When a 65-year-old man arrived with two months of worsening low back pain, numbness down his right leg, and cramping that forced him to stop after about 500 meters of walking, his MRI showed the expected culprits. There was canal narrowing at L4/5 and disc protrusions at two other levels.
But the axial images at L5/S1 showed something else: his thecal sac, the membrane holding the nerve roots, squeezed into the shape radiologists call the Y-sign by an overgrowth of ordinary fat inside the spinal canal. The condition is spinal epidural lipomatosis, and it is one of the more underrecognized causes of nerve compression in the lower back.
