CentralSensitizationIsModulatedFollowingTriggerPointAnesthetizationinPatientswithChronicPainfromWhiplashTrauma.ADouble-Blind,Placebo-Controlled,CrossoverStudy

Chronic pain after whiplash injury affects many people and often involves central sensitization, a condition where the nervous system becomes overly responsive so that normal touch or pressure feels painful. A key puzzle has been whether peripheral pain sources, specifically myofascial trigger points (tender knots in muscles), actually drive this central nervous system sensitivity or are merely a side effect of it. Previous research had produced conflicting answers. One study found that numbing painful muscles with local anesthetic did not change pain sensitivity in whiplash patients, while the authors' own earlier uncontrolled study suggested the opposite—rapid improvements in pain thresholds after trigger point injections.

This disagreement matters because if peripheral trigger points do modulate central sensitization, then treating them directly could offer a practical way to reduce widespread pain hypersensitivity, potentially even through surgical removal of the most problematic points.

The researchers recruited 31 patients who had suffered chronic neck and shoulder pain for more than a year after whiplash trauma, with clinical signs of central sensitization. Each participant had at least one distinct trigger point or tender area in the upper trapezius muscle. The study used a rigorous three-phase crossover design. In Phase I, the researchers mapped the most painful point and recorded baseline measurements.

In Phase II, participants were randomly assigned to receive either a real trigger point injection with 2 mL of bupivacaine (a local anesthetic) injected beneath the muscle fascia, or a sham injection of 0.5 mL bupivacaine given only into the skin. The intradermal sham was designed to feel similarly painful without reaching the muscle, and both participants and the assessor were blinded. Measurements were repeated within five minutes. In Phase III, after a washout period of at least 30 minutes, the sham group crossed over to receive the real injection, now knowing they had initially received placebo.

The outcomes measured were pressure pain thresholds at three distant body sites (shoulder, forearm, and lower leg), grip strength, maximum jaw opening, and overall pain on a 0-10 visual analog scale.

The real local anesthetic injection produced significant improvements in objective measures of central sensitization, while the sham injection did not. Pressure pain thresholds increased by 32.4% at the shoulder (P=0.03), 39.4% at the forearm (P=0.0001), and 51.9% at the lower leg (P=0.0006) after the real injection, but showed non-significant changes of 14.9%, 5.6%, and 13.5% after sham. Maximum jaw opening also improved significantly after real injection (5.1% increase, P=0.04) but not after sham (0.5% change, P=0.75). Grip strength did not change significantly in either group.

Notably, subjective overall pain improved significantly after both real and sham injections (38.0% and 30.4% decreases respectively, both P<0.01), confirming that expectation and placebo effects strongly influence pain reporting. In the crossover phase, when the sham group received the real injection, all six variables showed significant improvement: pressure pain thresholds rose 35.7% at the shoulder (P=0.0002), 42.0% at the forearm (P=0.0000), and 48.3% at the leg (P=0.003); grip strength improved 11.3% (P=0.02); jaw opening increased 8.2% (P=0.02); and pain dropped 36.8% (P=0.001). After crossover, no significant differences remained between the two original groups on any measure except grip strength.

This study provides strong evidence that myofascial trigger points can actively modulate central sensitization in at least some patients with chronic whiplash pain. The rapid, widespread increase in pressure pain thresholds after numbing a single trigger point—at body sites far from the injection—demonstrates that peripheral nociceptive input from trigger points maintains heightened central nervous system sensitivity. The dissociation between objective measures (which improved only with real anesthetic) and subjective pain ratings (which improved with both real and sham injections) highlights the importance of using objective outcomes in pain research and cautions that patient-reported pain relief alone can be misleading. Clinically, these findings support targeted trigger point interventions as a potential treatment for central sensitization, and they provide a scientific rationale for the authors' previous work on surgical removal of refractory trigger points.

For patients, this suggests that identifying and treating specific muscle trigger points may help reduce their overall bodily pain sensitivity, not just local pain. However, the small sample size and the specific population of whiplash patients limit generalizability, and the 30-minute washout period, while practical, means longer-term effects remain to be studied.