Figure 1. Theoretical mechanism for production of total spinal anesthesia after epidural saline injection. (A) Immediately after epidural injection, the local anesthetic solution (slashed lines) is confined predominately within the epidural space, although diffusion into the dural cuff regions and the superficial layers of the spinal cord undoubtedly occurs during the initial onset of sensory blockade. (B) As the duration of the procedure increases, local anesthetic accumulates in the spinal subarachnoid space. (C) Bulk injection of saline into the epidural space then compresses the spinal subarachnoid compartment, pushing local anesthetic-containing subarachnoid fluid into the intracranial compartment. Venous sinus blood volume (black) is diminished. To account for persistent dilated nonreactive pupils in the absence of extreme hypoxia, the local anesthetic must reach as far as the interpeduncular cistern, where the oculomotor nerve exits the mesencephalon. Although we have postulated that dural compression forces the subarachnoid fluid cephalad, other factors such as diffusion and pulsatile oscillations may contribute to the movement of local anesthetic-containing subarachnoid fluid into the skull. CSF = cerebrospinal fluid within the ventricular system; DS = dural sac; ES = epidural space; SC = spinal cord; SF = subarachnoid fluid.