The Role of Muscle Trigger Points in Chronic Whiplash-Associated Disorders with Neuropathic Pain Components: An...

Background/Objectives: The role of muscle trigger points (TrPs) in neuropathic pain (NP) components in whiplash-associated disorders (WAD) has not been investigated. Our aim was to systematically investigate if referred pain elicited by trigger points (TrPs) in neck musculature reproduces neuropathic pain (NP) characteristics in chronic whiplash-associated disorders (WAD) and to determine the association of TrPs with pain intensity, mechanosensitivity, and disability. Methods: An exploratory cross-sectional study was conducted (n = 64; chronic WAD: n = 32; age- and sex-matched healthy controls: n = 32). TrPs in upper trapezius, suboccipital, splenius capitis, levator scapulae, scalene, and sternocleidomastoid muscles were evaluated. Pain intensity, NP components, pain catastrophizing, and disability were assessed with an 11-point numerical pain rating scale (0-10), NP questionnaires (Douleur Neuropathique 4 [DN4], self-administered Leeds Assessment of Neuropathic Symptoms and Signs [S-LANSS], and Neuropathic Pain Symptom Inventory [NSPI]), the Pain Catastrophizing Scale, and the Neck Disability Index, respectively. Mechanosensitivity (pressure pain thresholds) was assessed bilaterally over C2-C3 and C5-6 zygapophyseal joints, second metacarpal, and tibialis anterior muscle. The Mann-Whitney U test and advanced chi-square (χ²) test, including rank-based ANCOVA adjusted for age and sex, were used for comparisons between groups. Additionally, multivariate analyses were also performed (rank-based MANCOVA adjusted for age, sex, and pain intensity). Spearman's rho (r_s) and LOESS regression analysis, corroborated with linear regression and/or polynomial regression coefficient analysis, were used to explore associations between clinical variables in WAD. Results: Significant differences in distribution of TrPs, with a significant effect of sex, were found between groups (p < 0.05). In WAD, a greater number of active TrPs, mostly prevalent in levator scapulae and suboccipital muscles, was associated with higher pain intensity, number and intensity of NP components, and disability (0.372 < r_s < 0.570, p < 0.05), or local mechanical hyperalgesia (r_s = -0.362, p < 0.05). Conclusions: Referred pain elicited by active TrPs in the neck muscles reproduced NP symptoms in chronic WAD. This study contributes to a new understanding of pain mechanisms in WAD, highlighting the role of active TrPs in generating or maintaining NP symptoms and sensitization processes.