Complaints of patients with chronic pain may increase when the weather changes. Several reports based on pain questionnaires have assessed pain sensitivity to meteorological factors, and conditions reported to be especially sensitive to weather changes are chronic pain in rheumatoid arthritis (Hollander 1962; Rasker et al. 1986; Guedi and Weinberger 1990; Gorin et al. 1999), nerve entrapment (Hendler et al. 1995), phantom limb pain (Mitchell 1877), osteoarthritis (Hollander 1962), fibromyalgia (Yunus et al. 1981; Fors and Sexton 2002), migraine (Anderson et al. 1965), postherpetic neuralgia (Nurmikko and Bowsher 1990), reflex sympathetic dystrophy (Hooshmand 1993; Hendler et al. 1995), and low-back pain (Shutty et al. 1992; Hendler et al. 1995). Various meteorological factors, such as barometric pressure, ambient temperature, humidity, sunshine, rain and storms, have been suspected to contribute to changes in pain (Hollander 1962; Sulman et al. 1970; Jamison et al. 1995). Hollander (1961) approached this issue experimentally using a climate-controlled room and showed that arthritic patients report more pain in response to the combination of increased humidity and decreased barometric pressure. All of the studies cited above demonstrated a consistent relation between changes in meteorological factors and pain intensity. Other studies, however, have failed to find a significant relationship between chronic pain and weather. For instance, Clark and Nicholl (1991) examined rheumatoid arthritis patients' pain levels daily for 30 days and found no correlation between pain intensity and barometric pressure or humidity. More recently, Redelmeier and Tversky (1996) failed to find any relationship between weather conditions and pain. They even suggested that psychological factors contribute to the belief that chronic pain is related to weather change. Does weather really influence chronic pain? Given the many contradictory results and opinions in this field, we were struck by the absence of controlled animal studies addressing the issue. Such studies could provide clues to help explain the contradiction described above as well as essential evidence for uncovering the mechanism(s) by which weather induces changes in pain. We decided, therefore, to conduct a behavioral animal study to determine whether changes in meteorological factors within the range of the natural environment aggravate pain-related behaviors in rats with chronic pain. In this article, the author summarizes the data from his group's recent behavioral animal studies (Sato et al. 1998, 1999, 2000, 2001, 2002; Nagao et al. 2000; Takahashi et al. 2001) and discusses the possible mechanisms by which weather changes aggravate chronic pain.
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