# Epidural Steroid Injections: Interlaminar Approach

## Introduction

The interlaminar epidural steroid injection (ILESI) is one of the most commonly performed interventional pain procedures. It delivers corticosteroid and local anesthetic to the epidural space via a posterior midline or paramedian approach between adjacent vertebral laminae. The technique depends on identifying the ligamentum flavum and using the loss-of-resistance (LOR) method to confirm entry into the epidural space. ILESI is performed at cervical, thoracic, and lumbar levels for the treatment of radicular pain, spinal stenosis, and disc herniation, and understanding the regional anatomic variations, contrast spread patterns, and complication profiles at each spinal level is essential for safe and effective practice.

## Relevant Anatomy

### Posterior Spinal Anatomy

The interlaminar window is the space between adjacent laminae, bounded superiorly and inferiorly by the laminae and laterally by the pedicles and facet joints. From posterior to anterior, the layers traversed during needle advancement are skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, and then the epidural space. The ligamentum flavum is a paired elastic ligament that typically fuses in the midline, though midline gaps exist in up to 10% of patients at lumbar levels. The epidural space itself contains fat, veins (Batson's venous plexus), nerve roots, and connective tissue.

### Regional Variations

Anatomy differs significantly across spinal regions. In the cervical spine, interlaminar windows are relatively large, but the spinal cord is present and the epidural space is narrowest at C5-C6 (approximately 1-2 mm posterior to the cord), making inadvertent dural puncture particularly dangerous. In the thoracic spine, the laminae overlap significantly (shingling), which narrows the interlaminar window, the spinal cord is present throughout, and steep angulation is required for midline access. In the lumbar spine, interlaminar windows are largest, especially at L4-L5 and L5-S1. The conus medullaris typically terminates at L1-L2, with only the cauda equina below, and the epidural space is widest posteriorly at L2-L3 (approximately 5-6 mm).

<image>Cross-sectional anatomic illustration at the lumbar spine level showing the layers traversed during an interlaminar epidural injection from posterior to anterior: skin, subcutaneous fat, supraspinous ligament, interspinous ligament, ligamentum flavum, posterior epidural space with fat and veins, dura mater, arachnoid mater, subarachnoid space with CSF and cauda equina nerve roots, with a needle shown at the ligamentum flavum with loss-of-resistance syringe attached</image>

## Technique

### Patient Positioning

For lumbar procedures, the patient is positioned prone on a fluoroscopy table with a pillow under the abdomen to reduce lumbar lordosis and open the interlaminar space. Seated or lateral decubitus positions are alternatives. For cervical procedures, the patient is prone with the neck flexed (using a face cradle or rolled towel) or seated with the neck flexed. For thoracic procedures, the patient is prone with the arms at the sides.

### Fluoroscopic Setup

An AP view is obtained first, with cranial-caudal tilt adjusted to square the target endplates. The target interlaminar space is identified, and the spinous processes should be midline, indicating no patient rotation. A lateral view is then used to assess depth and confirm epidural needle position before and after LOR. A contralateral oblique view (CLO) may be added to visualize the ventral epidural space and dorsal nerve root complex.

### Loss-of-Resistance Technique

The standard needle is a Tuohy needle (17- or 18-gauge) with a curved Huber tip that directs the catheter or injectate anteriorly. The needle is advanced through the interspinous ligament with a loss-of-resistance syringe attached, filled with saline, air, or a combination. LOR to saline is preferred by most pain practitioners because it carries a lower risk of pneumocephalus and patchy analgesia compared to air. Continuous or intermittent pressure is applied to the syringe plunger as the needle advances; a sudden decrease in resistance signals penetration of the ligamentum flavum and entry into the epidural space. The hanging drop technique is an alternative, in which a drop of saline placed at the needle hub is drawn in by negative epidural pressure -- this is more reliable in the cervical region.

### Midline vs. Paramedian Approach

The midline approach passes the needle through the interspinous ligament in the sagittal plane. It is straightforward but may be limited by calcified interspinous ligaments, spinous process hypertrophy, or prior surgery. The paramedian (parasagittal) approach enters lateral to the spinous process and angles medially, bypassing the supraspinous and interspinous ligaments to contact the ligamentum flavum directly. This approach is advantageous in patients with significant degenerative changes or at thoracic levels where laminar shingling limits midline access.

## Contrast Spread Patterns

After LOR is obtained, 1-3 mL of non-ionic contrast is injected under live fluoroscopy. A correct epidural pattern shows contrast spreading in a characteristic linear, segmental pattern outlining the epidural space, often filling nerve root sleeves. On AP view, contrast appears as paramedian columns; on lateral view, it outlines the posterior epidural space. Intravascular uptake appears as rapid contrast washout with vascular opacification and requires needle repositioning. A subdural pattern produces a smooth, thin contrast line wider than epidural spread but without nerve root sleeve filling. An intrathecal pattern shows dense, homogeneous myelographic spread, indicating dural puncture.

<image>Fluoroscopic image montage showing four panels: (1) AP view of correct epidural contrast spread with bilateral paramedian columns and nerve root sleeve filling at the lumbar level, (2) lateral view showing posterior epidural contrast dorsal to the thecal sac, (3) AP view of intravascular contrast uptake with vascular blush pattern, and (4) AP view of intrathecal contrast showing dense myelographic spread, each panel labeled with identifying features and clinical significance</image>

## Level-Specific Considerations

| Region | Common Level | Epidural Space Width | Spinal Cord | Typical Volume | Key Considerations |
|--------|-------------|---------------------|-------------|----------------|-------------------|
| Cervical | C7-T1 | 1-3 mm | Present | 3-5 mL | Lateral view mandatory; paramedian may be safer |
| Thoracic | Variable | 2-4 mm | Present | 3-5 mL | Steep cranial angulation; paramedian often needed |
| Lumbar | L4-L5, L5-S1 | 5-6 mm (at L2-L3) | Absent (cauda equina only) | 5-10 mL | Largest interlaminar windows; midline access easier |

### Cervical Interlaminar ESI

The most common level for cervical ILESI is C7-T1, which offers the largest interlaminar window and the absence of spinal cord at the C8 nerve root level in most patients. At midcervical levels, the posterior epidural space is very thin (1-3 mm), and the risk of spinal cord injury from dural puncture is significant. A lateral fluoroscopic view is mandatory to confirm needle depth and dorsal epidural position. A dorsal paramedian approach may be safer than midline to avoid the dorsal median septum and midline epidural veins. Injectate volume should be limited to 3-5 mL to avoid excessive epidural pressure.

### Thoracic Interlaminar ESI

Laminar shingling in the thoracic spine requires steeper cranial angulation and often a paramedian approach. The spinal cord is present throughout, and the epidural space is narrow (2-4 mm). Thoracic ILESI is less commonly performed than cervical or lumbar and is indicated primarily for thoracic radiculopathy and post-herpetic neuralgia. Lateral fluoroscopy is essential for depth confirmation.

### Lumbar Interlaminar ESI

Lumbar ILESI is most commonly performed at L4-L5 or L5-S1. Below the conus medullaris (L1-L2), the needle encounters cauda equina nerve roots rather than spinal cord, reducing but not eliminating the risk of neurologic injury. The larger interlaminar windows allow easier midline access, and the typical injectate volume is 5-10 mL including steroid, local anesthetic, and contrast.

## Evidence for Radiculopathy

The strongest evidence supports ILESI for acute and subacute lumbar radiculopathy due to disc herniation, with multiple systematic reviews demonstrating short-term (2-4 weeks) pain relief and functional improvement. Evidence for long-term benefit beyond three months is less robust. Cervical ILESI has moderate evidence for cervical radiculopathy. Evidence for spinal stenosis without radiculopathy is weaker. ILESI may reduce the need for surgical intervention in the short term for select patients with disc herniation and radiculopathy. Repeat injections are typically limited to 3-4 per year, as evidence does not support benefit from more frequent injections.

## Risk of Dural Puncture and Complications

### Dural Puncture

Inadvertent dural puncture with a Tuohy needle occurs in approximately 0.5-2% of cases in experienced hands. It is recognized by free flow of CSF through the needle or intrathecal contrast spread. Post-dural puncture headache (PDPH) follows in approximately 50-80% of cases and is characteristically positional (worse upright, improved supine), frontal or occipital, and may be accompanied by nausea, photophobia, and tinnitus. Initial management is conservative (hydration, caffeine, analgesics) for 24-48 hours, with an epidural blood patch offered if symptoms persist.

### Other Complications

Epidural hematoma is rare but potentially catastrophic, with risk increased by coagulopathy or anticoagulant use. Epidural abscess is also rare and presents with fever, back pain, and progressive neurologic deficits. Transient paresthesia during needle placement is common, but persistent nerve root injury is rare. Spinal cord injury is extremely rare at lumbar levels (below the conus) but is a genuine concern at cervical and thoracic levels. Systemic steroid effects -- hyperglycemia, adrenal suppression, fluid retention -- are possible, especially with particulate steroids.

<image>Anatomic sagittal section illustration of the lumbar spine showing the Tuohy needle traversing the ligamentum flavum into the epidural space at L4-L5, with detailed insets showing: (1) correct needle tip position in the posterior epidural space with epidural contrast spread, (2) inadvertent dural puncture with CSF leak and post-dural puncture headache mechanism, and (3) the epidural blood patch technique with autologous blood injected around the dural puncture site</image>

## Clinical Pearls

Needle position should always be confirmed with contrast injection under live fluoroscopy before injecting steroid; blind (non-image-guided) interlaminar ESI is no longer acceptable in modern pain practice. Loss-of-resistance to saline is preferred over air to reduce the risk of pneumocephalus and patchy block. At cervical levels, C7-T1 is the safest entry point, and dorsal epidural position must always be confirmed on lateral fluoroscopy before injecting. If intravascular uptake is noted on contrast injection, the needle must be repositioned and epidural spread confirmed before proceeding -- particulate steroid should never be injected into a vascular pattern. The frequency of epidural steroid injections should be limited to avoid cumulative systemic steroid effects, and the total steroid dose administered per year should be documented.

## References

1. Manchikanti L, Knezevic NN, Navani A, et al. Epidural interventions in the management of chronic spinal pain: American Society of Interventional Pain Physicians (ASIPP) comprehensive evidence-based guidelines. *Pain Physician*. 2021;24(S1):S27-S208.
2. Benyamin RM, Manchikanti L, Parr AT, et al. The effectiveness of lumbar interlaminar epidural injections in managing chronic low back and lower extremity pain. *Pain Physician*. 2012;15(4):E363-E404.
3. Stojanovic MP, Vu TN, Caneris O, Slezak J, Cohen SP, Sang CN. The role of fluoroscopy in cervical epidural steroid injections: an analysis of contrast dispersal patterns and the incidence of intravascular injection. *Spine*. 2002;27(5):509-514.
4. Rathmell JP, Benzon HT, Dreyfuss P, et al. Safeguards to prevent neurologic complications after epidural steroid injections. *Anesthesiology*. 2015;122(5):974-984.