Innervation of the Uterus and Cervix, Pain Pathways in Labour, and Labour Epidural Analgesia

1. Innervation of the Uterus and Cervix

Nerve Supply — Overview

Sympathetic Innervation

• Origin: Thoracolumbar sympathetic outflow (T10–L1)
• Pathway:
  • Preganglionic fibres originate from the lateral horn of the spinal cord at T10–L1
  • They pass through the sympathetic chain and splanchnic nerves
  • Synapse in the inferior mesenteric ganglion (and pelvic ganglia)
  • Postganglionic fibres travel via the hypogastric nerves → superior hypogastric plexus → inferior hypogastric (pelvic) plexus → Lee-Frankenhaüser plexus (uterovaginal plexus) at the base of the broad ligament
• Function: Motor innervation to uterine smooth muscle; mediates afferent pain signals from the uterine body and cervix

Parasympathetic Innervation

• Origin: Sacral parasympathetic outflow (S2–S4) via the pelvic splanchnic nerves (nervi erigentes)
• Pathway: Pelvic splanchnic nerves → inferior hypogastric plexus → uterovaginal plexus
• Function: Vasodilation, inhibition of uterine contraction in some contexts

Somatic Innervation (Perineum/Lower Vagina)

• The pudendal nerve (S2–S4) supplies the perineum, lower vagina, and vulva
• Relevant in the second stage of labour

Frankenhaüser's (Uterovaginal) Plexus

• Located lateral to the cervix, within the cardinal (Mackenrodt's) ligament
• Contains both sympathetic and parasympathetic fibres
• Acts as the main relay for uterine and cervical sensory signals

2. Pathway of Pain in Labour

First Stage of Labour (Cervical Dilation — T10 to L1)

"In the first stage of labor, pain is caused by uterine contractions, associated with dilation of the cervix and stretching of the lower uterine segment. Pain impulses are carried in visceral afferent type C fibres which accompany the sympathetic nerves. During the first stage of labor, pain is referred to the T10 to L1 spinal cord segments." — Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e, p. 3485

1. Stimulus: Uterine contractions → myometrial ischaemia + cervical dilation + stretching of lower uterine segment
2. Receptor: Free nerve endings in the myometrium and cervix
3. Afferent fibre: Unmyelinated visceral C fibres (slow pain) → travel with sympathetic nerves
4. Plexus: Uterovaginal plexus → inferior hypogastric plexus → hypogastric nerve → superior hypogastric plexus
5. Entry to cord: Via lumbar splanchnic nerves → sympathetic chain → dorsal horn of T10–L1 spinal segments
6. Referred pain: To lower abdomen and back (dermatomes T10–L1) — the "girdle" pattern of labour pain

Late First and Second Stage (Perineal Distension — S2 to S4)

"In the late first and second stages of labor, additional pain impulses from distention of the vaginal vault and perineum are carried by the pudendal nerves, composed of sacral fibres (S2 to S4)." — Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e, p. 3485

1. Stimulus: Descent of presenting part → distension of vaginal vault, perineum, pelvic floor
2. Afferent fibre: Somatic A-delta and C fibres in the pudendal nerve (S2–S4)
3. Entry to cord: Via dorsal roots of S2–S4
4. Referred pain: Perineum, inner thighs, buttocks

Summary Table

3. Method of Providing Labour Epidural Analgesia

Rationale

Timing

• Can be initiated at any cervical dilation if requested — there is no minimum dilation requirement
• Initiated once active labour is established or earlier if severe pain warrants it

Pre-procedure Preparation

• Informed consent obtained
• IV access established (16–18G cannula); IV fluid preload or co-load (500–1000 mL crystalloid)
• Monitoring: BP, SpO₂, FHR
• Resuscitation equipment checked

Patient Position

• Sitting (preferred for obese patients, better identification of midline) or lateral decubitus (left lateral)
• Flexed spine ("shrimp position") to open interspinous spaces

Epidural Catheter Placement — Technique

1. Identify the level: L2–L3 or L3–L4 interspace (using Tuffier's line — iliac crest = L4/L4–L5)
2. Skin preparation: Strict aseptic technique — chlorhexidine-based antiseptic
3. Local infiltration: 2–3 mL of 1% lidocaine at the chosen interspace
4. Epidural needle insertion: Tuohy needle (16G or 18G), bevel facing cephalad; advance through:
   • Skin → subcutaneous fat → supraspinous ligament → interspinous ligament → ligamentum flavum → epidural space
5. Loss of resistance (LOR): Using air or saline — resistance disappears as needle enters the epidural space
6. Depth: Typically 3.5–5.5 cm from skin to epidural space
7. Catheter insertion: Thread epidural catheter 3–5 cm into the epidural space (not more, to avoid unilateral or vascular placement)
8. Needle withdrawn, catheter secured with tape
9. Test dose: 3 mL of 1.5% lidocaine with 1:200,000 epinephrine (15 μg epinephrine) to exclude:
   • Intravascular placement (tachycardia >20 bpm within 30–60 sec = positive)
   • Intrathecal placement (rapid dense block within 2–3 min = positive)

Initiation of Analgesia

• Loading dose: 10–15 mL of dilute local anaesthetic-opioid mixture:
  • Bupivacaine 0.0625–0.1% + fentanyl 2 μg/mL (or sufentanil 0.5 μg/mL)
• Onset: 10–20 minutes
• Assess dermatomal level — target T10 bilaterally

Maintenance Options

Combined Spinal–Epidural (CSE) Technique

• Needle-through-needle: After identifying epidural space with Tuohy needle, a 127 mm pencil-point spinal needle is passed through it into the subarachnoid space
• Intrathecal injection: Fentanyl 10–25 μg ± bupivacaine 1.25–2.5 mg
• Onset: 2–5 minutes, duration 90–120 min with minimal motor block
• Spinal needle removed → epidural catheter inserted → maintained as above
• Advantage: rapid profound analgesia + flexibility of epidural for prolonged labour

Second Stage / Instrumental Delivery

• Sacral nerve roots (S2–S4) require more concentrated local anaesthetic
• 10 mL of bupivacaine 0.25–0.5% or lidocaine 1% or 2-chloroprocaine 2–3% via epidural catheter

Monitoring After Placement

• BP every 5 min for 20 min (risk of sympatholysis and hypotension)
• FHR monitoring (watch for transient non-reassuring patterns, especially after CSE)
• Assess motor block — Bromage scale
• Assess dermatomal spread every 30 min

Complications

• Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e, Chapter 41: Obstetric Anesthesia, pp. 3485–3491
• Miller's Anesthesia, 2-Volume Set, 10e