Spontaneous pain is a common complaint in chronic pain conditions. However, its properties have not been explored. Here we study temporal properties of spontaneous pain. We examine time variability of fluctuations of spontaneous pain in patients suffering from chronic back pain and chronic postherpetic neuropathy and contrast properties of these ratings to normal subjects' ratings of either acute thermal painful stimuli or of imagined back pain. Subjects are instructed to continuously rate their subjective assessment of the intensity of pain over a 6- to 12-min period. We observe that the fluctuations of spontaneous pain do not possess stable mean or variance, implying that these time series can be better characterized by fractal analysis. To this end, we apply time and frequency domain techniques to characterize variability of pain ratings with a single parameter: fractal dimension, D. We demonstrate that the majority of ratings of spontaneous pain by the patients have fractal properties, namely they show a power law relationship between variability and time-scale length; D is distinct between types of chronic pain, and from ratings of thermal stimulation or of imagined pain; and there is a correspondence between D for pain ratings and D for brain activity, in chronic back pain patients using fMRI. These results show that measures of variability of spontaneous pain differentiate between chronic pain conditions, and thus may have mechanistic and clinical utility.
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