Objectives: There is a need to find meaningful markers that can distinguish between unipolar and bipolar depression (UPD and BD respectively). In patients with UPD, unique and inconsistent patterns of pain perception and modulation have been widely described. At the same time, patterns of pain processing in BD have been poorly studied. A recent study showed that initial evaluation of pain intensity is elevated in UPD compared with healthy controls (HC). The aim of the present study was to compare the pain processing profile between UPD and BD. Methods: Participants were 40 UPD patients, 36 age- and sex-matched BD patients, and 35 healthy controls (HC). Thermal stimuli were used to determine sensory threshold and pain threshold. Pain 60 temperature (i.e., a temperature that elicits a pain rating of 60 out of 100) was the first noxious stimulus administered during the experimental session. Central pain inhibition was assessed using conditioned pain modulation (CPM). All participants completed questionnaires on sociodemographic and clinical information. Results: The only discriminatory experimental pain finding between UPD and BD was related to pain-60. Patients diagnosed with UPD had significantly lower pain-60 indices than patients with BD (p = .004). This finding was confounded by the level of anxiety symptoms. Conclusion: Patients diagnosed with UPD initially rated their pain intensity higher than patients with BPD and HC. Nonetheless, this difference becomes insignificant when controlling for the higher anxiety scores in the UPD group. Possibly, the higher levels of anxiety were manifested in a pronounced negative cognitive bias. This finding is important for the management of pain symptoms in patients with UPD and BD. Further studies should focus on anxiety as a mediator in pain processing. Limitations: We used the pain 60-temperature test, a method not yet established as an instrument for this purpose, to assess an evaluation bias. Antidepressants treatment of participants might have an antinociceptive effect. Physical and mental comorbidities of the participants may confound the results of our study.
- Bipolar disorder
- Conditioned pain modulation (CPM)
- Major depression
- Pain processing