• Mark C Bicket, Courtney Simmons, and Yi Zheng.
• Department of Anesthesiology, Critical Care, and Pain Medicine, Massachusetts General Hospital, Boston, MA.
• Pain Physician. 2016 Mar 1; 19 (3): 181-8.

BackgroundBack mice, or episacroiliac lipoma, represent a potentially treatable cause of low back pain that may be under-recognized in clinical practice. Despite being well characterized based on clinical history and physical examination findings, implementation of appropriate treatment may be delayed or missed based on a lack of familiarity with the diagnosis.ObjectivesIn this case report and literature review, we describe a 47-year-old woman with history of persistent low back pain who presented with a pain exacerbation consistent with a back mouse. The history, epidemiology, clinical characteristics, differential diagnosis, potential mechanisms for pain, and treatment options for back mice were then reviewed.Study DesignCase report and literature review.SettingAcademic university-based pain management center.ResultsStudies included one randomized clinical trial, 4 cross-sectional studies, 8 case reports or series, and 16 other publications prior to 1967.LimitationsA single case report.ConclusionsFirm, rubbery, mobile nodules that are located in characteristic regions of the sacroiliac, posterior superior iliac, and the lumbar paraspinal regions may represent fatty tissue that has herniated through fascial layers. When painful, these back mice may be confused with other causes of low back pain. In particular, the presence of point tenderness may mimic myofascial pain, and reports of radicular pain may imitate herniated nucleus pulposus. However, back mice may be distinguished from other entities based on findings from the history and physical examination such as absence of neurological deficit. Treatment consisting of injection of local anesthetic into the nodule with or without corticosteroid followed by repeated, direct needling has been reported to relieve pain in many case reports. The one clinical trial comparing injection of local anesthetic to normal saline, which did not include repeated needling, found only mild and transient benefit in the treatment group.

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