Background: Central sensitization in individuals with knee pain is associated with altered autonomic nervous system activity and descending pain modulation; however, the exact nature of these relationships remains unclear. We have hypothesized that chronic knee pain (CKP) disrupts the relationship between autonomic nervous system activity and conditioned pain modulation, whereas nonchronic knee pain (nCKP) does not.
Objectives: This study aimed to examine the association between autonomic nervous system activity and descending pain modulation as well as to identify factors contributing to dysfunction in the descending pain modulation system.
Study design: Observational study.
Setting: An institutional hospital.
Methods: Female patients with knee pain were recruited via advertisements and orthopedic referrals. Patients were categorized as having either CKP or nCKP, based on whether their pain persisted for 3 months or longer. Conditioned pain modulation was assessed using cold-water immersion (8-12°C for 3 minutes) as a measure of descending pain modulation. Autonomic nervous system activity, represented by low- and high-frequency normalized units (LFnorm and HFnorm), was assessed via heart rate variability. Changes in autonomic nervous system activity were calculated as post-immersion values minus pre-immersion values. Associations between conditioned pain modulation and changes in LFnorm and HFnorm units were analyzed using linear regression and Cohen's kappa. Path analysis was performed to explore factors affecting autonomic nervous system activity and conditioned pain modulation in patients with CKP.
Results: Of 96 patients, 90 (71 with CKP and 19 with nCKP) were analyzed. Changes in LFnorm were significantly associated with conditioned pain modulation only in patients with nCKP (P = 0.012). Among those with CKP, the baseline LFnorm was associated with pain catastrophizing and the number of pain sites.
Limitations: The study included only female patients, and the nCKP-pain group was relatively small.
Conclusions: Patients with nCKP exhibited significant relationships between conditioned pain modulation and sympathetic nervous system activity, suggesting that the phenomenon of descending pain modulation was intact. Conversely, in patients with CKP, those associations were absent, and sympathetic activity was rather influenced by pain catastrophizing and the number of pain sites. These findings suggest that chronic pain may impair descending pain modulation through altered autonomic nervous system function.
Keywords: autonomic nervous system; central sensitization; chronic pain; conditioned pain modulation; descending pain modulation; heart rate variability; nonchronic pain; parasympathetic nervous system; sympathetic nervous system; Knee pain.