Pain analysis in prediction of treatment outcome

HE PRIME goal of a pain analysis is to identify the T pathophysiology of the pain generating mechanisms, the understanding of which, is a pre requisite for the planning of adequate treatment. In the majority of patients the etiology and underlying cause of the pain or pains are obvious, and the pain is pharmacologically manageable by adhering to the simple concept of the “analgesic ladder” (1). However, sometimes the diagnosis of the type of pain or pain components present in the individual patient is uncertain and the history and clinical features of the pain or pains are difficult to interpret. The performance of diagnostic pharmacological tests within a Swedish multidisciplinary pain Clinic is suggested to be one of the contributions from anaesthesiology. The Swedish National Board of Health and Welfare has officially stated this in a report from 1994 (2) and the development of multidisciplinary pain care in Sweden now follows that outline. Pharmacological tests are based on studies which has shown the necessity of differentiating individual pain mechanisms as a pre-requisite for both the choice of pharmacological treatment and its outcome. In the first study within this area as late as 1988, it was shown that there was a lack of analgesic effect of opioids in neurophatic and idiophatic forms of pain (3). Relatively strong international reactions appeared based on the fact that until then, the indication for opioids was only based on pain intensity and not on its mechanism. In Sweden, instead of severe pain as an indication there is a suggestion from the medical products agency that the indication should be changed to severe opioidsensitive pains in order to remind doctors of the necessity to make a proper pain analysis before treatment. In recent years there has been a growing interest in the development and validation of different pharmacological tests. In principle the i.v. administration of a drug as a test can be performed following three different paradigms: 1. A fixed dose of the drug or saline, is infused i.v. double-blind and pain ratings are performed and at fixed intervals after infusion. Side-effects are repeatedly noted and rated. 2. The test drug is given in small and repeated doses at regular intervals with repeated assessment of pain intensity and side-effects. Often the test is started with one or two saline injections. A variety of this design is to slowly inject the drug i.v. in incremental doses until pain relief is obtained or side-effects become obvious. 3. During later years it has been possible to study treatment efficacy and toxicity of drugs at several different levels of concentration. The technique is called computer-controlled infusion (4). By giving drugs in an exponentially decreasing infusion speed, it is possible to achieve momentally stable drug levels in blood which can be maintained by giving smaller doses. The purpose of pharmacological tests are: 1. To identify pain generating pathophysiological mechanisms, or simply to establish a “pain diagnosis”. Typical examples are tests with i.v. phentolamine to acertain whether the pain is sympathetically dependent, opioids to indicate the likely hood that nociceptive mechanisms are involved and lidocaine in order to identify neurogenic pain. Adenosine and Ketamin have also come in focus in order to test central sensitization of any chronic pain syndrome (se below). 2. Second purpose of drug tests is to justify a subsequent treatment with the tested drug. The best examples are tests with opioids in cases of non-cancer related pain where a definite response, preferably with a dose-response feature and Naloxone-reversibility, by many clinicians is regarded as a pre-requisite for authorising such drugs for long-term use. (5). The outcome of pharmacological tests often serves as guidance for further treatment whether pharmacological, surgical or anaesthesiological. Of particular interest is the use of pharmacological tests to predict the outcome of specific treatment