Lumbar epidural lipomatosis: the “Y” sign of thecal sac compression

Purpose. We have identified a specific sign of lumbar thecal sac compression seen in patients with symptomatic lumbar epidural lipomatosis. Recognition of this sign will aid with proper diagnosis of this unusual disorder. Methods. Computerized Tomography (CT) and Magnetic Resonance Imaging (MRI) examinations of the lumbar spine performed in the evaluation of low back pain over a 24 mo period demonstrated nine patients with excessive fat in the epidural space. Myelography was performed on five of these patients. Detailed analysis of the shape and degree of thecal sac compression was made. Results. Eight of the nine patients with epidural lipomatosis demonstrated a characteristic type of thecal sac compression not seen with other spinal disorders. On cross sectional imaging, the thecal sac has a striking stellate appearance with three rays emanating from a central core. This produces a trifid shape resembling the letter “Y”. In two patients scanned following subsequent surgical removal of the epidural fat, the thecal sac had resumed its normal configuration. Conclusion. A seemingly specific and easily recognized type of compression of the thecal sac may occur with lumbar epidural lipomatosis. This creates a trifid appearance of the thecal sac; we have termed this the “Y” sign.