It is difficult to define when a pain becomes chronic. Pain lasting longer than the usual period of healing, or pain that is unlikely to resolve have been suggested, as well as various time scales, for example three or six months.5
These time scales are all arbitrary and in practice it may not be possible to be exact. As the mechanisms of the changes that occur after injury and surgery become better understood, details of definition, such as the time scale, will become less important. To illustrate this point, if a patient has an operation for varicose veins and the saphenous nerve is injured, then they will probably have pain immediately after the operation. This may persist from the time of the operation and possibly be permanent, as it is a neuropathic pain. When does it become chronic? In chronic pain after surgery, the difficulty is compounded because pain may have been one of the symptoms that the patient was complaining of prior to surgery, and in fact may have been the main reason for seeking medical help. For example, in patients who have had a cholecystectomy for right upper quadrant pain, the preexisting pain will confuse the issue and complicate understanding the process of development. Is the pain merely a continuation of the presurgical problem or is it a new problem? If it is new, was it caused by the surgery? In some cases, it is obvious that there is a new problem; for example, if there is nerve injury following open cholecystectomy the description of the pain will be quite different from the pain of gallstones and will be accompanied by sensory changes. Unfortunately, in many cases it is difficult to disentangle the pains, especially if the pain that the patient complained of prior to surgery was not helped by the operation. In many patients with abdominal pain, no obvious cause can be found, despite exhaustive investigations. Surgeons are often put under pressure by patients and their relatives, that "something must be done.'' Sometimes an incidental finding in one of the investigations may serve as a focus for an unfounded belief about the cause, which then leads to surgery. In cases where visceral hyperalgesia is part of the mechanism, surgery is likely to make the pain worse.6
A working definition of chronic postsurgical pain has been proposed,3 which suggests the following criteria:
• the pain developed after a surgical procedure;
• the pain is of at least two months duration;
• other causes for the pain should have been excluded (e.g. continuing malignancy or chronic infection);
• the possibility that the pain is continuing from a preexisting problem must be explored and exclusion attempted. (There is an obvious gray area here in that surgery may simply exacerbate a preexisting condition, but attributing escalating pain to the surgery is clearly not possible as natural deterioration cannot be ruled out.)
Unfortunately, most published studies do not attempt to define chronic pain after surgery, and in the studies where a definition is attempted, there are differences in timing and criteria used.7 Although the above definition is arbitrary, it is a starting point.
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