Abstract
Background and purpose
‘Central sensitization’ (CS) may play a major role in maintaining several chronic pain conditions. CS has been proposed to play a significant role in a range of musculoskeletal pain conditions, such as trapezius myalgia, fibromyalgia, temporomandibular disorders, and low back pain. Whether CS varies over time within an individual is not known. This study evaluated (1) whether there is an intraindividual association between clinical pain and signs of CS, and (2) whether there is an inter-individual association between clinical pain and signs of CS.
Methods
Twenty-seven sedentary workers (19 women, 8 men) with varying neck/shoulder pain participated in a pre-test and in two test sessions. On one of the test sessions the subjects had weak (or no) clinical pain (weak-pain day). On the other test session the subjects had stronger clinical pain (strong-pain day). As an indicator of ‘central sensitization’, we assessed the area of secondary pinprick hyperalgesia (tested by 84.4 g/mm^2^Von Frey hairs) in response to a first-degree burn to the volar fore-arm (contact heat, 46°C, 5 min). While in the lab, the subjects’ current clinical pain intensity (0–10 cm VAS) and distribution was assessed (PINT~lab~and PDIST~lab~). The subjects also rated their pain intensity and distribution retrospectively from the past 30 days (PINT~30~~d~and PDIST~30~~d~).
Results
PINT~lab~was lower on the weak-pain day (1.7 ± 1.5 cm) than on the strong-pain day (4.3 ± 1.6 cm). This was also the case for the other clinical pain measures (PDIST~lab~, PINT~30 d~and PDIST~30 d~) and indicated that the participants were successfully recruited at days that differed in clinical pain severity. Despite a significant intra-individual difference in clinical pain between days, the area of secondary hyperalgesia did not differ between__weak__-and__strong-pain days__(50.3 ± 13.5 cm^2^vs. 51.2 ± 12.6 cm^2^). Testing the inter-individual association between clinical pain and secondary hyperalgesia, we found a positive correlation between PINT~lab~and secondary hyperalgesia on the__weak-pain day__(rho = 0.6), but not on the__strong-pain day__(rho = 0.1). Given the stable secondary hyperalgesia across weak-and strong-pain days, this implies that subjects with a small secondary hyperalgesic area exhibited a relatively large variation in clinical pain between days, whereas subjects with a large secondary hyperalgesic area exhibited relatively small variation in clinical pain.
Conclusions
When subjects are observed across days, ‘central sensitization’, measured as the area of secondary hyperalgesia after a first-degree burn, does not seem to be important for clinical pain intensity per se, but may be important for clinical pain variation. Subjects with indication of low ‘central sensitization’ seem to exhibit larger variation in pain between “good” and “bad” days than subjects with indication of high ‘central sensitization’. The study indicates that ‘central sensitization’ does not explain intra-individual variations in clinical pain.
Implications
This study raises the question of the role of ‘central sensitization’ in clinical musculoskeletal pain disorders. Furthermore, a precise definition of the ‘central sensitization’ concept is called for.