Spinal arachnoid sleeves and their possible causative role in cauda equina syndrome and transient radicular irritation syndrome.

INTRODUCTION: We have previously described arachnoid sleeves around cauda equina nerve roots, but at that time we did not determine whether injections could be performed within those sleeves. The purpose of this observational study was to establish whether the entire distal orifice of a spinal needle can be accommodated within an arachnoid sleeve. MATERIALS AND METHODS: We carefully dissected the entire dural sacs off four fresh cadavers, opened them by longitudinal incision, and immersed them in saline. Under direct vision, we penetrated the cauda equina roots nerves traveling almost vertically downward at 30 locations each with a 27- and a 25-G pencil-point needle (60 punctures total). We captured the images with a stereoscopic camera. RESULTS: The nerve root offered no noticeable resistance to needle entry. Although the arachnoid sleeves could not be identified with the naked eye, they were translucent but visible under microscopy. In 21 of 30 attempts with a 27-gauge needle, and in 20 of 30 attempts with a 25-gauge needle, the distal orifice of the spinal needle was completely within the arachnoid sleeve. CONCLUSION: It seems possible to accommodate the distal orifice of a 25- or a 27-gauge pencil-point spinal needle completely within the space of the arachnoid sleeve. An injection within this sleeve could potentially lead to a neurological syndrome, as we have previously proposed.