Single-shot caudal epidural anesthesia via the sacral hiatus — the workhorse regional technique in children under general anesthesia
Caudal epidural block is the single most commonly performed regional anesthetic technique in children, precisely because the anatomy is so forgiving in this age group. The sacrum in infants and toddlers is flatter, less ossified, and covered by less subcutaneous fat than in adults, making the sacral hiatus a reliably palpable landmark — a triangle bounded by the two sacral cornua and the apex of the gluteal cleft.
The sacrum forms from the fusion of five sacral vertebrae. Failure of the laminae of S4 and (usually) S5 to fuse posteriorly leaves a natural bony gap — the sacral hiatus — at the caudal end of the sacral canal. This gap is sealed by the sacrococcygeal ligament, a fibrous membrane that is the direct caudal continuation of the ligamentum flavum.
The hiatus is flanked laterally by two bony prominences, the sacral cornua (remnants of the inferior articular processes of S5). In infants and children these three points — the two cornua and the apex of the natal cleft — form a palpable equilateral triangle that reliably marks the injection site, even before any imaging landmark is used.
An easy surface check: the sacral hiatus lies roughly at the level of a line joining the two posterior superior iliac spines (PSIS) extended caudally, and is typically 1–2 cm above the tip of the coccyx.
Two developmental facts drive both the ease and the risk of caudal block in small children:
• The dural sac (and the subarachnoid space containing CSF) terminates much lower in neonates and infants — around S3–S4 — compared with S1–S2 in adults. This means the distance between the sacral hiatus and the dura is short, so unintended deep needle advancement carries a real risk of dural puncture.
• The conus medullaris (the tapering end of the spinal cord itself) sits at approximately L3 at birth and ascends to the adult level of L1 only by around 1 year of age as the vertebral column outgrows the cord. This is why neuraxial punctures in neonates are always performed well below L3, and why the caudal route — entering far below the cord — is inherently safer for spinal-cord injury than lumbar approaches in this age group.
Both structures ascend relative to the vertebral column with growth, which is exactly why caudal landmarks become progressively less forgiving, and eventually less used, after roughly 6–8 years of age.
Because the dural sac can end as low as S3–S4 in a neonate, the safe zone for needle advancement past the "pop" is only a few millimeters — advancing the needle even 1–2 cm beyond the ligament click risks dural puncture in a small infant.
Caudal block is almost always placed as an adjunct to general anesthesia (children are anesthetized first, then the block is performed), providing intraoperative anesthetic sparing and several hours of postoperative analgesia without systemic opioids. Typical indications include:
• Circumcision and other penile/perineal procedures • Hypospadias repair • Inguinal herniorrhaphy and orchidopexy • Anorectal surgery (e.g. anoplasty for anorectal malformation) • Lower-limb orthopedic surgery (e.g. clubfoot repair, below-knee procedures) • Selected lower-abdominal procedures when extended (thoracolumbar) spread is achieved
Caudal block reduces volatile anesthetic requirements intraoperatively, blunts the stress response to surgical stimulation, and is associated with lower rates of emergence agitation and reduced postoperative opioid consumption compared with general anesthesia alone.
Caudal block is virtually always performed after induction of general anesthesia in children, since an awake or lightly sedated child cannot cooperate with positioning or tolerate needle placement. Careful positioning and a deliberate, low-force insertion technique are what keep this "blind" landmark-based approach both fast and safe.
The child is positioned lateral decubitus with the hips and knees flexed toward the chest ("fetal" position), or occasionally prone with a roll under the pelvis. Lateral positioning is preferred in most pediatric centers because it preserves airway access during a technique performed under general anesthesia with a supraglottic airway or endotracheal tube in place.
The sacral triangle (two cornua + natal cleft apex) is identified by palpation, the skin is prepped with antiseptic solution using full aseptic technique (cap, mask, sterile gloves — this is a neuraxial procedure), and sterile drapes are applied.
1. The needle (a short, 22–23G short-bevel needle, or in many centers a standard IV cannula used as an introducer for the "single-shot" caudal technique) is inserted through the skin at the apex of the sacral triangle at approximately 45° to the skin surface, directed cephalad.
2. The needle is advanced slowly until a change in resistance — the ligamentous "pop" — is felt as it pierces the sacrococcygeal ligament (see Stage 3).
3. Immediately after the pop, the needle angle is flattened to about 20–30° (nearly parallel to the skin/sacrum) to align with the sacral canal, preventing the needle from angling upward into the dural sac.
4. The needle is advanced only a further 2–4 mm into the sacral canal — enough to ensure the needle tip and bevel are fully within the epidural space, but not so far as to approach the dural sac, which may end as low as S3 in an infant.
Because caudal block is traditionally a blind, landmark-guided technique, several adjuncts are increasingly used to confirm correct placement before the full dose is given:
• Ultrasound guidance: a linear probe over the sacral hiatus can directly visualize the ligament, needle tip, and the anterior "bulge" of the dura as fluid is injected — increasingly considered best practice, especially in trainees or anatomically atypical patients • Nerve stimulation (rarely used) or ECG-guided caudal catheter techniques for extended catheter placement • "Whoosh test": auscultation with a stethoscope over the lumbar spine while injecting a small bolus of air or saline — a whooshing sound confirms epidural placement (largely superseded by ultrasound) • Loss-of-resistance to saline/air using a glass or low-resistance syringe, analogous to lumbar epidural technique
The tactile "pop" as the needle crosses the sacrococcygeal ligament is the defining sensory endpoint of caudal block — the moment the operator commits to flattening the needle and entering the sacral epidural space. Recognizing it correctly, and stopping appropriately afterward, is the single technical step that separates a smooth block from a complication.
The sacrococcygeal ligament is a taut fibrous sheet. As the advancing needle tip loads it under tension, resistance rises progressively; once the ligament is punctured, resistance drops abruptly as the tip enters the fat- and venous-plexus-filled sacral epidural space. This is felt as a distinct, often audible "pop" or "click" — similar in character (though usually crisper) to the loss-of-resistance sensation felt when a lumbar epidural needle crosses the ligamentum flavum, because the two ligaments are histologically continuous.
Immediately after the pop, the needle should not be advanced further at the original 45° angle — doing so risks driving the tip anteriorly into the ventral sacral canal wall, the presacral venous plexus, or (in small infants) toward the low-lying dural sac or even the rectum anteriorly.
A convincing pop alone is not proof of correct placement — several confirmatory signs are sought together:
• Free, easy advancement of the needle a further 2–4 mm without bony resistance • Absence of subcutaneous swelling when a small test injection is given (swelling suggests the needle tip is still superficial/subcutaneous, not truly in the epidural space) • Free flow on injection without high resistance (high resistance suggests the needle is still within or against the ligament, in bone, or subperiosteal) • On ultrasound, visible anterior displacement of the dura/fluid spread within the sacral canal during a test injection
A "false pop" can occur if the needle strikes and slides off the sacral periosteum lateral to the true hiatus, mimicking the ligament click. This is the most common cause of a failed block despite a felt "pop," and is why post-pop confirmatory signs (free advancement, no subcutaneous swelling, free injection) are always sought together — not the pop in isolation.
In neonates and young infants, the sacrococcygeal ligament is thinner and less calcified, so the pop can be subtler; the sacral canal itself is also proportionally shorter, so the margin for error between "just past the ligament" and "too deep" is narrower in absolute millimeters. Sacral canal length (hiatus to S1) is roughly 20–30 mm in a neonate versus 40–50 mm in an older child, reinforcing why the post-pop advance is deliberately limited to only a few millimeters across the entire pediatric age range.
The sacral canal is richly supplied by the epidural venous plexus, and — especially in small infants — sits close to the low-lying dural sac. Before a single milliliter of local anesthetic is injected, aspiration and a test dose are used to rule out that the needle tip has entered a vein or the subarachnoid/subdural space, since either scenario converts a routine block into a life-threatening event.
Before injecting, the syringe is gently aspirated (never with high negative pressure, which can collapse a small vein around the needle tip and produce a false-negative). Two findings must be actively excluded:
• Blood — indicates the needle tip lies within the epidural venous plexus, a network of thin-walled veins that runs along the sacral canal and is a well-recognized site of accidental cannulation, especially with slightly-too-deep or off-midline needle placement
• Clear fluid (CSF) — indicates dural puncture and potential intrathecal placement; this is more likely in small infants because the dural sac can end as low as S3–S4
A negative aspiration reduces but does not eliminate risk: aspiration alone has a meaningful false-negative rate, particularly for intravascular placement, because a vein wall can appose the needle bevel.
Because aspiration alone is unreliable, a test dose containing epinephrine 0.5 mcg/kg (typically as part of a 1:200,000 epinephrine-containing local anesthetic solution) is injected first, with continuous ECG and pulse-oximeter monitoring:
• A positive response — heart rate increase of more than 10 beats per minute, or T-wave amplitude changes on ECG, occurring within roughly 90 seconds — suggests intravascular injection, and the needle should be repositioned before proceeding
• Under volatile anesthesia (particularly sevoflurane and halothane), the test dose is less reliable because these agents blunt the sympathetic/chronotropic response to epinephrine — a recognized limitation that means a "negative" test dose does not fully exclude intravascular placement
Because no single sign is perfectly sensitive, the modern approach layers aspiration, an epinephrine test dose, incremental injection, and (increasingly) real-time ultrasound visualization of fluid spread.
Regardless of a negative aspiration and test dose, the full calculated volume of local anesthetic is never given as a single rapid bolus. Best practice is:
• Inject in increments (e.g., 3–5 mL at a time in larger children, proportionally less in infants) • Re-aspirate between increments • Inject slowly overall, typically across 60–90 seconds for the full dose • Watch continuously for hemodynamic changes (heart rate, blood pressure, arrhythmia on the ECG trace) and for any unexpected rise in injection resistance or subcutaneous swelling
This incremental strategy is the single most effective way to limit the dose delivered before an unrecognized intravascular or intrathecal injection is detected — turning a potential large-bolus catastrophe into a small, manageable exposure.
Layered safety — aspiration + epinephrine test dose + slow incremental injection with continuous monitoring — is what keeps the historically higher complication rate of caudal block (relative to peripheral nerve blocks) clinically low in modern practice.
Unlike most peripheral nerve blocks, the level of sensory block achieved by a single-shot caudal injection is governed primarily by the volume of local anesthetic instilled, not the concentration. The Armitage formula (1979) remains the standard bedside method for choosing an injectate volume matched to the required dermatomal spread, while total dose (mg/kg) is kept within recognized safety limits by adjusting concentration.
Armitage (1979) proposed that the injected volume, scaled to body weight, predicts the cephalad extent of sensory block far more reliably than in adult epidural dosing, because the pediatric epidural space is loosely packed with fat and has minimal segmental resistance to spread:
• 0.5 mL/kg → lumbosacral spread (roughly S2–L1): sufficient for circumcision, anal/perineal, and lower-limb procedures • 1.0 mL/kg → thoracolumbar spread (to around T10, the umbilical dermatome): needed for inguinal hernia repair and orchidopexy • 1.25 mL/kg → mid-thoracic spread (to around T6–T7): needed for upper-abdominal procedures such as pyloromyotomy
Most centers cap the total single-shot volume at approximately 20 mL regardless of the formula result, and many avoid caudal single-shot dosing altogether above roughly 20–25 kg body weight, switching instead to a lumbar or thoracic epidural/catheter technique if extended cephalad spread is needed.
Because Armitage volumes scale up sharply for higher spread levels, concentration must scale down to keep the total milligram dose under the systemic toxicity threshold. A commonly used pairing in practice:
• 0.5 mL/kg (sacral/lumbosacral) → bupivacaine/levobupivacaine 0.25% (2.5 mg/mL) or ropivacaine 0.2% • 1.0 mL/kg (thoracolumbar) → bupivacaine/levobupivacaine 0.2% (2 mg/mL) • 1.25 mL/kg (mid-thoracic) → bupivacaine/levobupivacaine 0.125% (1.25 mg/mL)
With this scaling, total dose stays close to 1.25–2 mg/kg across all three spread levels — safely under the accepted single-shot ceiling of ~2.5 mg/kg for bupivacaine/levobupivacaine (ropivacaine has a wider therapeutic margin and is often dosed similarly for equivalent efficacy with a theoretically lower cardiotoxic risk).
Volume determines how far the block spreads; concentration (together with total volume) determines the total milligram dose and thus systemic toxicity risk. Choosing a higher Armitage volume for extended spread always requires diluting the local anesthetic to stay under the mg/kg ceiling — volume and concentration are adjusted together, never volume alone.
Sensory and motor block onset after caudal injection is typically apparent within 5–10 minutes, with surgical anesthesia established by 15–20 minutes — well timed against the induction and positioning period of a general anesthetic.
Plain single-shot bupivacaine/ropivacaine caudal blocks provide postoperative analgesia for roughly 4–8 hours. Additives can meaningfully extend this without increasing local anesthetic dose:
• Clonidine 1–2 mcg/kg — extends analgesia to 8–12+ hours, modest sedation, no respiratory depression • Preservative-free ketamine (S-ketamine) 0.5 mg/kg — historically used but now less favored given neurotoxicity concerns in some preclinical data • Dexmedetomidine 0.5–1 mcg/kg — increasingly preferred, extends analgesia with a good safety profile
Opioids (e.g. caudal morphine) are largely avoided for routine day-case caudal blocks due to the risk of delayed respiratory depression, reserved instead for major surgery with planned postoperative monitoring.
| Product | Indication | Trial Design | Key Result |
|---|---|---|---|
| 0.5 mL/kg | Lumbosacral (S2–L1) | Bupivacaine/levobupivacaine 0.25% or ropivacaine 0.2% | Circumcision, anal/perineal & lower-limb surgery |
| 1.0 mL/kg | Thoracolumbar (T10) | Bupivacaine/levobupivacaine 0.2% | Inguinal hernia repair, orchidopexy, hypospadias |
| 1.25 mL/kg | Mid-thoracic (T6–T7) | Bupivacaine/levobupivacaine 0.125% | Upper-abdominal surgery (e.g. pyloromyotomy) |
| Total volume cap | ~20 mL (all levels) | Applied regardless of formula result above this weight-derived value | Limits absolute mg dose in larger children |
Caudal block has an excellent overall safety record, but pediatric regional anesthesia registries consistently show it carries a higher complication rate than peripheral nerve blocks — precisely because it is a neuraxial technique performed blindly through richly vascular, anatomically compact tissue. Recognizing and preventing its specific complications is essential to safe practice.
The most feared complication is unrecognized intravascular injection leading to LAST — CNS signs (perioral tingling, tinnitus, agitation, seizures) followed by cardiovascular signs (arrhythmia, conduction block, hypotension, cardiac arrest). Under general anesthesia, early CNS warning signs are masked, so cardiovascular signs (dysrhythmia on the ECG trace, sudden hypotension) are often the first — and only — clue.
Management follows ASRA (American Society of Regional Anesthesia) LAST guidelines: • Stop injecting immediately, call for help, secure the airway with 100% oxygen • Treat seizures with benzodiazepines (avoid propofol if any hemodynamic instability) • Give 20% lipid emulsion: 1.5 mL/kg bolus over 1 minute, followed by an infusion of 0.25 mL/kg/min, with bolus repeatable if instability persists • Modified CPR (smaller epinephrine boluses, avoid vasopressin) if cardiac arrest occurs, prolonged resuscitation efforts as lipid emulsion can restore cardiac function even after delay
Prevention (aspiration, epinephrine test dose, incremental slow injection, weight-based dose ceilings) is far more effective than any treatment, which is why every step in Stages 3–5 exists specifically to reduce this risk.
LAST from caudal block is rare in absolute terms but is the complication with the highest potential severity — this is precisely why the technique layers multiple independent safety checks (aspiration, test dose, incremental injection, dose ceiling) rather than relying on any single one.
Because the dural sac can end as low as S3–S4 in infants, advancing the needle too far past the ligamentous pop can puncture the dura. If unrecognized and the full epidural-intended volume is injected intrathecally, the result is a total spinal block: rapid onset of dense motor block, hypotension, bradycardia, and potentially respiratory arrest requiring immediate airway support and hemodynamic resuscitation.
Recognition relies on: aspirating and seeing clear fluid (send for glucose/temperature confirmation if uncertain), an unexpectedly rapid or unusually dense block onset, and — if ultrasound is used — direct visualization of the needle tip relative to the dura. Limiting the post-pop advance to 2–4 mm and using ultrasound guidance in higher-risk patients (small infants, prior sacral surgery) are the principal preventive measures.
• Subcutaneous or intraosseous injection — from a needle tip that never truly entered the epidural space; produces local swelling and a failed block, but carries little systemic risk
• Infection/epidural abscess — rare with single-shot technique and strict asepsis, but caudal injection sites are close to the perianal region, so meticulous skin prep and sterile technique are mandatory, and risk rises with indwelling caudal catheters left for extended postoperative analgesia
• Rectal perforation — a rare but serious historical complication from a needle directed too far anteriorly and too deep, more often reported in neonates; avoided by respecting the flattened post-pop trajectory and limited depth of advance
• Urinary retention and transient lower-limb motor block/weakness — expected pharmacologic effects of local anesthetic on sacral nerve roots, not true complications, but should be anticipated and explained to families as they can delay same-day discharge
• Delayed detection of compartment syndrome — dense sensory block from caudal anesthesia can mask early pain signals after certain orthopedic procedures (e.g. below-knee casting), requiring specific vigilance and parent education for warning signs beyond typical postoperative pain