Increased affective bias revealed using experimental graded heat stimuli in young depressed adults: evidence of "emotional allodynia"

Abstract

Objective: To examine the hypothesis that young adults with major depressive disorder (MDD) would show increased affective bias to painful and nonpainful experimental heat stimuli, as evidenced by an increased responsiveness to warm and hot temperatures. Pain and depression often occur together. Pain is both a sensation and an affective experience. Similarly, depression is associated frequently with somatic symptoms as well as emotional dysphoria. Existing evidence indicates that MDD may be associated with altered pain processing. However, the extent to which alterations in experimentally controlled heat pain sensations are related to increased affective bias in MDD is unknown.

Method: Graded nonnoxious and noxious heat stimuli were delivered randomly with a thermode applied to the volar surface of the left arm of 15 unmedicated subjects with current MDD and 15 age- and gender-matched healthy comparison subjects. MDD and non-MDD subjects rated the intensity and unpleasantness of all stimuli.

Results: Two main results were observed. First, MDD relative to non-MDD subjects showed decreased heat pain thresholds. Second, a significantly increased affective bias (unpleasantness/intensity) was observed in subjects with MDD, particularly over the range of nonnoxious heat stimuli. This bias was independent of the change in sensory pain thresholds.

Conclusion: These findings represent corroborative evidence of abnormal affective heat pain processing in young adults with MDD, and suggest that MDD is associated with "emotional allodynia," a qualitatively altered negative emotional response to normally nonaversive thermal stimuli.

Figure 1. Psychometric Functions. A. Intensity (left) and unpleasantness (right) ratings to temperature stimuli in MDD (closed shapes) and Comparison (CON, open shapes) Subjects

Repeated measures ANOVA showed significant effect of group and temperature for both measures, and significant group × temperature interaction for the unpleasantness measure (p’s < 0.01). B. Affective Bias ratings during temperature stimuli in MDD (closed shapes) and Comparison (CON, open shapes) Subjects. Affective bias to heat stimulation was calculated by dividing the individual subject’ unpleasantness scores by the corresponding intensity scores (See Methods). Repeated measures ANOVA showed significant main effect of group, temperature and significant group × temperature interaction (p’s<0.05). Arrows indicate heat pain intensity thresholds for MDD (solid) and CON (broken) group (MDD: 41.7±3.3; CON: 43.9±1.1, p<0.05, t(28)=2.4).