Delayed onset and resolution of pain: some observations and implications.

Occasionally, pain after disease or trauma develops only after a prolonged interval. Examples include late-onset pains which first occur months or years following a stroke, spinal cord lesion or amputation of a limb; a previously experienced pain that is recalled years later; and latent pain triggered for the first time by a further insult in the same area. Late-onset pains may develop gradually or suddenly, and may be brief or long standing. Pains which develop after an innocuous insult may be associated with slowly evolving sensory changes. However, even long-standing pains, particularly those of nociceptive origin, may resolve sometimes after many years. Resolution, which again can occur gradually or suddenly, may be spontaneous or follow development of another disorder or after therapeutic intervention. The duration of this pain relief can range from minutes to an indefinite period. These clinical phenomena, and the mechanisms, including genetic factors, subserving them, have been little studied. It is postulated that mechanisms implicated in acute pain may not be the same as those that subserve pain that develops after a long interval. Those late-onset pains which develop slowly after innocuous lesions may be associated with a variety of slow anatomical, physiological and biochemical changes. In late-onset pains that follow a painful insult, however, memory of the former pain and threshold triggering factors may be particularly important. Further studies of these neglected conditions may lead to understanding of as yet unknown processes subserving pain and to novel approaches to treatment.