Methods/Techniques to Identify the Epidural Space

1. Loss of Resistance (LOR) Technique - Most Common

• LOR to air - simpler; easier to recognize accidental dural puncture (instant headache if air enters intrathecally); risk of epidural air causing patchy block or air embolism
• LOR to saline - associated with fewer dural punctures, fewer post-dural puncture headaches (PDPH), fewer paresthesias, and easier catheter threading; a small air bubble is often included to act as a compressible indicator

• Intermittent (Bromage technique): Advance needle millimeter-by-millimeter, tapping the plunger between each advance; plunger "bounces" or "recoils" in ligamentum flavum, then collapses on entering epidural space
• Continuous pressure (Doughty/son-of-Doughty): Constant thumb pressure on plunger while advancing needle - needle stops advancing when plunger collapses on entering epidural space

2. Hanging Drop Technique (Gutierrez Sign)

• After entering the interspinous ligament and removing the stylet, a drop of saline is placed at the needle hub
• The needle is slowly advanced; as long as the tip is within the ligament, the drop "hangs"
• When the tip just enters the epidural space, the drop is aspirated ("sucked in") into the hub

• Requires true negative pressure; this is more reliably present in the thoracic region than lumbar (thoracic epidural space has more consistent negative pressure due to transmitted negative intrathoracic pressure)
• If the needle becomes plugged with tissue, the drop is not sucked in - risking inadvertent dural puncture
• Preferred by some for paramedian approach and cervical epidurals
• Negative pressure may be absent in obese patients or those with increased intra-abdominal pressure

3. Ultrasound Guidance

• Identifies spinous processes, intervertebral levels, midline, interspinous window, and paramedian interlaminar window
• Estimates skin-to-epidural space distance before needle insertion
• Particularly useful in: obese patients, scoliosis, previous spinal surgery, difficult surface landmarks

• Visualization of ligamentum flavum and epidural space (harder than spinal structures due to acoustic shadowing from bone)
• More impressive in pediatric patients where vertebral ossification is incomplete - can visualize needle, catheter tip, dural displacement, and extent of cranial spread during injection

4. Epidural Pressure Waveform / Manometry

• A transducer connected to the epidural needle via saline-filled tubing can detect the characteristic pulsatile pressure waveform of the epidural space (transmitted cardiovascular and respiratory pulsations)
• A negative pressure or a pressure close to zero, with pulsations, confirms epidural location
• Used more commonly to confirm catheter tip position rather than primary identification
• Helps differentiate epidural from subarachnoid placement

5. Nerve Stimulation (Epidural Nerve Stimulation Test - Tsui Test)

• A low-current electrical stimulus (1-15 mA) is applied through a conducting epidural catheter
• Correct epidural placement produces bilateral segmental motor responses (e.g., intercostal muscle contraction at appropriate dermatomal level)
• Intrathecal placement requires much lower current to elicit response (< 1 mA)
• Intravascular placement produces no motor response
• Particularly useful in pediatric patients and for confirming thoracic epidural catheter placement
• Does not confirm epidural vs. subdural placement reliably without other checks

Summary Table for Quick Revision

• LOR to saline is currently preferred over air (meta-analysis shows similar outcomes but saline associated with fewer complications in many observational studies)
• Hanging drop is more reliable at the thoracic level than lumbar
• "Experienced practitioners sense the give as the needle traverses ligamentum flavum and use LOR/hanging drop only as confirmation" - (Morgan & Mikhail)
• Always inject a test dose (e.g., lidocaine 3 mL with epinephrine 1:200,000) after identifying the space to rule out intravascular or intrathecal placement

Modifications of the Loss of Resistance (LOR) Technique

A. Medium Used in the LOR Syringe

1. LOR to Air

• Simplicity - easy to set up
• Accidental or intentional dural puncture is immediately obvious (instant severe headache from intracranial air; the patient reports this instantly)
• Easier to distinguish epidural from intravascular placement

• Large volumes of air (>2-3 mL) in the epidural space can cause patchy or unilateral block (air pocket disrupts drug spread)
• Pneumocephalus if dural puncture occurs
• Risk of venous air embolism (rare)
• Air is compressible - the endpoint feel can be less crisp

2. LOR to Saline

• Fewer dural punctures reported
• Fewer PDPHs (post-dural puncture headaches)
• Easier catheter threading (saline hydrodissects the space)
• Fewer paresthesias
• Fewer intravascular catheter placements
• Better analgesia and fewer unblocked segments in some studies

• Harder to recognize accidental dural puncture - saline can be confused with CSF
• Cannot use the "instant headache" sign
• A meta-analysis found no statistically significant outcome differences between air and saline - "any technique done well is better than the best technique done poorly"

3. LOR to Saline + Air Bubble

• The air bubble acts as a visual and tactile indicator - it compresses when the needle tip is in the ligamentum flavum and decompresses (disappears) on entering the epidural space
• Prevents the plunger from sticking to the syringe wall
• Combines benefits of both - tactile feel of saline with the compressibility indicator of air

B. Technique of Needle Advancement

4. Intermittent (Step-by-Step) Technique

• Advance the needle 2-3 mm at a time
• Between each advance, firmly tap or compress the plunger of the LOR syringe
• In ligamentum flavum: plunger recoils/bounces (firm gritty resistance)
• On entering epidural space: plunger collapses - loss of resistance felt

5. Bromage Continuous Pressure Technique

• Nondominant hand - make a fist, place carpal-metacarpal joints on patient's back, grip needle shaft between thumb and forefinger
• Dominant hand - applies continuous pressure to the syringe plunger

• Needle is advanced by rolling the fist forward (extending the wrist, rocking forward on knuckles)
• Continuous pressure maintained on the plunger throughout
• Needle advances automatically stops when the plunger collapses on entering the epidural space

6. Doughty Continuous Pressure Technique

• Nondominant hand - braces the epidural needle against the patient's back (controls advance)
• Dominant hand - grips the barrel of the LOR syringe; the metacarpal head of the index finger rests on the end of the plunger

• Dominant hand drives the needle forward by balancing driving pressure against the bracing resistance from the nondominant hand
• The metacarpal head pressure on the plunger allows immediate tactile perception of LOR

7. "Son-of-Doughty" / One-Handed Continuous Pressure Technique

• Nondominant hand braced against patient's back, advancing the needle
• Dominant hand applies direct thumb pressure to the plunger of a saline-filled syringe
• Needle advances continuously; the plunger collapses the instant the epidural space is entered, and the needle stops advancing automatically

Summary of Modifications