Posterior epidural fibrotic mass associated with Baastrup’s disease

A few reports have demonstrated rare cases of Baastrup’s disease that involve epidural cysts that cause dural compression. However, there have been no reports of a midline epidural fibrotic mass being associated with Baastrup’s disease. A 60-year-old man presented with neurogenic claudication that had lasted for 5 years. Radiography showed anterolisthesis at the L4–L5 level, magnetic resonance imaging demonstrated severe stenosis due to a posterior noncystic mass, and the linear fluid signal tracked into the posterior epidural space at the L4–L5 level. A cleft in the ligamentum flavum was identified by probe at surgery, and this enabled the probe to be inserted into the epidural space without excising ligamentum flavum. Histological analysis showed that the fibrotic mass consisted of a collagen matrix that had a cystic component and exhibited a peripheral inflammatory reaction. This report shows that it is possible for an extended epidural cystic mass that occurs in Baastrup’s disease to change over time through peripheral inflammation into a cyst-containing fibrotic mass.