Predictors of episodic migraine transformation to chronic migraine: A systematic review and meta-analysis of observational cohort studies

Background and purpose An estimated 2.5-3.1% of people with episodic migraine develop chronic migraine in a year. Several risk factors are associated with an increased risk for this transformation. We conducted a systematic review and meta-analysis to provide quantitative and qualitative data on predictors of this transformation. Methods An electronic search was conducted for published, prospective, cohort studies that reported risk factors for chronic migraine among people with episodic migraine. Risk of bias was assessed using the Newcastle-Ottawa Quality Assessment Scale. Quality of evidence was determined according to the Grading of Recommendations Assessment, Development and Evaluation (GRADE) guidelines. Effect estimates were retrieved and summarized using risk ratios. Results Of 5695 identified publications, 11 were eligible for inclusion. The pooled analysis (GRADE system) found "high" evidence for monthly headache day frequency >= 10 (risk ratio = 5.95), "moderate" evidence for depression (risk ratio = 1.58), monthly headache day frequency >= 5 (risk ratio = 3.18), and annual household income >= $50,000 (risk ratio = 0.65) and "very low" evidence for allodynia (risk ratio = 1.40) and medication overuse (risk ratio = 8.82) in predicting progression to chronic migraine. Conclusions High frequency episodic migraine and depression have high quality evidence as predictors of the transformation from episodic migraine to chronic migraine, while annual household income over $50,000 may be protective.