A Facet-Associated Cyst with Aneurysmal Bone Cyst Morphology Causing T10–T11 Compressive Myelopathy Mimicking Cauda Equina Syndrome in an Octogenarian

Aneurysmal bone cysts (ABCs) are benign, locally aggressive osseous lesions that predominantly affect children and young adults; ABC morphology in a facet-associated cystic lesion in an octogenarian is highly unusual and may overlap radiologically with a degenerative facet or synovial cyst. An 81-year-old woman presented with a 2-month history of progressive bilateral lower-limb weakness, sensory disturbance, and gait deterioration, followed by acute saddle anaesthesia and bowel dysfunction, prompting concern for cauda equina syndrome. Examination showed bilateral MRC 3/5 weakness, mildly increased lower-limb tone, brisk reflexes, and bilateral Hoffmann signs. Lumbar MRI did not demonstrate cauda equina compression. Dedicated thoracic MRI revealed severe multifactorial T10–T11 canal stenosis with marked cord compression, together with a small right posterolateral extradural lesion adjacent to the facet/posterior elements. Posterior T10–T11 decompression, including laminectomy and flavectomy, with excision of the cystic lesion resulted in neurological improvement. Histopathology demonstrated blood-filled cystic spaces separated by fibrovascular septa containing osteoclast-like multinucleated giant cells and reactive bone, without malignant features, consistent with ABC morphology. This case emphasizes careful neurological localization when cauda equina-like symptoms coexist with upper motor neuron signs and highlights an unusual facet-associated cystic lesion with ABC morphology contributing to multifactorial thoracic spinal stenosis in an elderly patient.