PR_122: Postinjection Soreness in Myofascial Pain Syndrome: Its Course, Associated Factors, and the Effect of Acetaminophen

Abstract

Objectives: To demonstrate incidence, time course, and influencing factors of postinjection soreness and to assess the effect of acetaminophen to prevent postinjection soreness. Design: Randomized, double-blind study. Setting: Outpatient clinic of a university hospital. Participants: 29 patients (8 men, 21 women) with myofascial pain syndrome of the shoulder girdle muscles. Intervention: Trigger point injection with lidocaine. Main Outcome Measures: Visual analog scale (VAS), Disability of Shoulder, Arm and Hand (DASH), time of maximal soreness after injection, and time of pain improvement. Results: 17 (58.6%) patients experienced postinjection soreness. The mean onset of pain was 6 hours after the injection, which was most severe at 22 hours and started to improve at 47 hours after injection. The pain was more severe in women. In particular, women with chronic (>3mo) pain showed significantly more soreness. The presence of postinjection soreness did not make any differences in the improvement of VAS and DASH after 7 days. Stratified analysis by sex and onset of pain revealed no significant difference in the incidence and severity of postinjection soreness between acetaminophen and placebo medication. Conclusions: The incidence of soreness after trigger point injection was 58.6%. The soreness began at 6 hours after injection and reduced at 57 hours after injection. Significant associated factors were women and chronicity (>3mo) of the disease. Acetaminophen had no effect on relieving postinjection soreness.