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Case 26

Right paracentral disc prolapse

Right sided sciatica

This man complained of right sided sciatica. A T2 weighted axial MRI of the lumbar spine is shown.

Findings

The image was obtained at the L5/S1 disc space. There is a low signal mass protruding posteriorly and to the right from the posterior disc margin (white arrow head). This causes only minor compresion on the anterior margin of the theca (the bright, CSF containing sac in the spinal canal). The nerve roots within the theca are visible around its posterolateral margins and are not affected. However the neural foramen on the right is obliterated - compare with the other side where the higher signal fat, and the lower signal S1 nerve root (white arrow) are clearly seen. Note on the labelled image how the L5 root (black arrow) is also spared, as it is quite lateral at this level.

Diagnosis

Right paracentral disc prolapse at L5/S1

Discussion

Thus a paracentral disc prolapse at for example L5/S1 will normally miss the L5 root, catch the S1 root and spare the roots still within the theca. A central disc prolapse, which is less common, will compress thecal nerve roots. The even rarer lateral disc prolapse may occasionally catch the L5 nerve root.

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