Benner classification of arachnoiditis (1978):

0. Extradural compression (spondylosis/stenosis)

1. Local post-operative changes

2. Arachnoiditis at a single level

3. Arachnoiditis at multiple levels

4. Myelographic block secondary to arachnoiditis

5. Arachnoiditis progressive or ascending greater than 2 levels above operative site.

The radiologists classify arachnoiditis according to Delamarter's MRI classification:

Type I: central clumping of nerve roots

Type II: peripheral adhesion of nerve roots to the theca ("empty sac")

Type III: complete opacification of the thecal sac, extending over at least one vertebral level

Aldrete describes:

Class I: nerve root sleeves alone

Class II: extends to involve subarachnoid space