Changes in nociceptive flexion reflex threshold across the menstrual cycle in healthy women

Objective: We assessed the influence of changes in steroid hormones across the menstrual cycle on the spinal nociceptive reflex. Method: We studied in 14 healthy women during the follicular and luteal phase the nociceptive flexion reflex (RIII reflex), an objective neurophysiological method that allows exploring possible abnormal functioning of the pain-control system. The basal body temperature (BBT) was used to evaluate the different phases of the ovarian cycle. The menstrual distress questionnaire (MDQ) was also applied for monitoring somatic and psychological symptoms during the cycle. Results: During the luteal phase, the threshold of the RIII reflex (Tr) and the psychophysical threshold for pain (Tp) were both significantly reduced compared with the follicular phase. Moreover, the reflex threshold in the luteal phase was negatively correlated to the total MDQ score of the recording day. Conclusions: A higher sensitivity to pain stimuli was observed during the luteal phase of the menstrual cycle, which probably results from a reduction in the inhibitory descending control on spinal nociceptive flexion reflex. Complex neuromodulatory interactions of ovarian steroids with other systems of neurotransmission (especially serotonergic) may account for these observations.