A member recently sent me a private message of an MRI that shows the presence of epidural fat at L5/S1 that "completely narrows the thecal sac". I thought that I would give my answer here so that other members can see and perhaps comment.
Some epidural fat is normally present in the spinal canal, particularly in the sacral spinal canal where there is space, as suggested in the following diagram:
MIDLINE SAGITTAL VIEW OF THE LUMBAR SPINE
The posterior epidural space (shown in orange) is segmented by areas where the dura contacts bone. The posterior epidural space compartments have their greatest anteroposterior dimension at their superior end. The anterior dura is fused with the posterior longitudinal ligament and the annular ligament at the level of each intervertebral disc, which divide the anterior epidural space into vertical segments. The posterior longitudinal ligament separates from the anterior dura at the lumbosacral junction and the anterior epidural space becomes filled with fat more inferiorly.
Some epidural fat is normal. In fact, the epidural fat is typically soft and cushions the spinal cord in the canal. Epidural fat is typically absent from the cervical spine but may appear in the lower thoracic spine and usually is present in the lumbar and sacral spinal canal, usually in front of or in back of the spinal cord and in the sleeves of the spinal roots (Source). However, to have the spinal thecal sac enclosed by fat is abnormal. Lumbar stenosis (a narrowing of the spinal canal in the lumbosacral spine) is a common cause of back pain and neurological symptoms. Most of the time, it is due to narrowed bony canal and requires surgical decompression. However, in some cases, as in the one described above, epidural fat is the cause. It is interesting that in many cases of spinal stenosis, there is usually a loss of epidural fat (Source).
Spinal epidural lipomatosis is sometimes associated with steroid long term steroid use or endocrinopathies such as Cushing's disease. There have been suggestions that it is associated with obesity. The attached is a paper from Hong Kong published in 2002, describing a 24-year old man who was diagnosed with spinal epidural lipomatosis around L4/5 level. He was put on a diet and lost 3 kg to 91.5 kg with no change in his symptoms. Although he had spondylosis of his spine at L4/5, injection of local anesthetics into the L4/5 joints did not reduce pain. The patient refused surgery and so was continued on a weight reduction program. The paper described a "Y-shaped sign" that is supposedly indicative of compression of the thecal sac.
The choices of therapy are relatively straight forward. If the condition is associated with symptoms such as pain or neurological loss, particularly related to the position of the fat, the following course of action is reasonable. First, investigate for the possibility of endocrinopathy (such as Cushing's disease that can lead to accumulations of fat in various parts of the body), hypothyroidism (Source) or steroid use (which should be tapered). If obesity is present, efforts should be made to reduce weight. Second, if the above do not reduce symptoms, the only other option is surgery. The surgery is relatively simple. Epidural fat is relatively easy to remove and can even be done without unroofing the entire lumbosacral canal. Please remember that the spinal cord ends at just below the L1 vertebra and the thecal sac is filled with spinal roots at L5/S1 level.