Lipoma of the filum terminale
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Lipoma of the filum terminale, also known as fatty filum terminale or filar lipoma, is a relatively common finding on imaging of the lumbar spine, and in most cases is an incidental finding of no clinical concern. However, in some patients it may be associated with signs and symptoms of tethered cord syndrome. In such cases, it is usually associated with a thickened filum and a low-lying conus.
Fat is seen within the filum terminale in ~5% of relevant examinations.
Lipoma of the filum terminale is formed as a result of a developmental error in mesodermal cell migration.8
A thin filum (<2 mm in diameter at the L5/S1 level) is rarely symptomatic. When the filum is thickened (with or without fat) it is much more likely to be associated with a low lying cord (tip of the conus at or below the mid point of L2) and cord tethering. As such careful assessment of the position of the conus is essential 5.
If large enough, then fat density (-90 to -30 HU) can be seen below the level of the conus. If small, and depending on the quality of the CT scanner, the size of the patient, and the amount of quantum mottle, it may be difficult to identify.
The abnormality typically is linear and extends over some distance. Signal follows that of fat on all sequences and can demonstrate chemical shift artifact on T2* / gradient weighted sequences.
fat saturated sequences: signal loss demonstrated
T1 C+ (Gd): no enhancement
Treatment and prognosis
When patients have tethered cord syndrome and a low lying conus surgical intervention would be considered appropriate by most, with the filum sectioned either just at the tip of the conus or lower down in the lumbar theca 4.
In asymptomatic patients, management is equally simple, in that nothing need be done.
Difficulty arises in patients who have some symptoms suggesting tethered cord syndrome, but whose conus terminates at a normal level. Controversy as to the benefits of division of a fatty filum in such patients exists 4.
There is little or no differential when the presence of fat is confirmed, however other filum terminale lesions can be considered only to be eliminated.
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