Epidural lipomatosis is not uncommon to see and is an important diagnosis to make as it can be symptomatic.

Marked increase in fat in the epidural space (Green arrows) with the thecal sac compressed into a characteristic trefoil shape (Pink arrow).

The trefoil appearance of the thecal is a very characteristic appearance in the lumbar spine with epidural lipomatosis.

However this appearance is not seen in the thoracic spine where there will just be excess epidural fat.

In the cervical spine epidural lipomatosis is not seen.

• The trefoil appearance refers to a three lobed structure arising from a common centre.
  
  
• The sac compresses into that shape because Dentate ligaments attach to the 3 margins preventing the sites of attachment from being compressed.

For the characteristic MRI appearance of Lumbar Epidural lipomatosis, see the images and explanation below.