Which statement is NOT true regarding spinal anatomy and spinal anesthesia?

Which statement is NOT true regarding spinal anatomy and spinal anesthesia?

• A. The addition of phenylephrine to lidocaine will prolong spinal anesthesia
• B. A high thoracic sensory block will result in total sympathetic blockade
• C. The largest vertebral interspace is L5-S1
• D. The dural sac extends to the S4-S5 interspace

Answer: (D)

The most important idea here is where the dural (thecal) sac ends in adults and how that relates to spinal anesthesia. In the adult spine, the thecal sac containing CSF ends around the level of the second sacral vertebra (S2). The actual spinal cord ends higher, around L1–L2, with the lower part of the space occupied by the cauda equina. So the statement that the dural sac extends to the S4–S5 interspace is not true—the dural sac ends much higher at approximately S2. This distinction matters clinically because intrathecal anesthesia is given into the lumbar subarachnoid space well above the sacral end of the thecal sac, typically at L3–L4 or L4–L5, to ensure CSF is reached within the thecal sac. The other statements align with common knowledge: adding phenylephrine can prolong spinal anesthesia by vasoconstriction reducing vascular uptake of the local anesthetic; a high thoracic block can produce substantial, potentially near-total, sympathetic blockade with hypotension; and the largest vertebral interspace in the adult is at L5–S1 due to lumbar anatomy.

The most important idea here is where the dural (thecal) sac ends in adults and how that relates to spinal anesthesia. In the adult spine, the thecal sac containing CSF ends around the level of the second sacral vertebra (S2). The actual spinal cord ends higher, around L1–L2, with the lower part of the space occupied by the cauda equina. So the statement that the dural sac extends to the S4–S5 interspace is not true—the dural sac ends much higher at approximately S2.

This distinction matters clinically because intrathecal anesthesia is given into the lumbar subarachnoid space well above the sacral end of the thecal sac, typically at L3–L4 or L4–L5, to ensure CSF is reached within the thecal sac. The other statements align with common knowledge: adding phenylephrine can prolong spinal anesthesia by vasoconstriction reducing vascular uptake of the local anesthetic; a high thoracic block can produce substantial, potentially near-total, sympathetic blockade with hypotension; and the largest vertebral interspace in the adult is at L5–S1 due to lumbar anatomy.