|M.Sc Student||Honigman Liat|
|Subject||Inter-Relations between Offset Analgesia and Conditioned|
|Department||Department of Medicine||Supervisors||Professor David Yarnitsky|
|Dr. Irit Weisman-Foegel|
|Full Thesis text|
Endogenous pain modulation is evaluated psychophysically using two basic paradigms: conditioned pain modulation (CPM) expressing spatial filtering, and offset analgesia (OA) expressing temporal filtering.
In this study we examined the inter-relations and possible additive effect between these two paradigms, asking whether they are subserved by a single mechanism. Furthermore, we investigated the effect of gender on the association between CPM and OA, and the role of pain-related psychological factors on OA and CPM.
This study included 29 healthy subjects (15 males and 14 females) who underwent randomly four series of thermal stimuli that were given to the dominant forearm: OA evoked by three-temperature stimulus train (T1=49°C, T2=50°C, T3=49°C; 5s, 5s, 20s respectively), 30s constant 49°C stimulus ('control train') and two CPM paradigms (30s of OA or constant 49°C stimulus followed by simultaneous immersion of the non-dominant hand in 46°C hot water). Subjects were asked to report their pain intensity on a numerical pain scale six times during the test stimulus (every 5s) and twice during the conditioning stimulus (at 10 and 20s). Differences in pain rating between: (I) 5ths and 15thsec in the OA train versus the 'control train' were taken as OA effect, and (II) before and during hand immersion versus the 'control train' were taken as CPM effect.
The findings of this study are that there is an additive effect of CPM on OA for the whole group (P=0.003) and a positive correlation between CPM and OA for males only (r=0.62, P=0.01). Males showed greater effect then women for each of the mechanisms as well as an additive effect of CPM on OA (P=0.003), and a trend for additive effect of OA on CPM (P=0.07). Females, however, showed no effect for any of the mechanisms nor any additive relations between them. There was also a negative correlation between results of the pain sensitivity questionnaire and OA (r=0.38, P=0.038), which was stronger in females (r=0.53, P=0.04).
In summary, the findings of this study demonstrate for the first time that the two endogenous analgesia (EA) mechanisms, CPM and OA, have inter-relations of additive nature. Since both mechanisms rely on brainstem analgesia centers, the additive effect might result from the different dimensions they apply; the longer and relatively subtler effect of CPM might be augmented by the relatively robust yet shorter effect of the OA. Future pain psychophysical studies might use both to depict full picture of pain inhibition capacity.