Chapter 21. Neuraxial Anatomy (Anatomy Relevant to Neuraxial Anesthesia)
In most humans, what level do the dural sac and spinal cord terminate?
A. In infants, dural sac at S4 and spinal cord at L3. In adults, dural sac at S2 and spinal cord at L1.
B. In adults and infants, dural sac at S2 and spinal cord at L1.
C. In infants, the spinal cord is tethered to the dural sac and they both terminate at S4. Shortly after birth, the filum terminale ablates and in adulthood, dural sac terminates at S4 and spinal cord at L1.
D. In infants and adults, dural sac terminates at S4. In infants, spinal cord is at L1 and grows down to L3 as an adult.
A is correct. In the embryo, the spinal cord terminates in the sacral area. During further development, the vertebral column grows faster than the spinal cord leading to the variation in its position as the individual ages. At birth, the conus medullaris terminates at L3 and dura mater terminates at S4. The conus medullaris settles in its final position by 2 months of age as the vertebral column outpaces the spinal cord in growth. From 2 months into adulthood, the conus medullaris finds its terminal position at L1 when the individual is in an upright and erect position and the dural sac terminates distally at S2. Because the dural sac has a more distal termination, inadvertent intrathecal puncture when attempting caudal epidurals in premature infants is more likely than in older children or adults.
B is incorrect. After the infant is born, the neuraxiom continues to grow and mature. The bony structures of the spine grow at a different rate than the contents of the spinal column. As a result, the levels of the dural sac and spinal cord do not remain at the same level from birth into adulthood.
C is incorrect. A tethered spinal cord is a pathological disorder. This can occur when the filum terminale is thickened and attached to the sacrum or a lipoma growth on the filum terminale tethers the spinal cord to the surrounding sacrum. This syndrome is closely associated with spina bifida.
D is incorrect. Because the vertebral column grows faster than the spinal cord, the spinal cord terminates lower at birth, L3, and higher in adulthood, L1. Studies corroborate the majority of spinal cords terminate at the level of L1, L1–L2 disc, and upper L2. There are variations of spinal cord termination as high as T12 and as low as upper L3 in adults.
What is the correct palpable landmark and its corresponding vertebral level?