Rethinking headache chronification

Abstract

The objective of this series is to examine several threats to the interpretation of headache chronification studies that arise from methodological issues. The study of headache chronification has extensively used longitudinal designs with 2 or more measurement occasions. Unfortunately, application of these designs, when combined with the common practice of extreme score selection as well as the extant challenges in measuring headache frequency rates (eg, unreliability, regression to the mean), induces substantive threats to accurate interpretation of findings. Partitioning the amount of observed variance in rates of chronification and remission attributable to regression artifacts is a critical yet previously overlooked step to learning more about headache as a potentially progressive disease. In this series on rethinking headache chronification, we provide an overview of methodological issues in this area (this paper), highlight the influence of rounding error on estimates of headache frequency (second paper), examine the influence of random error and regression artifacts on estimates of chronification and remission (third paper), and consider future directions for this line of research (fourth paper).

Figure 1

Several examples of regression to the mean. When there is a perfect correlation (r = 1.0) between two time measurement occasions (Time 1 and Time 2), there is no regression to the mean (standardized Z-scores remain consistent, top panel). However, when the the two measurement occasions are imperfectly related (ie, an individual has a different score at Time 1 and Time 2), as will be the case when the the variable being measured is imperfectly reliable, regression to the mean is inevitable. In the middle panel, Time 1 and Time 2 scores that are correlated r = 0.80 exhibit less regression to the mean than scores that are correlated r = 0.50 (standardized Z-scores regress to the mean, lower panel).

Figure 2

This series on headache chronification will examine the current view of headache chronification (Hypothesis 1 [H1]) as a disease wherein a subset of individuals exhibit increased headache frequency (up arrow) on a second measurement occasion vs a novel hypothesis that posits that at least some degree of the headache chronification/remission rates are due to statistical artifacts (up and down arrows) and not durable change (Hypothesis 2 [H2]). T1 = Time 1; T2 = Time 2.