities to those with CM in a statistical model.

This was later similarly undertaken by Silberstein SD, et al who found similar cut-off points (4). We think that the paper by Chalmer et al. would benefit from comparing their cohort to other similar studies.