Pudendal, Lumbar Paravertebral Sympathetic, and Paracervical Nerve Blocks

The ventral rami of S2–S4 form the pudendal nerve and the pelvic splanchnic nerves. The pudendal nerve exits the pelvis via the greater sciatic foramen (i.e., below the piriformis, medial to the pudendal artery, and lateral to the ischial spine) and then enters the perineum via the lesser sciatic foramen and Alcock’s canal. Because of the pudendal nerve’s motor capacity (pelvic and perineal muscles), blockade can theoretically prolong the second stage of labor. An older trial concluded that pudendal nerve block only slightly prolonged the second stage. The paracervical nerve is a synapse point immediately posterolateral to the cervicouterine junction, comprised of visceral afferent fibers from the cervix, lower uterine segment, and upper vagina that join the lumbar sympathetic chain at L2–L3 ascend and then enter the spinal cord at T10-L1. Because these fibers are purely sensory visceral, it is specifically for analgesia during the first stage of labor (i.e., cervical dilation).