Capsaicin-induced pain and sensitisation in the postpartum period

Background: Recovery from Caesarean delivery in women and surgical nerve injury in animals after delivery is more rapid than expected, an effect reversed in animals by spinal injection of an oxytocin receptor antagonist. We hypothesised that endogenous modulation of acute pain is altered postpartum. Methods: Endogenous inhibition of acute pain in a conditioned pain modulation paradigm or endogenous sensitisation by topical capsaicin was tested in women who were breastfeeding 10–14 days after Caesarean delivery and age-matched controls (n=80 total: 20 per group and 20 per test). The study was powered to detect a difference in area of hyperalgesia after capsaicin of 33%. Capsaicin-evoked pain was recorded in women, and capsaicin-evoked mechanical hypersensitivity was measured in rats 48 h after delivery and in age-matched female and male animals. Results: There was no effect of the postpartum period in the endogenous sensitisation assay in women, and the conditioned pain modulation assay failed to produce analgesia in either group. Postpartum women, however, reported less intense pain than controls at the end of topical capsaicin exposure (1.3 [1.4] vs 2.0 [2.0] on 0–10 verbal scale), and acute hypersensitivity after capsaicin was less in postpartum than control rats (withdrawal threshold 25 [15] vs 3.6 [1] g). Conclusions: These results agree with a recent report that oxytocin may desensitise the transient receptor potential for vanilloid-1 channel, although other explanations, including hormone effects, are possible. These results do not, however, support the inhibition of capsaicin-evoked spinal sensitisation in the postpartum period. Clinical trial registration: NCT01843517.