It is important to emphasize the difficulties in the description and in the assessment of painful symptoms. Pain is a very personal experience and there is marked variation in the description of symptoms between patients with similar pathological lesions. This has important implications for trials of therapies for neuropathy and, as stated by Huskison (11): "pain is a personal psychological experience and an observer can play no legitimate part in its direct management." Thus, any trial of treatments for pain must rely upon the patients' response to questions, questionnaires, or other measures. This principle has not been followed in all trials: for example, it is inadmissible to rely on the "physician's overall impression" of the patient's response as was the case in one trial (12). In recent years, a number of valid measures for the assessment of chronic neuropathic pain and for the evaluation of treatment responses have been developed and tested. One or more of the following methods are usually used in clinical trials of new treatments for painful neuropathy (Table 3).
One of the most common measures of neuropathic pain in clinical trials is the visual analog scale (VAS), originally described by Scott and Huskison (13). The VAS is a straight line the ends of which are the extreme limits of the sensation being assessed. The VAS has been shown to be a satisfactory method for assessing pain or the relief of pain. The line is normally 10 cm in length and is frequently referred to as the 10 cm VAS. A VAS with descriptive terms placed along the length of the line is known as a verbal descriptor scale. Thus a 10 cm VAS with the terms "mild, moderate, and severe" along the base of the line is known as a 10 cm verbal descriptor scale.
There are three major classes of word descriptors in the Mcgill pain questionnaire (MPQ) originally described by Melzack (14): sensory qualities, affective qualities, and evaluative words. The MPQ was originally designed to provide more quantitative measures of clinical pain, changes which can be evaluated statistically. Masson et al. (15) later showed that the MPQ was a useful aid in the differential diagnosis of painful diabetic neuropathic symptomatology.
Galer and Jensen (16) developed a neuropathic pain scale as they felt that previous measures, such as the VAS did not adequately assess the experience of neuropathic pain. Therefore, this scale includes two items that assess the overall dimension of pain intensity and pain unpleasantness.
A number of simple symptom screening questionnaires are available to record symptom quality and severity. These include a simplified neuropathy symptom score that was used in European prevalence studies and might also be useful in clinical practice (17,18), and the Michigan neuropathy screening instrument, which is a brief 15 item questionnaire that can be administered to patients as a screening tool for neuropathy (19). Other similar symptom scoring systems have also been described (20). Finally, some of the newer condition specific QoL measures also include symptom scoring scales as described in the following paragraph.
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