First step in evaluation of any spinal lesion is to determine the compartment of origin:

• Intramedullary (w/n substance of SC)

• Intradural extramedullary (w/n thecal sac but outside cord)

• Extradural (outside the thecal sac)

INFLAMMATORY LESIONS

Intradural extramedullary lesions are located w/n the dura but outside the SC, usually involving the nerve roots or meninges and projecting into the SAS. A subset of this category are lesions of the conus medullaris, cauda equina, and filum terminale which have a distinct ddx.

Intradural extramedullary lesions may deviate/compress the SC. CSF is seen above and below the lesion as well as possibly as a cleft b/n the lesion and cord.

Nerve sheath tumors and meningiomas comprise 90% of intradural extramedullary neoplasms although which is the single most common tumor varies by source. Leptomeningeal mets are less common.

Myxopapillary ependymomas are the MC tumor in the cauda equina region. Spinal paragangliomas are less common but also almost always arise in this region.

CONGENITAL / DEVELOPMENTAL

Extradural lesions are located external to the dura, including the vertebra and epidural space.

• Degenerative lesions (osteophytes and herniated discs) are the MC lesions. Others include neoplasms and infection.

• MC extradural neoplasms that caue epidural SC compression are carcinoma mets, plasmacytoma/MM and lymphomas.

• Primary extradural tumors are rare.

More sagittaly oriented Lr-L5 facets are more likely to have degenerative listhesis.

>4mm anterolisthesis is abnormal.

Isthmic spondylolisthesis: wide canal sign (sagittal canal ratio [AP diameter of canal at the index level / diameter of canal at L1 ] >1.25 is reliable predictor of presence of pars interarticularis defects.