The Problem of Subdural Placement in Myelography

Abstract

Subdural placement of contrast medium is anathema to the myelographer. It immediately compromises the diagnostic accuracy of the procedure and jeopardizes the success of future examination of the spinal subarachnoid space (1–6). In the literature, allusions to extra-arachnoidal injection of radiopaque oil are invariably coupled to warnings that in such instances interpretation of findings is difficult and often erroneous. The purpose of this paper is to consider the cause, appearance, effect, and possible prevention of subdural placement of contrast medium. Material A total of 140 myelographic examinations via lumbar puncture were attempted or performed between January 1959 and April 1961 at the University of Florida Teaching Hospital and Clinics. In 9 instances, the test dose was completely subdural in position, and in most of these the procedure was discontinued. In 10 cases, the instillation was in part subdural, sufficient in amount to interfere with radiologic study. In several instances, no test dose had been used. The incidence of faulty placement was 13 per cent (19 of 140 examinations). The radiographs were reviewed to determine whether any anatomic or technical features were common to the 19 cases with faulty placement of the medium. No spinal bony abnormalities were found, and in all cases the interpedicular distance was normal at the level of lumbar puncture. No herniated disks were present at the injection level. The needle bevel was as often coronal as sagittal and was in the center of the anteroposterior diameter of the spinal canal more often than it was shallow or deep. Where spot-films in anteroposterior projection were available and included the needle, the tip was as frequently midline as lateral in position. Recent lumbar puncture as a cause of subdural injections on myelography, due to leakage of cerebrospinal fluid into the subdural space, has been emphasized (3, 4, 6, 7). Lumbar puncture had been done within two weeks in only 4 of our cases of faulty placement. On 6 occasions, successful subarachnoid myelograms were obtained within one week of lumbar puncture, and subarachnoid placement was accomplished in 5 patients within three days after subdural instillation of a test dose of contrast medium. Puncture of the cisterna magna for myelography is usually considered an easy technical procedure, although carrying some risk. Cisternal injection was performed 9 times during the period under review, and subdural injection of the contrast medium occurred 4 times despite the large size of the normal cisternal subarachnoid space. The mechanism was probably as illustrated in Figures 1, Band C. Mechanisms of Subdural Injection Ordinarily, the dura mater and arachnoid are separated by a tiny cleft containing a minute amount of lubricating fluid.