Abstract
Pain complaints are frequently described by depressed patients, and are mostly attributed to abnormal pain perception and modulation. The present study aimed to assess whether a unique pain processing profile differentiates depressed patients from healthy controls. Participants were 25 patients suffering from a moderate-severe unipolar depressive episode and 25 age and sex-matched healthy controls. Thermal stimuli were used to assess sensory threshold and pain threshold. Pain-60 temperature (temperature that induces pain ratings of 60 out of 100) was the first noxious stimuli to be administered during the experimental session. Central pain inhibition was assessed via conditioned pain modulation (CPM) and the degree of central nervous system excitability was assessed via mechanical temporal summation. Depressed patients reported higher levels of pain compared with healthy controls, and a significantly higher perceived pain during the last month. Additionally, they displayed significantly lower pain-60 temperature values compared with healthy controls (p = 0.01). Otherwise, no significant group differences were found in measures of pain perception and modulation. Our results suggest that the initial evaluation of pain intensity among depressed patients, as validated by pain-60 temperature values, is increased compared with healthy controls, and might be the mediator between depression and pain complaints. Possibly, depressed patients' negative bias in the processing of pain is similar to their processing pattern of facial expression or future events. Further studies are necessary in order to establish the mechanisms underlying the excessive pain complaints reported by patients with unipolar depression.
Highlights
Pain symptoms are extremely common in depressed patients [1], and up to 80% of patients who present in primary care settings with major depression suffer from physical symptoms such as pain [2]
Depressed patients had significantly lower pain-60 temperature scores than the healthy controls (HC) (p = 0.01), indicating that their appraisal of the intensity of pain was increased in comparison to the HC
Depressed patients in our sample displayed lower pain-60 temperature values compared to those manifested by HC
Summary
Pain symptoms are extremely common in depressed patients [1], and up to 80% of patients who present in primary care settings with major depression suffer from physical symptoms such as pain [2]. A significant line of evidence shows increased thresholds for experimentally-induced pain (i.e., reduced perception of phasic cutaneous heat pain) in patients with major depressive disorder. The “paradox of pain” in depressed individuals is manifested in decreased experimentally-induced pain sensitivity combined with a high incidence of reported endogenous pain [5]. At the core of additional hypothesis in the literature [7, 8] is the assumption that a deficit in central pain inhibition plays an essential role in the pathophysiology of pain symptoms in depression and might explain their high incidence. Prior studies [3, 9] did not support this assumption, and failed to demonstrate decreased central pain inhibition among depressed patients when compared to healthy controls (HC). An alternative explanation is that the excessive pain symptoms among depressed patients are a result of the patients’ negative evaluation-bias in processing complex sensory input, just as is their processing pattern of facial expression and rating of future events [10]
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