Introduction and objectives
Myofascial Pain Syndrome (MPS) of the Quadratus Lumborum muscle (QL) is a frequent cause of chronic low back pain. With this study, we aimed to assess the efficacy of ultrasound-guided infiltration with 0.25% levobupivacaine and 40 mg triamcinolone for MPS of the QL.

Methods
Observational and retrospective study of participants submitted to ultrasound-guided infiltration of the QL muscle from January 1, 2015 to June 31, 2019. Pain intensity was assessed using the five-point pain Numeric Rating Scale (NRS): pre-intervention, at 72 hours, 1 month, 3 months and 6 months post-intervention. Additional data collected were demographic characteristics, opioid consumption, and adverse effects.

Results
We assessed 90 participants with mean age of 55.2 years. Sixty-eight percent of participants were female. Compared to the pre-intervention assessment, there was an improvement in pain at 72 hours (Mean Difference [MD = 3.085]; 95% CI: 2.200–3.970,p < 0.05), at the 1 st month (MD = 2.644; 95% CI: 1.667–3.621, p <  0.05), at the 3rdmonth (MD = 2.017; 95% CI: 0.202–2.729, p <  0.05) and at the 6thmonth (MD = 1.339; 95% CI 0.378–2.300, p <  0.05), post-intervention. No statistically significant differences in opioid consumption were observed. No adverse effects associated with the technique were reported.

Conclusions
Ultrasound-guided infiltration of the QL muscle is a safe and effective procedure for the treatment of pain in the QL MPS within 6 months post-intervention.