Posterior Epidural Cyst Associated with Baastrup Disease

Background:

• Baastrup disease is characterized by bony sclerosis along adjacent spinous processes and is related to repetitive flexion and extension. In the elderly, the tips of the spinous processes may undergo hypertrophy with interspinous degeneration with bursitis. Sometimes, it presents with a ventral extension of bursal fluid as postero-central epidural cyst leading to neurogenic claudication.

Clinical Presentation:

• Midline tenderness localized in the lumbar region and increased pain during spinal extension, which is alleviated by flexion
• Signs of spinal canal stenosis and neurogenic claudication

Key Diagnostic Features:

• Approximation of spinous processes with enlargement, flattening, and reactive sclerosis of the apposing interspinous surfaces
• T1/T2 hypointensity due to sclerosis and ligament flavum hypertrophy
• T2 hyperintensity due to fluid and edema along the interspinous ligament
• Associated juxta-articular cysts
• Enhancement of the enlarged interspinous ligament bursa and/or degenerated cysts
• Some patients may develop subsequent anterolisthesis 

Differential Diagnoses:

• Ligamentum flavum or facet synovial cyst: Typically seen as a calcified cystic lesion adjacent to a facet joint, may also show adjacent facet joint arthropathy and presence of gas. T1-hyperintense if hemorrhage, hypointense on both T1- and T2-weighted images if calcification or gas; sometimes shows peripheral contrast enhancement
• Abscess: Phlegmonous stage usually shows homogeneously enhancing area correlating to granulomatous thickened tissue with embedded micro-abscess without significant pus collection; liquid abscess stage shows a low-T1, high-T2 signal region, with restricted diffusion and variable peripheral contrast enhancement. Associated soft tissue inflammation and diskitis-osteomyelitis can coexist.

Treatment:

• Oral analgesics and nonsteroidal anti-inflammatory drugs (NSAIDs)
• Local steroid injection into the interspinous processes/ligament region 
• Surgical excision of part of the involved spinous processes and interspinous process decompression devices