Where is the pain? Spatial patterns of pain co-occurrence in a population-based study of 4833 pain drawings...

Spatial pain patterns are widely used as diagnostic tools, yet population-level estimates, such as the prevalence of pain in specific body regions and likelihood of their co-occurrence, are lacking. Despite this, bilateral limb pain is considered relatively uncommon. Baseline data from a population-based Danish cohort were analysed. Twenty-one pain drawing regions, coded as binary "pain"/"no-pain" variables, were entered into an Ising model. Conditional dependencies between pairs of painful regions were quantified, while accounting for the pain state of other regions. Four-week prevalence of pain was also calculated for body regions. Of 4833 analysed pain drawings, 34.7% (1676) reported bilateral (upper or lower) limb pain and 32.3% (1561) reported symmetrical (mirrored) bilateral limb pain. Strongest positive edge weights of the Ising model were between mirrored contralateral regions; the strongest being between left and right hips (mean: 3.86, 95% confidence interval: 3.84-3.87). Next strongest edge weights were between spatially adjacent ipsilateral regions; the strongest being between the right hip and right buttock (mean: 2.72, 95% confidence interval: 2.71-2.74). Negative edge weights, indicating inhibitory relationships, were consistently seen between nonmirrored contralateral regions, the strongest being between regions adjacent to their mirrored contralateral counterparts. In conclusion, bilateral limb pain, particularly in mirrored regions, is more prevalent than previously thought. Pain co-occurrence is facilitated between mirrored contralateral regions and, to a lesser degree, between adjacent ipsilateral regions. An inhibitory effect occurs between nonmirrored contralateral regions, diminishing with increasing distance from the mirrored region. Potential inhibition between mirrored contralateral regions is likely overshadowed by the more dominant facilitation.