Hemorrhagic lumbar synovial cyst mimicking a neoplastic process: illustrative case.

backgroundSynovial cysts with extensive hemorrhage, marked mass effect, and tumor-like appearance are rare and may mimic neoplastic processes. OBSERVATIONS: A 56-year-old man presented with progressive mechanical low back pain, neurogenic claudication, gait instability, and bilateral weakness. MRI revealed a 2.0-cm lesion arising from the right L3 facet joint, causing severe central canal and right foraminal stenosis with displacement of the cauda equina. Imaging features suggested pigmented villonodular synovitis. Intraoperatively, however, the lesion appeared fleshy, partially encapsulated, and dural adherent, resembling a nerve sheath tumor, with necrotic regions suggesting malignancy. Frozen-section analysis demonstrated benign fibroconnective tissue with marked reactive and degenerative changes, including necrosis and hemosiderin deposition, without evidence of neoplasia. Intraoperative confirmation of benign pathology shifted operative priorities toward anatomical preservation rather than extent of resection; gross-total removal was nevertheless achieved and the patient had complete neurological recovery. Permanent pathology was most consistent with a degenerative synovial cyst with hemorrhage. LESSONS: This case highlights the importance of integrating imaging findings, intraoperative appearance, and histopathological evaluation rather than relying on a single diagnostic modality. The combination of a focused differential and confirmation of benign pathology guided the extent of resection, balancing maximal lesion removal with preservation of adjacent structural elements. https://thejns.org/doi/10.3171/CASE26317.