[CT-epidurography. A comparison of conventional and CT-epidurography with contrast medium injection through a thoracic peridural catheter].

Epidurography with contrast medium is used to verify the correct position of an epidural catheter and to detect malpositioning. There is great variability in the distribution of contrast medium according to the individual morphology of the epidural space and the way it is injected. Results of investigations of the anatomy of the spinal canal and epidural space performed with anatomic specimens, epiduroscopy, and conventional and computed tomographic (CT) epidurography are sometimes contradictory. We have performed CT epidurography in approximately 30 patients to date. Insights regarding the distribution of contrast medium in the epidural space can help to interpret conventional epidurographies and to explain special features, such as the "rail-road-track phenomenon" or the plica mediana dorsalis. METHODS. Patients scheduled for major abdominal operations were studied. Catheters were introduced into the thoracic epidural space on the day before the operation. Conventional epidurographies were performed routinely after insertion of the catheters with a mobile X-ray apparatus, usually in the recovery room, using 5 ml iopamidol (Solutrast 250 M). Selected patients (good mental state and stable psychic condition), after written consent and with approval of the local ethical committee, were investigated with CT epidurography using the same contrast medium (partly diluted). From among the CT scans performed until now, five characteristic images are presented and compared to conventional epidurographies. RESULTS. When small volumes (5 ml) of iopamidol were injected slowly, we regularly observed a railroad-track phenomenon in the AP images of conventional epidurographies. After rapid injection of larger volumes (10-15 ml), the spread was more homogeneous and sometimes outline a lighter zone of contrast distribution in the midline. CT epidurography in these cases (5 out of 30 patients) revealed a plica mediana dorsalis. In the majority of the CT scans of the thoracic epidural space its anterior (ventral) compartment was unfilled by contrast medium. According to the findings of several investigators, dura mater and ligamentum longitudinal posterius have grown together, so that an epidural space does not exist in ventral thoracic segments of the spinal canal. In lower thoracic and lumbar segments both structures are separate, and a wide anterior epidural space was present and filled with contrast medium. In other cases the dorsal and dorsolateral epidural space was completely filled, but the contrast medium stopped behind the spinal nerves and surrounding dural sheaths, as if these structures, together with the connective tissue strands between them, formed a membrane that spread in a frontal plane. A railroad-track phenomenon is interpreted in the literature as the X-ray correlate of contrast medium spread in the subdural space, indicating an incorrect catheter position. According to conventional epidurographies and confirmed by CT scans, this feature is also seen regularly in cases with correct catheter position. Small volumes of iopamidol spread preferentially into the wider parts of the epidural space between the gaps of the vertebral arcs, whereas the dura mater and periosteal layers touch at the inner side of the pedicules and laminae, allowing the existence of only a potential epidural space. This rhythmic widening and narrowing of the epidural space is the explanation of the "railroad track" in AP contrast epidurographies. CONCLUSIONS. CT epidurography is a valuable tool to provide better insight into the morphology of the epidural space when filled with fluid (contrast medium, local anaesthetics), complementing findings using epiduroscopy and anatomic specimens. It stands to reason that this time- and cost-expensive method can never replace conventional epidurographies, but can help to interpret them properly.