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MRI ARACHNOIDITIS: NORMAL NERVE ROOT DISTRIBUTION – WHAT TO LOOK FOR (VIDEO)

The Dx

/ Spine Degeneration

NORMAL NERVE ROOT DISTRIBUTION: WHAT TO LOOK FOR

Before you can spot abnormal nerve roots, you need to know what normal looks like

This is part 1 of our series on the MRI appearance of arachnoiditis. Arachnoiditis is not rare — it's often thought of only in terms of infection, but it's relatively common in post-operative spines.
In this video we cover the pathogenesis of arachnoiditis — why it happens — and the normal distribution of nerve roots in the lumbar spine, since knowing normal is what lets you recognise abnormal.
Part 2 of this series covers normal contrast enhancement patterns, which you need to know before interpreting arachnoiditis studies given with contrast.

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▾ The pathogenesis

Arachnoiditis is inflammation of the arachnoid membrane, made up of the dura, arachnoid, and pia.
These layers are avascular — important to know, since it affects what you'd expect to see with contrast.
Acute inflammation, infection, or hemorrhage leads to fibrin and exudate being deposited on the nerve root sheaths within the arachnoid.
Over time this produces collagen formation, and the nerve roots start to stick together.
Similar collagen change within the subarachnoid space can form septae.

▾ Causes, and how common it is

Postoperative change is probably the most common cause in many practices; infection is a common cause in other countries.
Up to around 20% of post-laminectomy or post-fusion patients develop some form of arachnoiditis.
Radiculitis or early septae formation postoperatively is actually quite common, but doesn't always persist — the persistent cases make up that ~20% figure.
Surgery doesn't need to go through the dural space to cause it: even extradural surgery can result in arachnoiditis or radiculitis, apparently through changes in vascularity.
Other causes include trauma, hemorrhage, and injections into the subarachnoid space (e.g. spinal tap or puncture).

▾ Malignant arachnoiditis is different

This series doesn't cover malignant arachnoiditis specifically, since it isn't a true arachnoiditis.
It's tumour cells lining the arachnoid, not an inflammatory or infective process.

▾ At the conus

Proximally, at the level of the conus, the nerve roots are centred close together, bunched around it.

▾ At the mid-lumbar level (around L3)

The nerve roots start to separate out, becoming progressively further apart from each other as you move distally — this is the cauda equina ("horse's tail") appearance.

▾ At the S1 level

The nerve roots are fully separated out, distributed around the margin of the spinal canal. Seeing them bunched together at this level — instead of separated peripherally — is abnormal.

At the S1 level, the nerve roots lie peripherally, right up against the edge of the thecal sac.
There should still be a visible rim of CSF separating the nerve root from the thecal sac margin — not the nerve root stuck directly against the dura.
Sagittal images show the nerve roots fanning out — the cauda equina appearance.
Axial images are more useful than sagittal for assessing whether a nerve root distribution is normal.

Arachnoiditis is inflammation of the arachnoid membrane (dura, arachnoid, pia) — avascular layers, which matters when later interpreting contrast enhancement.
Postoperative change is a common cause; up to around 20% of post-laminectomy or fusion patients develop some form of arachnoiditis, and even extradural surgery can trigger it.
Normal nerve roots are bunched centrally near the conus and progressively separate more distally, lying peripherally by S1 — but should still show a rim of CSF separating them from the thecal sac margin.
A bunched-up nerve root pattern at S1 is abnormal; axial images are more useful than sagittal for assessing this.

▾ What is arachnoiditis?

Inflammation of the arachnoid membrane — the dura, arachnoid, and pia — which are avascular layers.

▾ What is the most common cause of arachnoiditis?

Postoperative change is the most common cause in many practices, though infection is more common in some countries.
Other causes include trauma, hemorrhage, and injections into the subarachnoid space.

▾ What does a normal nerve root distribution look like at the S1 level?

The nerve roots lie peripherally, distributed around the margin of the spinal canal, with a visible rim of CSF separating them from the thecal sac — not bunched together, and not stuck directly against the dura.