Complete Freund's adjuvant-induced hyperalgesia: a human perception

Abstract

Much of our current understanding about chronic pain and the mechanisms of nociception has been derived from animal models (Bennett GJ. Animal models of neuropathic pain. In: Gebhart, GF, Hammond DL, Jensen TS, editors. Progress in pain research and management, vol. 2, Proceedings of the 7th World Congress of Pain. Seattle, WA: IASP Press, 1994. pp. 495–510; Dubner R, Methods of assessing pain in animals. In: Wall PD, Melzack R, editors. Textbook of pain, vol. 3. Edinburgh: Churchill Livingstone, 1994. pp. 293–302). It has been argued in some cases that animals do not perceive ‘pain’ as humans do, and thus extrapolation of the results of studies in animals is invalid. Clearly, the animal models used in the laboratory do not approach the complexity of chronic pain encountered in the clinical setting. Human pain perception is more complex since it encompasses lesion variability, as well as psychosocial, cultural, developmental, and environmental variables. Where parallels exist, it is possible to gain insight into certain aspects of human pain syndromes that are likely to lead to improved therapeutic opportunities for individual patients. One such model that is frequently used in animals to study pain associated with inflammation is the subcutaneous injection of complete Freund's adjuvant (CFA). For ethical reasons, however, little information is available from humans concerning pain associated with this form of inflammation. Due to an inadvertent subcutaneous injection of CFA into the terminal phalanx of this investigator, a study with an N of 1, was conducted to compare the subjective effects of CFA-induced inflammation with objective measurements.