Anterior Thecal Sac Impression on MRI: Causes, Significance, and Management

An anterior thecal sac impression (or ventral epidural compression) is an MRI finding where the front of the dural sac (thecal sac) is indented, typically by a posterior disc protrusion/herniation.

Anatomy

• Thecal sac = dura mater enclosing CSF, spinal cord (ends ~L1–L2), and cauda equina nerve roots.
• Anterior boundary = posterior longitudinal ligament (PLL) + posterior vertebral body/disc.

Common Causes

• Disc herniation/protrusion: Nucleus pulposus ± annulus fibrosis extends posteriorly. Most common at C5–6, C6–7, L4–5, L5–S1.
• Disc bulge: Circumferential annulus extension (>50% circumference) — may impress sac without frank herniation.
• Osteophytes/spondylosis: Chronic degenerative spurs.
• Synovial cyst: Facet joint origin.
• Ligamentum flavum hypertrophy: Usually posterior, but can contribute circumferentially.

Clinical Significance

• Asymptomatic: Common incidental finding (30%+ of asymptomatic adults).
• Symptomatic: Compression of spinal cord (myelopathy) or nerve roots (radiculopathy).

Symptoms by Level

Management

• Conservative (most): NSAIDs, PT (McKenzie, core stabilization), activity modification, epidural steroid injections. 80–90% improve in 6–12 weeks.
• Surgical indications: Progressive myelopathy, cauda equina syndrome (emergency), intractable radiculopathy >6–12 weeks with concordant imaging, progressive neurologic deficit.
• Procedures: Microdiscectomy, laminectomy, ACDF, corpectomy, disc arthroplasty.

Impression alone ≠ surgery. Clinical correlation is mandatory.

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Region	Cord Compression (Myelopathy)	Root Compression (Radiculopathy)
Cervical	Gait imbalance, hand clumsiness, hyperreflexia, Babinski, Lhermitte's	Neck/shoulder/arm pain, numbness, weakness in dermatomal/myotomal pattern
Thoracic	Spastic paraparesis, sensory level, bowel/bladder dysfunction	Truncal pain, band-like sensory changes
Lumbar	Conus medullaris syndrome (saddle anesthesia, urinary retention)	Sciatica (L4–S1 distributions)