MR characteristics of a primary melanoma of the quadrigeminal plate.

and (3) paresis , loss of sensation, and low levels of protein in the CSF are more common. Unenhanced CT examinations can show an intramedullary mass with zones of low density suggestive of a cystic component; if the cord is collapsed , syringomyelia can reliably be assumed to be present. If the cord is not collapsed , which occurs more frequently, it is impossible to differentiate syringomyelia from a cystic intramedullary tumor [3). Contrast-enhanced CT also can be used to confirm the existence of an intramedullary cyst. After intrathecal injection of metrizamide, opacification of a region of low density confirms the cystic component of the lesion. Sometimes, opacification occurs immediately because the cyst communicates directly with the subarachnoid space. In other cases , opacification occurs after several hours (up to 24 hr) when metrizamide arrives at the cyst by an indirect route (i.e., via the fourth ventricle or by diffusion across the cord). Whether delayed or undelayed, the occurrence of opacification is not useful in differentiating syringomyelia from cystic tumor or tumor associated with syringomyelia. What does matter is verification of the presence of syringomyelic cavities at each level of cord enlargement because this rules out the existence of a tumor. Evidence of an enlarged cord zone without cavities in its interior strongly suggests formation of a tumor. This was true for our case in which an enlargement of the spinal cord from C3 to C7 was apparent, and the cavity from the inferior part of C4 as far as C7 was filled with contrast material. When the diagnosis is doubtful despite the criteria mentioned, direct puncture of the cystic cavities and contrast filling (syringogram) may be helpful [8). An irregular or nodular wall strongly suggests a tumor enclosing the syringomyelic cavity. Intraoperative echography also can show the cystic nature of these lesions, and the superior and inferior contours are better identified and localized by this method than by CT. Large central cysts with smooth margins are more suggestive of syringomyelia, whereas tumoral cysts are much smaller and marked by eccentric localization and irregular contours [2, 5, 6, 9).