Spine: Compartments, Cord Signal, and Degenerative Pitfalls
Core concepts
Approach the spine by compartment: extradural (bone/disc), intradural extramedullary (meninges, nerve roots), and intramedullary (cord). For degenerative disease, know the difference between disc bulge (broad, symmetric) and herniation (focal), and how to describe central canal, lateral recess, and foraminal stenosis using axial and sagittal images. For trauma, understand the three-column model and ligamentous injury signs (interspinous edema, facet malalignment). For cord signal abnormality, separate intramedullary tumor (expansion, syrinx, enhancement), transverse myelitis (long-segment, central), and infarction (anterior cord, restricted diffusion). Recognize the classic intradural extramedullary lesions: schwannoma (often foraminal, enhancing) and meningioma (dural-based, broad base).
Traps
Examiners test modic changes vs. infection (endplate destruction, disc-centered vs. bone-centered), epidural abscess vs. phlegmon (peripheral enhancement, restricted diffusion), and cord compression grading. A frequent trap is mislabeling a conjoined nerve root or perineural cyst as a disc herniation. Also watch for CSF flow artifact mimicking cord signal change.
Worked rule of thumb
When a lesion is intradural extramedullary, localize by dural relationship: broad dural base with a dural tail favors meningioma; a dumbbell shape through a neural foramen favors schwannoma. Always check enhancement and foraminal widening to confirm.