A 23-year-old primigravida woman has a vaginal delivery of a 3600-g (7 lb 15 oz) infant after a protracted labor. The patient has had no pain medications or epidural analgesia during delivery. She sustains a second-degree perineal laceration, and repair is planned. Prior to administering local anesthesia, the physician palpates for distinct bony protrusions located on the posterolateral vaginal sidewalls; a firm band can be felt running medially and posteriorly from each of the bilateral bony protrusions. The anesthetic agent is injected around the bony prominence. Which of the following nerves is most likely blocked by this injection?

Question

A 23-year-old primigravida woman has a vaginal delivery of a 3600-g (7 lb 15 oz) infant after a protracted labor.  The patient has had no pain medications or epidural analgesia during delivery.  She sustains a second-degree perineal laceration, and repair is planned.  Prior to administering local anesthesia, the physician palpates for distinct bony protrusions located on the posterolateral vaginal sidewalls; a firm band can be felt running medially and posteriorly from each of the bilateral bony protrusions.  The anesthetic agent is injected around the bony prominence.  Which of the following nerves is most likely blocked by this injection?

Quizplus · Accepted Answer

11ec160b_3573_42a2_9249_f1b58aaaf05c_MD0001_00 Local anesthesia may be administered during labor, delivery, and obstetric perineal laceration repair in patients who decline neuraxial anesthesia or who have progressed too far in labor to place an epidural safely. Pudendal nerve blocks are performed because the pudendal nerve provides sensory innervation to the perineum and external genitalia. The pudendal nerve arises from the S2-S4 nerve roots and also provides motor innervation to the external anal and urethral sphincters. Important landmarks palpable on vaginal examination include the ischial spine (a bony protrusion located posterolateral to the vaginal sidewall) and sacrospinous ligament (a firm band running medially and posteriorly from the ischial spine to the sacrum). When administering a pudendal nerve block, the physician palpates the ischial spine on vaginal examination and injects the anesthetic transvaginally and immediately medial to the ischial spine through the sacrospinous ligament. The internal pudendal artery and inferior gluteal artery are directly lateral to the pudendal nerve; inadvertent injection into these vessels can lead to hematoma or arrhythmia from intravascular infiltration of the local anesthetic (eg, lidocaine). (Choice A) The genitofemoral nerve courses along the psoas muscle, and its branches innervate the labia majora and medial thigh. It is at risk for injury by abdominal retractors during laparotomy but is not accessible transvaginally. (Choice B) The iliohypogastric nerve is vulnerable to injury where it pierces the anterior abdominal musculature. Damage (eg, during cesarean delivery) can result in decreased sensation, burning pain, and paresthesia of the suprapubic skin. (Choices C and E) The inferior gluteal nerve innervates the gluteus maximus and can be injured during hip surgery, leading to impaired hip extension. The posterior femoral cutaneous nerve provides sensory innervation to the posterior thigh, buttock, and posterior labia. Neither nerve is accessible transvaginally, and blockade would not provide adequate anesthesia to the perineum and external genitalia. (Choice D) The obturator nerve provides motor and sensory innervation to the medial thigh. Injury during pelvic surgery (eg, retroperitoneal pelvic lymph node dissection) can cause weakness of hip adduction and decreased sensation of the medial thigh. Educational objective: A pudendal nerve block provides anesthesia to most of the perineum and external genitalia. It is performed by transvaginal injection of a local anesthetic medial to the ischial spine and through the sacrospinous ligament. __________ References: Pudendal nerve block. (http://www.ncbi.nlm.nih.gov/pubmed/31855362)

A 23-Year-Old Primigravida Woman Has a Vaginal Delivery of a 3600-G