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.
Causes of adhesive arachnoiditis include severe trauma, previous surgery, haemorrhage, radiotherapy, myelography, meningitis (both microbial and chemical), and intrathecal misplacement of methylprednisolone acetate and/or other neurotoxic agents 3.
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