Unintentional subdural placement of epidural catheters during attempted epidural anesthesia: an anatomic study of spinal subdural compartment.

BACKGROUND: Although infrequent, subdural block is a complication of epidural anesthesia with obvious implications. Knowledge of the spinal subdural compartment (dura-arachnoid interface) may help elucidate controversies arising from evidence that subdural catheter placement is feasible and may be difficult to identify clinically. METHODS: Samples of arachnoid lamina obtained during in vivo lumbosacral surgery (n = 4) and from cadavers (n = 6) were obtained and prepared for transmission electron microscopy and scanning electron microscopy. Subdural spaces were artificially produced in suitable samples, and an epidural catheter was inserted between the arachnoid and dura to compare the dimensions of meninges in relation to epidural catheters. RESULTS: Scanning electron microscopy of the dural sac showed areas of continuity between the arachnoid lamina and dura mater and other parts with both membranes separated by a subdural space. Transmission electron microscopy allowed the study of such border zones, where alternating cellular and collagen layers could be seen. A layer rich in collagen fibers and some fibroblasts separated arachnoid and neurothelial cells (dural border cells). Few specialized membrane junctions were found among cells adjacent to collagen fibers. Dura mater had an average thickness of 260 to 400 µm, with a dural lamina of approximately 4 to 6 µm. In areas where the arachnoid appeared separated from the dural lamina, its thickness measured 35 to 45 µm. Catheters with a diameter of 700 µm were successfully inserted inside the subdural space, between the dura mater and the arachnoid lamina. CONCLUSIONS: Dura mater and arachnoid layers act as a single unit but may be pulled apart by traction forces during cadaver processing of the dural sac or in vivo placement of catheters. This generates subdural spaces, either parallel or concentric, because of the minimal resistance offered by the tissue, which may be explained by its few specialized membrane junctions.

The ultrastructure of the human spinal nerve root cuff in the lumbar spine.

BACKGROUND: Spinal nerve root cuffs may be relevant in selective nerve root and epidural blockade. METHODS: We examined the ultrastructural aspects of spinal nerve root cuffs, such as their cellular and fibrillar components, using special histological staining methods, transmission and scanning electron microscopy, from six human cadavers. RESULTS: The morphology of the spinal nerve root cuff resembles that of the spinal subdural compartment. Cells gather together in compact layers due to specialized junctions. The thickness of its cellular layers is 5 to 8 microns; cells appear oriented parallel to the direction of their own nerve roots. The fibrillar component, made largely of collagen fibers, is found in the outer part of the spinal nerve root cuff and measures 100 to 150 microns. Numerous adipocytes separate dural laminas in concentric layers, extending from the dural sac to the spinal nerve root ganglia. However, adipocytes are not found within the thickness of the dural sac. CONCLUSIONS: The presence of few capillaries and the short distance between fat and axons may affect the passage of epidurally injected substances towards nerve root axons.

Ultrastructural findings in human spinal pia mater in relation to subarachnoid anesthesia.

UNLABELLED: We examined ultrastructural details such as the cellular component and membrane thickness of human spinal pia mater with the aim of determining whether fenestrations are present. We hypothesized that pia mater is not a continuous membrane but, instead, that there are fenestrations across the pial cellular membrane. The lumbar dural sac from 7 fresh human cadavers was removed, and samples from lumbar spinal pia mater were studied by special staining techniques, immunohistochemistry, and transmission and scanning electron microscopy. A pial layer made by flat overlapping cells and subpial tissue was identified. We found fenestrations in samples from human spinal pia mater at the thoracic-lumbar junction, conus medullaris, and nerve root levels, but these fenestrations did not appear at the thoracic level. We speculate whether the presence of fenestrations in human spinal pia mater at the level of the lumbar spinal cord and at the nerve root levels has any influence on the transfer of local anesthetics across this membrane. IMPLICATIONS: The ultrastructural anatomy of the human pia mater, such as pial cells, membrane thickness, and subpial tissue at different levels of the thoracic and lumbar spinal cord and nerve roots, was studied by special staining techniques, immunohistochemistry, and transmission and scanning electron microscopy. Fenestrations were found in samples at the thoracic-lumbar junction, conus medullaris, and nerve root levels. No fenestrations were found in samples at the thoracic level. At present, we cannot determine the significance of these findings.