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Empty thecal sac sign

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.

Pathology

Causes

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.

Radiographic features

MRI

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.

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