The main clues favoring an extradural tumor are extrinsic dural sac compression and tumor extension into the neural foramen. With severe tumor compression of the dural sac, however, it can be diffcult to tell if the dural/cord compression is extrinsic (by an extradural tumor) or due to a space occupying lesion within the dura (by an intradural tumor). The clue that points toward an extradural tumor lies in the appearance of the subarachnoid space at the margin of the tumor; due to extrinsic compression of the dura, the subarachnoid space between the dura and spinal cord is obliterated at its margin of the tumor. With an intradural tumor, the dura bulges out instead with a convex appearance, while the cord is compressed by the tumor, resulting in widening of the subarachnoid space at the edge of the tumor.