Differential Processing of Physical and Psychological Pain during Acute Suicidality

placed supine, a point 5 inches below the patella in the medial border of both tibias was identified, and pressure was applied using a hand-held gauge with a 1-cm 2 rubber tip (Wagner Instruments, Greenwich, Conn., USA) alternately 3 times per leg. The subjects were instructed to verbally report when sensation changed from pressure to pain or discomfort. The mean pressure pain threshold was averaged from the 6 measurements [7] . Table 1 shows the participants’ demographic and clinical characteristics. There were no significant differences in age, gender, race, or years of education between the groups. No significant differences in diagnosis, presence of anxiety, substance use disorders, medications, or specifically antidepressant treatment between the 3 patient groups were noted using the χ 2 test. ANOVA showed that depressive symptoms were severer in attempters and ideators than in nonsuicidal depressed patients, which in turn showed higher depression scores than healthy controls (F 3, 179 = 169.55, p < 0.001). All depressed subjects reported higher anxiety (F 3, 179 = 50.03, p < 0.001), hopelessness (F 3, 179 = 11.15, p < 0.001), sleep abnormalities (F 3, 179 = 57.88, p < 0.001), and poorer attention and speed processing (Trail-Making Test A; F 3, 179 = 9.37, p < 0.001) than healthy controls. ANOVA showed higher psychological pain in attempters and ideators compared with depressed nonsuicidal subjects, who in turn reported more psychological pain than healthy controls (F 3, 179 = 54.35, p < 0.001). A general linear model showed a gradient in pressure pain threshold with the highest values in attempters compared with ideators or depressed nonsuicidal patients, both of whom in turn showed higher values than healthy controls (F 3, 179 = 7.07, p < 0.001). Linear regression analysis between time since attempt and pressure pain threshold was not significant ( r 2 = 0.001; p = 0.877). Stepwise linear regression analysis showed that pressure pain threshold was only correlated with usual psychological pain in the last 15 days, hopelessness, and current physical pain [ r 2 = 0.245; p = 0.002, β(psychological pain) = 0.76, β(hopelessness) = –0.51, β(physical pain) = –0.41]. On a follow-up assessment, performed 3–5 days after the initial interview, both attempters (n = 16) and ideators (n = 14) showed reductions in severity of depression (t = 7.52, p < 0.001), suicidal ideation (t = 4.87, p < 0.001), and psychological pain (t = 4.8, p < 0.001) in a paired t test. In contrast, only attempters showed a decrease in pressure pain threshold (t = 2.34, p = 0.037), while ideators’ measures did not change (t = –0.44, p = 0.67). Our main findings were: increased pressure pain threshold in depressed subjects compared with healthy controls, with the highest threshold in attempters; pressure pain threshold in attempters was reduced to ideators’ levels within 5 days; high levels of psychological pain were found in attempters and ideators, and pressure pain threshold was correlated with hopelessness and psychological and physical pain. A key barrier to suicide prevention is the limited understanding of its neurobiological mechanisms. Investigating pain regulation in suicide may be fruitful in this regard for several reasons: pain syndromes are associated with depression and suicide; pain biology is relatively well understood, and there are well-established methods for studying pain regulation in humans. For example, survivors of a suicide attempt show a higher pain threshold and tolerance [1] . Data in depressed people are less consistent but support a decreased sensitivity to experimentally induced physical pain [2] . Psychological pain, considered cardinal for suicide [3] , is found in both recent suicide attempters and depressed patients with suicidal ideation [4] . It is noteworthy that a remarkable overlap in the subjective response and neurobiology of physical pain and social rejection has been described [5] . We hypothesized that recent suicide attempters would show an elevated experimental pressure pain threshold while endorsing high psychological pain, and their pain threshold would normalize with the resolution of the suicidal crisis. To characterize pain processing in suicide, 4 groups of individuals between the ages of 18 and 65 years were studied: recent suicide attempters (score of ≥ 2 on the Columbia Suicide Severity Rating Scale actual lethality subscale; n = 52), suicide ideators (n = 64), nonsuicidal depressed patients (n = 24), and healthy controls (n = 40). Participants were recruited between 2015 and 2016 from University of Arkansas for Medical Sciences inpatient and outpatient services and the community. Subjects in clinical groups fulfilled DSM-IV criteria for major depressive episode. Exclusion criteria were: (a) inability to read and write English; (b) dementia, neurovascular or neurodegenerative conditions; (c) use of opioid or nonsteroidal anti-inflammatory agents, and (d) history of nonsuicidal self-harm. The University of Arkansas for Medical Sciences Institutional Review Board approved all procedures. After written informed consent, the subjects completed an interview, behavioral ratings (Columbia Suicide Severity Rating Scale, Beck Depression Inventory, Beck Anxiety Inventory, Beck Hopelessness Scale, Trail-Making Test, and the Physical and Psychological Pain Scale [6] ), and pressure pain threshold measurement. The pressure pain threshold was measured with patients Received: April 29, 2016 Accepted after revision: September 6, 2016 Published online: February 10, 2017