Center for Neuroplasticity and Pain

Ph.D. Defense by Megan Elizabeth McPhee Christensen

Megan Elizabeth McPhee Christensen will defend her Ph.D. thesis “Temporal Changes in Pro-nociceptive and Anti-nociceptive mechanisms in Relation to the Experience of Low Back Pain: Evidence from Experimental and Clinical Models” on Tuesday 1 December at 13:00

Last modified: 30.11.2020


13.00 Opening by the Moderator Thomas Graven-Nielsen
13.05 PhD lecture by Megan Elizabeth McPhee Christensen 
13.50 Break
14.00 Questions and comments from the Committee
          Questions and comments from the audience at the Moderator’s discretion
16.00 Conclusion of the session by the Moderator


The Faculty Council has appointed the following adjudication committee to evaluate the thesis and the associated lecture: 

Dr. Petra Schweinhardt, University of Zurich, Switzerland

Associate Professor Robert R. Edwards, Pain Management Center at Brigham & Women’s Hospital, Chestnut Hill, MA, USA

Associate Professor Carsten Dahl Mørch, Aalborg University, Denmark

Professor Thomas Graven-Nielsen, Aalborg University, Denmark


Low back pain (LBP) has afflicted humans for thousands of years and today remains a leading cause of disability globally. The vast majority of LBP cases are classified as non-specific, meaning no clear pathophysiology has been identified. During the past decades, increased focus has been on elucidating underlying mechanisms and drivers of LBP. As such, measures of pain sensitivity, such as local and widespread hyperalgesia, temporal summation of pain (TSP), and conditioned pain modulation (CPM)), have gained special interest and may reflect mechanisms that play an important role in LBP. A plethora of studies have now investigated either cross-sectional differences in pain sensitivity between LBP patients and controls, or the utility of these measures after pain onset in predicting longer-term prognosis. Unfortunately, results have been highly inconsistent, and it has been unclear to what extent alterations in these mechanisms are present among LBP populations and whether alterations are present prior to pain onset or develop and/or fluctuate over time.

The present thesis set out to further investigate this temporal relationship between pain sensitivity and LBP experience in a systematic review and three experimental studies. In the systematic review and meta-analysis, clear differences in TSP and CPM were demonstrated overall between LBP patients and controls, though the magnitude of these differences was small. Alterations in TSP were weakly related to pain severity, while CPM impairment showed relation to both pain duration and severity. Study I highlighted that mild experimental LBP (delayed onset muscle soreness) provoked reductions in local and distant PPTs but was not sufficient in intensity or duration to significantly effect TSP or CPM. Baseline pain-free TSP did, however, show some relation to the severity of LBP developed. Study II showed that, during a recurrent LBP episode, patients demonstrated local and widespread pressure hyperalgesia and facilitated TSP compared to controls, but this resolved when the patients were pain-free. On the contrary, CPM was impaired in recurrent LBP patients compared to controls overall regardless of pain status. Finally, Study III demonstrated similar patterns of change in PPTs and TSP in relation to pain status as Study I-II, though CPM did not appear to be impaired in this group and remained unchanged across the study period. Unfortunately, the transcranial direct current stimulation paradigm selected was ineffective though, perhaps due to the already functioning descending inhibition in this group.

Taken together, findings from the present work would suggest that local and widespread hyperalgesia to pressure is primarily a consequence of the presence of LBP. Similarly, though TSP may have a small degree of predictive value for prognosis when assessed in a pain-free state, it seems that the facilitation observed in patients is mostly consequential to ongoing pain. CPM, on the other hand, seemed less impacted by pain presence per se and instead seems to deteriorate over time in LBP patients. The strength and generalisability of these conclusions are, however, limited by the considerable inter- and intra-individual variation in these measures, the selectivity of recruitment and the small experimental samples included. Nonetheless, this work has provided a comprehensive approach to understanding the influence of LBP presence on outcomes, which can easily be applied to various other outcomes and conditions. This work has also clarified certain aspects of the relationship between measures of pain sensitivity and LBP presence, suggesting these measures may be important in tracking fluctuations in LBP conditions and/or predicting pain and treatment prognosis, though future work is required to further explore these potential utilities.