Dural sac: Anatomy

The dura mater is the outermost of the spinal meninges. It is a thick, fibroelastic membrane composed of collagen and elastin fibers that is a continuation of the cranial dura. At its cephalad origin, it fuses with the periosteum of the skull at the foramen magnum, and it continues caudally in a circumferential manner to its termination at approximately S2 in the adult. Here it fuses with the filum terminale and eventually the periosteum of the coccyx. Laterally, it follows the spinal nerve roots out to about the level of the intervertebral foramina, where it blends with the epineurium of the roots. The dural sac and closely adherent arachnoid mater on its inner surface, are a barrier to drug migration into and out of the cerebrospinal fluid during epidural anesthesia. The termination of the sac well below the conus medullaris (classically found at L1 in the adult) also provides a degree of safety when performing spinal anesthesia, offering the anesthesiologist multiple spinal levels for intrathecal drug administration with low risk for direct spinal cord trauma.

Age-related anatomic variations exist in the location of the caudal termination of the dural sac that can affect neuraxial anesthesia. During early development in utero, the spinal cord exists throughout the entire spinal canal. Differential growth of the vertebrae compared to the nerves and cord account for a relative ascension of the cord within the canal over time. By birth the sac ends at S3 or S4, with the conus medullaris terminating at the L3 or L4 level. It is not until approximately 1 year of age that anatomic relationships resemble that of the adult. It is thus possible to enter the dural sac during caudal anesthesia in very young infants. If spinal anesthesia is used in this population, a low approach to entering the dural sac is warranted to avoid the cord.