Spinal epidural lipomatosis

One reason for a false positive reading of epidural fat interposition, in suspected spondylolysis, is epidural lipomatosis.

Epidural lipomatosis is a condition in which there is excess fat in the epidural space producing thecal sac compression. Typically the amount of fat is twice or thrice as much as the thecal sac area and qualitatively the diagnosis is made when a stellate or irregular shaped thecal sac is seen, usually at the L5/S1 level. Googling gave me one great example with pictures and another slide from a lecture published on the net.

Qualitatively too, this condition can be graded. Borre DG et al from Argentina have published an article on this subject in European Radiology, with multiple illustrations, showing how we can grade this condition from Gd 0 (normal) to Grade III. Basically if the AP diameter of the fat (anterior and posterior to the thecal sac) is three times that of the thecal sac AP diameter, it is significant Gd III lipomatosis. All the patients with Gd III lipomatosis in their series were symptomatic. Typically they will also have an abnormal shape of the thecal sac.

There is another short but informative article by Fassett & Schmidt discussing etiology, diagnosis and treatment.