Dynamic mechanical allodynia in peripheral neuropathic pain : Psychological observations

Introduction and aim: Pain due to a light moving mechanical stimulus, dynamic mechanical allodynia, is a protruding symptom/sign in subgroups of patients with peripheral neuropathic pain and frequently as troublesome as spontaneous ongoing pain. The objective of this thesis was to survey psychophysical details of dynamic mechanical allodynia using a novel semi-quantitative method. In addition, the psychophysical characteristics of dynamic mechanical allodynia in the secondary hyperalgesic zone after an intradermal injection of capsaicin were probed with regard to similarities and differences of that phenomenon compared to such allodynia in peripheral neuropathic pain. Methods: Using a semi-quantitative method brush-evoked allodynia was induced in the innervation territory of the lesioned nervous structure in patients by lightly stroking different distances of the skin 2 or 4 times with brushes of different widths or while varying stroking velocity or brushing force. In study III the patients were also examined in the area outside the flare after an intradermal capsaicin injection in the corresponding contralateral site to the area of painful neuropathy, i.e., in the secondary hyperalgesic area. Age- and sex-matched controls injected with identical amounts of capsaicin were examined in a corresponding area. In all studies the intensity and duration of brush-evoked allodynia was recorded using a computerized visual analogue scale. The total brush-evoked pain intensity, including painful aftersensation was calculated as the area under the curve. Following each stimulus, the subjects selected pain descriptors from a validated instrument. In study II the repeatability of brushevoked allodynia was examined within and between days in patients with peripheral neuropathic pain. Results: Significantly increased total brush-evoked pain intensity was demonstrated with increased brushing length and number of strokes, higher brushing force and lower stroking velocity but not while altering brush width. Lack of influence of brush width was further underlined by the finding that brushing of equivalent skin areas resulted in higher total evoked pain intensity if brushing the skin with a thin brush over a longer distance than a thick brush over a shorter distance. A very good repeatability of brush-evoked allodynia within and between days was reported using this semiquantitative method. In patients similarities were found in the relationship between brush-evoked allodynia and temporo-spatial stimulus parameters comparing the capsaicin-induced secondary hyperalgesic area with the area of painful neuropathy. Only 3/9 controls (compared to 8/9 patients) reported brush-evoked pain after capsaicin injection. In all studies the frequency of preferred sensorydiscriminative and affective pain descriptors for the brush-evoked pain indicated some similarities, in particular the choice of affective pain descriptors such as annoying and troublesome. Conclusions: Our findings demonstrated dynamic mechanical allodynia to be a partially graded phenomenon in peripheral neuropathic pain conditions since stimulus parameters such as increased brushing length, increased number of strokes, lower stroking velocity and increased brushing force significantly increased the total brush-evoked pain intensity. However, alterations of the brush width within a limited range did not significantly change the total brush-evoked pain intensity. In addition, dynamic mechanical allodynia in the capsaicin-induced secondary hyperalgesic zone in patients seemingly well reflected perceptual details of such allodynia in the neuropathic condition. In healthy controls, only one-third developed brush-evoked allodynia in the potential secondary hyperalgesic area. Such a low hit frequency calls into question the value of the capsaicin model when aiming at studying dynamic mechanical allodynia. Taken together, these results substantiate the usefulness of this semiquantitative assessment method in studies on dynamic mechanical allodynia, including longitudinal treatment studies