Residency · Residency · Chronic Pain Management
Caudal Epidural Injections
Introduction
The caudal epidural injection provides access to the lumbar and sacral epidural space through the sacral hiatus, a natural bony deficiency at the distal end of the sacrum. This approach avoids traversing the ligamentum flavum and the risks associated with interlaminar techniques at lumbar levels, making it technically straightforward and broadly applicable. It is particularly useful in patients with prior lumbar surgery, severe spinal stenosis, or anatomy that precludes interlaminar or transforaminal approaches. Caudal epidural injections deliver corticosteroids, local anesthetics, and other agents for the treatment of lumbosacral radiculopathy, spinal stenosis, post-laminectomy syndrome, and sacral nerve root pathology.
Sacral Hiatus Anatomy
Bony Landmarks
The sacral hiatus is formed by the failure of fusion of the S5 (and sometimes S4) posterior laminae, creating an inverted U-shaped or V-shaped opening in the dorsal sacral wall. It is bounded laterally by the sacral cornua, two bony prominences that represent the inferior articular processes of S5. The hiatus is covered posteriorly by the sacrococcygeal ligament, which is an extension of the ligamentum flavum. The hiatus typically measures 10-20 mm in length and 5-15 mm in width, though significant anatomic variation exists.
Contents of the Sacral Canal
The dural sac (thecal sac) typically terminates at the S2 level (range S1-S3) in adults. Below the dural sac, the sacral canal contains the sacral and coccygeal nerve roots (filum terminale), epidural fat, and the sacral epidural venous plexus. The anterior wall of the sacral canal contains the sacral nerve root foramina.
Anatomic Variations
The sacral hiatus is absent in approximately 5-8% of individuals due to complete posterior sacral fusion. In approximately 1-2%, the hiatus extends to S3 or higher, increasing the risk of dural puncture. Bony spurs, calcification of the sacrococcygeal ligament, and narrow canal diameter can make cannulation difficult. Obesity significantly increases the difficulty of palpating the sacral cornua.
<image>Posterior view anatomic illustration of the sacrum showing the sacral hiatus with labeled sacral cornua, sacrococcygeal ligament, and the inverted-U shape of the hiatus opening, with a cross-sectional inset at the S4-S5 level showing the sacral canal contents including epidural fat, sacral nerve roots, filum terminale, and epidural veins, and dotted lines indicating the typical dural sac termination at S2</image>
Technique
Patient Positioning
The patient is placed prone on the fluoroscopy table with a pillow under the pelvis to reduce lumbosacral lordosis. The legs are slightly abducted with toes turned inward to relax the gluteal muscles. A lateral decubitus position is an alternative for patients who cannot tolerate prone positioning.
Landmark Identification
The sacral cornua are palpated as two bony prominences at the inferior sacrum, approximately at the level of the natal cleft. The sacral hiatus is located in the midline between and slightly cephalad to the cornua. Fluoroscopic confirmation on a lateral view is essential: the sacral hiatus is identified as the bony defect at the distal dorsal sacrum, with the sacrococcygeal junction serving as a reference.
Needle Insertion
Using sterile technique, a 22-gauge spinal needle or 18-gauge Tuohy needle is directed at a 45-degree angle to the skin, aiming toward the sacral canal. The needle penetrates the sacrococcygeal ligament with a characteristic "pop" or loss of resistance. Once through the ligament, the needle angle is reduced to approximately 15-30 degrees and the needle is advanced 2-3 cm into the sacral canal to position the tip at approximately the S3 level. The needle should not be advanced above the S3 level to avoid dural puncture, since the dural sac terminates nearby.
Fluoroscopic Confirmation
On lateral view, the needle tip should be within the sacral canal, posterior to the anterior sacral wall, and below the S2-S3 level. On AP view, the needle should be in the midline. Three to five milliliters of contrast is injected under live fluoroscopy to confirm epidural spread. The expected pattern is linear, cephalad spread within the sacral and lower lumbar epidural space, often filling sacral nerve root sleeves. Intravascular, intrathecal, and subcutaneous contrast spread must all be ruled out before proceeding.
Injection
After confirming epidural contrast spread, the therapeutic solution is injected. A typical injectate consists of corticosteroid (triamcinolone 40-80 mg or dexamethasone 8-10 mg) with local anesthetic (0.25% bupivacaine or 1% lidocaine). Total volume is usually 10-20 mL, with higher volumes used to promote cephalad spread to the lumbar epidural space. Larger volumes may be necessary to reach L4-L5 or L5-S1 from the caudal approach.
Indications
Lumbosacral Radiculopathy
Disc herniation at L5-S1 and S1 radiculopathy are particularly amenable to caudal epidural injection due to proximity. The caudal approach is useful when transforaminal access is technically difficult, as in cases of severe foraminal stenosis or post-surgical hardware. It may be less target-specific than a transforaminal injection but provides broader coverage of the lower lumbosacral nerve roots.
Spinal Stenosis
Central and lateral recess stenosis at L4-L5 and L5-S1 can be addressed with caudal epidural steroid delivery. Higher-volume injections (15-20 mL) promote spread to the stenotic levels. This approach is particularly useful in patients with multilevel stenosis where targeted injection at a single level would be insufficient.
Post-Laminectomy Syndrome (Failed Back Surgery Syndrome)
Epidural fibrosis and altered anatomy after lumbar surgery may preclude safe interlaminar or transforaminal access. The caudal approach enters below the surgical site, reducing the risk of needle misplacement into surgical defects. Epidural adhesiolysis (the Racz procedure) can be performed via the caudal route, with catheter advancement through scar tissue.
Other Indications
Additional indications include sacral nerve root pathology (S2-S4 radiculopathy), coccydynia (with ganglia impar block performed via the same access), and use as a diagnostic tool for differentiating pain generators in the lumbosacral spine.
<image>Lateral fluoroscopic view illustration showing correct caudal epidural needle placement through the sacral hiatus into the sacral canal, with the needle tip at the S3 level, contrast spread visible as a linear column extending cephalad through the sacral canal to the lower lumbar epidural space with bilateral S1 and S2 nerve root sleeve filling, and annotations showing the dural sac termination at S2, the sacral canal boundaries, and the angle of needle insertion through the sacrococcygeal ligament</image>
Comparison with Other Epidural Approaches
Caudal vs. Interlaminar
The caudal approach is technically easier than the interlaminar approach and carries a very low risk of dural puncture (provided the needle is kept below S3), compared to a 0.5-2% dural puncture rate with the interlaminar technique. However, the caudal approach has lower target specificity, providing broad coverage rather than segmental precision. It requires higher volumes (10-20 mL versus 5-10 mL for interlaminar) to achieve adequate spread. Its major advantage is in post-surgical anatomy, where it is the safer approach because it enters below the level of surgical alteration.
| Feature | Caudal | Interlaminar | Transforaminal |
|---|---|---|---|
| Target specificity | Low (broad coverage) | Moderate (segmental) | High (single nerve root) |
| Volume required | 10–20 mL | 5–10 mL | 1–3 mL |
| Dural puncture risk | Very low (if below S3) | 0.5–2% | Low |
| Vascular injection risk | Low | Low | 8–23% |
| Catastrophic complication risk | Very low | Low | Spinal cord infarction possible |
| Post-surgical anatomy | Excellent (enters below surgical site) | May be limited by scar | May be limited by hardware/fibrosis |
| Ventral epidural spread | Variable | Variable | Reliable |
Caudal vs. Transforaminal
The transforaminal approach offers high nerve root selectivity and reliable ventral epidural spread, whereas the caudal approach has low selectivity and variable ventral spread. The caudal approach has a low vascular injection risk compared to the 8-23% rate seen with transforaminal injections, and its catastrophic complication risk is very low compared to the transforaminal risk of spinal cord infarction. The caudal approach is excellent for post-surgical cases, while the transforaminal approach may be limited by hardware or fibrosis.
Complications
The most common technical failure is subcutaneous injection, where the needle tip passes anterior to the sacrococcygeal ligament but remains outside the sacral canal. Intravascular injection is possible because the epidural venous plexus is prominent in the sacral canal -- this is why contrast must always be injected first. Dural puncture risk is very low if the needle is kept below S3 but may occur with high sacral hiatus variants or excessive needle advancement. Infection (sacral epidural abscess or meningitis) is rare with aseptic technique. Rectal perforation, caused by needle advancement through the anterior sacral wall, is extremely rare. Transient neurologic symptoms such as lower extremity weakness or numbness from local anesthetic spread typically resolve within hours.
<image>Comparative anatomic diagram showing sagittal views of the lumbar spine with three epidural injection approaches side by side: (1) caudal approach through the sacral hiatus with needle and contrast spread pattern, (2) interlaminar approach at L4-L5 with loss-of-resistance needle and contrast pattern, and (3) transforaminal approach at L5-S1 with needle in the neural foramen and contrast outlining the nerve root, each labeled with key advantages and limitations</image>
Clinical Pearls
The sacral cornua are the most reliable surface landmarks for identifying the sacral hiatus, but lateral fluoroscopy should always be used to confirm position before advancing the needle. The needle must never be advanced above the S3 level to avoid dural puncture, and lateral fluoroscopy should be used to monitor needle depth in real time. Higher injectate volumes (15-20 mL) are needed to achieve adequate cephalad spread to the L4-L5 level from the caudal approach, and insufficient volume is a common cause of treatment failure. The caudal approach is the safest epidural access route in patients with prior lumbar surgery, as it enters below the level of surgical alteration. Contrast should always be injected under live fluoroscopy to rule out intravascular, intrathecal, or subcutaneous placement before delivering the therapeutic injectate.
References
- Manchikanti L, Cash KA, McManus CD, Pampati V, Singh V. A preliminary report of a randomized double-blind, active-controlled trial of fluoroscopic lumbar interlaminar epidural injections in managing chronic lumbar discogenic pain without disc herniation or radiculitis. Pain Physician. 2010;13(4):E279-E292.
- Conn A, Buenaventura RM, Datta S, Abdi S, Diwan S. Systematic review of caudal epidural injections in the management of chronic low back pain. Pain Physician. 2009;12(1):109-135.
- Stitz MY, Sommer HM. Accuracy of blind versus fluoroscopically guided caudal epidural injection. Spine. 1999;24(13):1371-1376.
- Manchikanti L, Singh V, Cash KA, Pampati V, Datta S. Management of pain of post lumbar surgery syndrome: one-year results of a randomized, double-blind, active controlled trial of fluoroscopic caudal epidural injections. Pain Physician. 2010;13(6):509-521.
