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On the borderline of pain

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Pain is one of the most complex sensory experiences. The International Association for the Study of Pain defines pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage”, which highlights the fact that pain is a subjective phenomenon with a negative connotation. But there are no rules without exceptions.

Borderline personality disorder (BPD) is characterized by emotion dysregulation, disturbances in social interaction, and an unstable sense of self. One particular attribute of BPD is a reduction in pain sensitivity. During so-called dissociation, i.e., a state of severe detachment from reality, pain sensitivity is often reduced even further in BPD, especially in emotionally stressful situations. A common strategy of BPD patients to overcome the unpleasant experience of dissociation is self-harm. Interestingly, the act of inflicting cuts, burns, or punches to their own body is usually not reported as being painful.

The mechanisms involved in the reduced pain sensitivity in BPD remain unknown. Previous results indicated that alterations in the central, rather than the peripheral, nervous system cause the attenuation and/or emotional re-evaluation of perceived pain. However, since the induction of pain in previous studies always relied on the application of nociceptive input, i.e., on stimulation which activates the nociceptors in the skin, it is difficult to differentiate between peripheral and central alterations.

The thermal grill illusion (TGI) offers the fascinating possibility to induce a vivid pain experience in the absence of nociceptive input via the simultaneous application of innocuous cool (e.g., 16°C) and warm temperatures (e.g., 40°C). In this setup, while the temperatures by themselves do not elicit pain, their combination induces an unpleasant and painful heat sensation. You might know a similar phenomenon when coming home from a winter walk while having forgotten your gloves and thus trying to acclimatize your cold hands by holding them under warm running water: suddenly, a painful heat sensation flashes through your limbs, which disappears after your hands warm up. The origins of this kind of illusion likely rely on the integration of non-nociceptive, but conflicting, thermal input in the central nervous system, which inhibits the cold and exaggerates the warm component of the incoming information.

In our study, which has recently been published in PAIN, my colleagues and I investigated pain perception in female patients with a current diagnosis of BPD. A group of female participants with BPD in remission was also included in order to differentiate between trait and state influences of the disorder. Both groups were compared to a sample of sex-matched healthy controls. We assessed the participants’ heat pain thresholds and related these to their pain intensity and unpleasantness ratings in the TGI experiment. If pain perception in BPD is altered at a central stage of the nervous system, then patients with current BPD would be expected to show reduced sensitivity to both heat pain and the TGI compared with healthy controls. Remission of the disorder would be expected to be accompanied by a normalization of pain perception.

We found that patients with current BPD reported significantly elevated heat pain thresholds compared to remitted BPD patients and healthy controls. More interestingly, our analyses revealed the same pattern for the TGI, i.e., current BPD patients reported about 40% less intense pain and unpleasantness towards the stimulation compared to the healthy controls, while remitted patients differed less than 5% in both measures from the control group. This finding indicates that central integration processes underlying the TGI appear to be distorted in current BPD, which normalizes during the course of remission. Further, we found the TGI pain intensity to be negatively associated with the level of dissociation in current BPD as assessed by questionnaires: the higher the level of dissociation, the less illusory pain was induced. Interestingly, we found a decoupling of TGI pain intensity and unpleasantness ratings only in the current BPD group. This contradicts the everyday experience that pain is rated more unpleasant the more intensely it is perceived. This is of special importance for BPD, since it might help to understand the use of self-harm: if the sensory component of pain is uncoupled from its aversive evaluation, the inhibition threshold to perform such a behavior might be reduced.

Illusions, whatever sensory domains they involve, help to understand how the brain is creating its representation of reality. By using an illusion of pain, we demonstrated that current BPD is accompanied by alterations in sensory integration related to nociception and that central mechanisms are likely involved in the disorder-specific insensitivity to pain. Dissociation might reflect a modulating factor. Since both reduced pain sensitivity and the disposition for dissociation are associated with the experience of traumatic events in early life, both phenomena might represent dysfunctional coping strategies with shared underlying mechanisms in BPD.

About Robin Bekrater-Bodmann

Robin Bekrater-BodmannRobin Bekrater-Bodmann, PhD, is a psychologist at the Department of Cognitive and Clinical Neuroscience at the Central Institute of Mental Health in Mannheim, Germany. His main research interest is the interaction between pain and body perception in health and disease as well as the use of bodily illusions to reveal the underlying processes. Any questions about his work can be directed to him at r.bekrater-bodmann@zi-mannheim.de

Reference

Bekrater-Bodmann R, Chung BY, Richter I, Wicking M, Foell J, Mancke F, Schmahl C, & Flor H (2015). Deficits in pain perception in borderline personality disorder: results from the thermal grill illusion. Pain 156 (10), 2048-92. PMID: 26098439

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