We still have no data to change the diagnostic criteria for chronic migraine

To the Editor:

We read with great interest the article by Chalmer and colleagues on the proposed new diagnostic criteria of chronic migraine (CM) (1). The authors found no differences between patients with CM and patients with high frequency episodic migraine (HFEM), 8–14 migraine days per month, in the demographic profile, disability, or disease chronicity. Therefore, they concluded both groups are comparable and proposed new diagnostic criteria for CM to include those currently diagnosed as HFEM. We have concerns about the authors’ interpretation of the data and believe they do not support their conclusion (2,3).

First, the comparison of the demographic profile was limited to the following variables: Sex, age, age of onset, and self-reported provocation by triggers. Several variables previously shown to be different between patients with CM and episodic migraine (EM), including body-mass index, education, ethnicity, household income, or marital status, all have not been included in the current study for comparison (4,5).

Second, the authors did not show the data on the disease frequency using the headache/migraine days per month. Instead, they used lifetime/previous year attacks. The information of lifetime attacks is hard to interpret when the disease duration is unknown. The attack frequency over the previous year is an oxymoron: In the CM group, only 31% of patients had >36 migraine attacks in the previous year. That means, 69% of the CM patients had on average ≤3 migraine attacks monthly over the previous year?

Third, the authors claimed that HFEM is comparable to CM in disability. The four disability measures in the current study are defined by the receipt of social security benefits, including early retirement pension, receipt of sickness benefit, cash assistance, and rehabilitation benefit. For the following reasons, this claim requires further evidence:

CM patients are in a higher proportion in receipt of an early retirement pension than HFEM patients (33.5% vs. 20.8%, p = 0.001).

The odds ratios for all four disability measures are actually similar, ranging from 1.59 to 1.78. The case number may be simply too small to have the power to allow any statistical difference for the following: Sickness benefit, 15 (9.9%) vs. 9 (5.7%), p = 0.207; cash assistance, 25 (16.4%) vs. 13 (9.2%), p = 0.139; rehabilitation benefit, 18 (11.9%) vs. 12 (7.5%), p = 1.

The claiming of social security benefits strongly depends on the social security system in a country and is influenced by many other confounding factors besides disability. An alternative of using Migraine Disability Assessment (MIDAS) or Headache Impact Test (HIT-6) would have been more convincing, whereas both have been well established for the comparison between EM and CM (6,7).

Fourth, the authors claimed that the chronicity is comparable between both groups. An indicator of a chronic disease in the current study is the use of triptans. However, the authors did not show that CM and HFEM are comparable. The patients with HFEM actually consumed “more” triptans than those with CM. This is perplexing. When the efficacy is comparable and the patients with CM have definitely more migraine attacks than those with HFEM, why would they consume less triptans? Whether potential confounding factors including the use of preventive medication or comorbid medication overuse headache (MOH) are comparable should be examined (8,9).

Lastly, the inclusion of the second cohort in the current study provides no additional information either to support or refute the claim of the authors:

Both cohorts are completely different. One is a tertiary headache referral center; the other is a cohort of medical students even in a different country.

The authors did not validate their findings with the second cohort in the study. Whether the findings can be generalized in the second cohort remains unknown.

We agree with the authors that the disease burden of HFEM is very high and requires more recognition. These patients can be by all means similar to those with CM and certainly deserve more clinical awareness and proper treatment. However, the current study provides insufficient evidence to support the claim that HFEM and CM are comparable in demographic profile, chronicity or disability. The suggestion of proposed new diagnostic criteria for CM to include those with HFEM is therefore questionable and premature.