Inadvertent extra-arachnoid injections in myelography.

Abstract

In the performance of positive contrast myelography, difficulty is occasionally experienced in ascertaining whether contrast medium is in the subarachnoid, the subdural, or epidural space. Inadvertent extra-arachnoid injections of contrast medium must be recognized if errors in diagnosis are to be avoided. Contrast medium in the subdural space has been described as showing sluggish movement, characteristic oil accumulations, and incomplete filling defects (1). Myelograms after epidural injections have shown “streaky shadows along the spinal canal and for some distance along the nerve roots” (2). In order to assess these reported appearances more accurately, a series of consecutive myelograms was re-evaluated by the authors. Method and Material After a joint review by both authors of the roentgenograms of 244 myelographic examinations for 1961 at the University of California's Radiology Department, San Francisco, the studies were classified into four groups: 1. The contrast medium was all in the subarachnoid space. 2. The contrast medium was inadvertently introduced entirely into the epidural or subdural space. 3. The contrast medium was introduced partly into the subarachnoid and partly into the extra-arachnoid space. 4. The space into which contrast medium was introduced was not recognizable on the basis of the available roentgenograms. Seven criteria were applied in this study to indicate that contrast medium had entered the extra-arachnoid space: 1. Lack of definition of spinal cord: The spinal cord is not clearly defined. When the medium is in the subdural space, the Pantopaque column is blurred and its periphery somewhat reticulated (Figs. 1 and 4). This blurring is particularly noticeable in the cervical area, where the contrast agent reaches the concave dorsal aspect of the vertebral bodies. 2. Position of contrast material in canal: Posterior placement of contrast medium in the spinal canal seen on the horizontal-beam lateral views indicates a subdural injection (Fig. 2). 3. Excessive extension of the contrast material along the nerve roots: The nerve roots are outlined for a greater distance when the contrast medium is in the extra-arachnoid rather than in the subarachnoid space. This criterion is more applicable to epidural injection (Fig. 3). 4. Absence of globule formation: The possibility of an extra-arachnoid injection is considered if the end of the Pantopaque column is smooth or does not show globule formation. Globules are more pronounced when only a small amount of contrast material is in the subarachnoid space (Fig. 4). 5. “Stringiness” of column: Irregular defects in the Pantopaque column are noted with subdural injections (Fig. 3). 6. Nonvisualization of the vertebral or basilar arteries: Contrast material in the extra-arachnoid space, carried to the high cervical level, will fail to show an outline of the basilar and vertebral arteries (Fig. 5). These vessels are seen normally if Pantopaque is in the subarachnoid space.