[1688] Injection Site Pseudosarcomas in Piriformis Syndrome

DC Phan, X Fan, OTM Chan, E Himmelfarb, SI Bannykh. Cedar-Sinai Medical Center, Los Angeles, CA

Background: Treatment for patients with refractory nerve entrapment and piriformis syndromes often includes localized injection of anesthetic and/or corticosteroids. The histologic appearances of these injection sites have not been well-described. We have recently seen three cases in which the exuberant reactive fibroblastic/myofibroblastic proliferation at these injections sites mimicked a sarcoma. As the clinical history provided on pathology requisition forms is frequently incomplete, the lesions may easily be misinterpreted as neoplastic.
Design: 266 cases with muscle resection for piriformis syndrome were evaluated. Three cases including, 2 males and 1 female aged 36, 44 and 59, respectively showed exuberant reactive fibroblastic/myofibroblastic proliferation. All had received local injections of corticosteroids and/or anesthetics. As the patients continued to have pain, partial resection of the piriformis muscle was subsequently performed for nerve release.
Results: The histologic findings in all 3 cases were similar and included: a florid, highly cellular, cytologically atypical and mitotically active spindle cell proliferation diffusely infiltrating between skeletal myofibers and associated with myonecrosis/phagocytosis, degenerating and regenerating myofibers and a mixed inflammatory infiltrates. In the most striking index case the MIB-1 labeling index was >80%. Immunohistochemically, the proliferating cells were of three categories: 1) MyoD1+/desmin+/SMA+ skeletal myocytes, MyoD1-/Desmin-/SMA-/caldesmon+ myofibroblasts and 3) quadruple negative presumed fibroblasts. No staining of the lesional cells for CD31 and S100. Features arguing against a neoplastic process were the gross and clinical absence of a mass lesion and the pattern of diffuse infiltration between skeletal muscle fibers. However, the reparative etiology of the lesions was confirmed only after investigation of the clinical history. Confirmatory, in one of the cases the affected muscle was re-excised with findings of unremarkable muscle with a possible fibrosis.
Conclusions: Misinterpretation of a florid reactive process as a sarcoma is one of the leading causes of medical malpractice lawsuits in the field of soft tissue pathology (nodular fasciitis being the most commonly cited example). We herein report another clinical scenario in which this pitfall may be encountered.
Category: Neuropathology

Tuesday, March 23, 2010 1:00 PM

Poster Session IV # 245, Tuesday Afternoon


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