Hypothesis concerning the anatomical basis of cauda equina syndrome and transient nerve root irritation after spinal anesthesia

OBJECTIVE: Cauda equine syndrome is a rare neurological complication associated with subarachnoid anesthesia, and particularly with the use of 5% hyperbaric lidocaine and small gauge catheters. Our aim was to study a possible anatomical factor that might impede adequate dilution of local anesthetic and explain the development of cauda equine syndrome and transitory radicular irritation. MATERIAL AND METHOD: The spinal dura matters and their contents from two male human cadavers were examined after organs had been extracted for transplantation. Both men had recently died at ages 56 and 65 years of age. Samples were fixed in a glutaraldehyde phosphate buffer and dehydrated in acetone, which was then removed by critical point elimination. The samples were then metallized with gold and observed under a scanning electron microscope. RESULTS: We found that one portion of the arachnoids was more compact and another was lax. The compact portion had a laminar structure formed by the fusion of fibers and cell components lining the inner surface of the dural mater. The lax portion was comprised of a weblike network of filaments and few cells bodies. This portion extended from the compact inner arachnoid lamina to the cell plane of the pia mater, where it dispersed, sending out compact arachnoid projections that wrapped around structures in the subarachnoid space. We termed these wrappings "arachnoid sheaths".