Modulating the development of secondary hyperalgesia by manipulating attention through a high load working memory task

Central sensitisation (CS), one of the causes of ongoing and amplified pain in some chronic pain patients, refers to the physiological changes that occur in nociceptive neurons in the spinal cord as a result of their intense and repeated activation. CS can be assessed at the peripheral level by the development of secondary hyperalgesia, defined as increased sensitivity to peripheral stimuli, often pinprick mechanical stimuli, that extend around the site of stimulation or injury. Several researchers have attempted to modulate central sensitisation. To this end, studies have examined the effect of selective attention on the development of experimentally induced secondary hyperalgesia, as selective attention modulates pain perception and nociceptive system activity. Among these studies, it has been shown that diverting attention from nociceptive stimuli, which induce secondary hyperalgesia, by a cognitive task reduces the development of secondary hyperalgesia, especially when the task requires an attentional load, or effort, to perform. However, as this observation is rather controversial, our experiment aimed to improve the knowledge on this subject. In our experiment, secondary hyperalgesia was induced by high-frequency electrical stimulation (HFS) and attention was diverted either to a difficult, high-load task or to an equivalent easy, low-load task. Ninety-two healthy volunteers participated in this study. They were either assigned to the difficult or easy group. To see whether the load modulated the development of secondary hyperalgesia, we compared, between groups, (1) the change in sensitivity to pinprick mechanical stimuli between before and after the application of HFS but also (2) the extent of hypersensitivity after HFS. As our hypothesis suggested that high-load tasks reduce the development of secondary hyperalgesia, we expected that the increase in mechanical sensitivity and its extent would be greater in the easy group than in the difficult group. Unfortunately, our results did not confirm our hypothesis as there were no significant differences between the two groups. However, we cannot conclude that the initial hypothesis was wrong since the results were very close to the significance level (i.e., P-value = 0.05) and the graphs followed the hypothesised trend.