The most common disorder seen while performing myelography is the extradural defect. Ventral extradural defects at the level of the disc space are caused by a variety of conditions, including disc bulges, disc herniations, posterior endplate spur formation, epidural hematomas, inflammatory processes, and epidural tumor, with or without associated adjacent bone involvement. Anterolateral extradural defects at the level of the disc space are routinely caused by posterolateral disc herniations, which will also result in nerve root displacement or nerve root sleeve truncation and the classic double-density sign seen on a lateral radiograph with the patient in the prone position (...Fig 2.3:18. ). Posterior and posterolateral extradural defects at or near the level of the disc space are usually the result of chronic degenerative changes of the posterior elements, including hypertrophic facet disease, ligamentum flavum hypertrophy, and the occasional synovial cyst arising from a markedly degenerated facet joint. Less common causes include metastatic disease to the posterior elements in association with a posterior epidural tumor component. An anterior or anterolateral extradural defect not limited to the disc space level could be the result of a migrated free disc fragment, epidural hematoma, inflammatory process, or metastasis. Far lateral disc herniations within the lateral aspect of the neural canal that extend beyond filling of the nerve root sleeve are radiographically silent on myelography. Similarly, a significant percentage of disc bulges and herniations at the L5-S1 level are nondescript using conventional myelography owing to the rather prominent anterior epidural space at this level that allows for posterior extension of disc material without associated thecal sac compression or nerve root displacement.
Intramedullary lesions causing enlargement of the cord or conus with concomitant narrowing of the adjacent subarachnoid space include primary cord neoplasms such as ependymomas, astrocytomas, and hemangioblastomas; metastases; inflammatory conditions such as sarcoidosis and abscess formation; cord hematomas and infarcts; vascular malformations; and congenital lipomas, dermoids, and epidermoids. Although these groups of lesions will most certainly cause widening of the silhouette of the cord or conus, the intrinsic pathology and extent of these intramedullary lesions are far better imaged with MRI.
Intradural extramedullary lesions such as meningiomas and the majority of neurofibromas can be adequately studied with myelography followed by CT. Whereas the intrinsic characteristics of these lesions is best evaluated with
Figure 23-18 A, Prone cross-table lateral view of lumbar myelogram. Prominent ventral impressions upon the contrast column are present at L3-4 and L4-5. Much milder ventral impressions are seen at L2-3 and L5-S1. A so-called double-density sign consistent with an eccentric posterior disc herniation is present at L3-L4 and L4-B, Prone left anterior oblique view of lumbar myelogram. Left-sided filling defects are present at L3-4 and L4-5 secondary to eccentric posterior disc herniations at both C, /Axial CT scans at and just below L3-4 disc space demonstrate moderate-sized broad-based central and left-sided posterior disc herniation extending into left lateral recess. This eccentric posterior disc herniation would account for the double density sig A.seen in
MRI, myelography and postmyelography CT still play an important role in the presurgical evaluation of these lesions. Subtle cases of arachnoiditis or the determination of the level of a dural tear in a postoperative pseudomeningocele is as adequately visualized with myelography and postmyelography CT as with MRI ( ...
Fig 2.3:19. ). Myelography and postmyelography CT are excellent diagnostic tools in evaluating patients with brachial plexus traction injuries presenting with nerve root sleeve tears and nerve root avulsion injuries. The communication of intradural and extradural arachnoid cysts, sacral cysts, and Tarlov cysts with the subarachnoid space can be accurately evaluated with myelography and postmyelography CT.
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