The empty thecal sac sign or empty sac sign is when the thecal sac appears empty on MRI of the lumbar spine, best seen on T2-weighted images. If the empty thecal sac sign is present, a diagnosis of adhesive arachnoiditis can be made.
There is usually no Gadolinium contrast enhancement. Delamarter et al 1 described three categories of MRI appearances consistent with chronic adhesive arachnoiditis. Groups 1 and 2 were mild and group 3 severe. Groups 1 and 2 were consistent with adhesions developing between nerve roots and the parietal arachnoid.
The adherent nerve roots were seen centrally within the CSF in group 1. Whilst in group 2 the nerve roots were adherent peripherally to the theca, which leads to the appearance of an empty thecal sac.
- 1. Delamarter RB, Ross JS, Masaryk TJ et-al. Diagnosis of lumbar arachnoiditis by magnetic resonance imaging. Spine. 1990;15 (4): 304-10. - Pubmed citation
- 2. Ross JS, Masaryk TJ, Modic MT et-al. MR imaging of lumbar arachnoiditis. AJR Am J Roentgenol. 1987;149 (5): 1025-32. AJR Am J Roentgenol (abstract) - Pubmed citation
- 3. Lima RM, Navarro LH, Carness JM et-al. Clinical and histological effects of the intrathecal administration of methylprednisolone in dogs. Pain Physician. 2011;13 (5): 493-501. Pubmed citation