What is segmental spinal anesthesia, how is it performed, and how are the block levels assessed?

Segmental Spinal Anesthesia: Definition, Technique, and Assessment

Segmental spinal anesthesia is a neuraxial technique that produces a limited, targeted band of sensory and motor blockade by using small doses of local anesthetic (typically hyperbaric or isobaric solutions) to block specific spinal nerve roots, rather than producing extensive cephalad spread. This approach aims to provide adequate surgical anesthesia while minimizing hemodynamic side effects and preserving function in unaffected dermatomes 1.

What is Segmental Spinal Anesthesia?

Segmental spinal anesthesia differs from traditional spinal anesthesia in its goal of creating a restricted zone of blockade rather than extensive spread. The technique relies on:

• Controlled drug delivery: Using reduced volumes and concentrations of local anesthetic
• Baricity manipulation: Hyperbaric solutions (e.g., 0.75% bupivacaine in dextrose, 5% ultracaine) tend to settle dependently, while isobaric solutions (0.5% bupivacaine) spread more predictably 1
• Patient positioning: Maintaining specific positioning during and after injection to limit spread
• Injection rate control: Slower injection rates (0.02 mL/s) help control distribution 2

The pharmacology demonstrates that hyperbaric bupivacaine 0.75% and ultracaine 5% provide equivalent efficacy with shorter latent periods (3.14 ± 0.16 minutes for ultracaine), while isobaric bupivacaine 0.5% offers the longest duration (215 ± 45 minutes) 1.

How to Perform Segmental Spinal Anesthesia

Positioning Strategy

Spinal flexion in the lateral decubitus position is critical for achieving more restricted unilateral blockade. With full spinal flexion, the cauda equina becomes tightened and suspended in the middle of the subarachnoid space, which significantly reduces early spread to the nondependent side 2.

Technique steps:

1. Position the patient in lateral decubitus with the operative side dependent
2. Achieve full spinal flexion (hips and knees flexed, chin to chest)
3. Identify the injection site at L3-4 or L4-5 interspace
4. Use a 25-gauge Quincke needle for controlled injection
5. Inject slowly at 0.02 mL/s (or 0.1 mL/s for standard technique) 2, 3
6. Maintain lateral position with spinal flexion for 15 minutes after injection
7. Turn to supine position gently after the desired segmental block is established

Drug Selection and Dosing

For lower body/limb surgery:

• Hyperbaric bupivacaine 0.75%: 8-10 mg for restricted block 2, 1
• Ultracaine 5%: Provides shortest onset but similar duration to hyperbaric bupivacaine 1
• Isobaric bupivacaine 0.5%: For longest duration when extended analgesia needed 1

Critical caveat: Even with optimal technique using 8 mg hyperbaric bupivacaine and maintained spinal flexion, strict unilateral sensory block is difficult to achieve—93.7% of patients still developed some contralateral spread after returning to supine position 2.

Advanced Techniques

Continuous spinal anesthesia via microcatheter improves safety and control compared to single injection, allowing titration of segmental blockade and extending duration as needed 1, 4. This technique involves placing an intrathecal catheter for repeated dosing intraoperatively and postoperatively 4.

How to Check Block Levels

Assessment Methods and Their Differences

The method of sensory testing significantly affects the documented block level—you must specify both the stimulus used AND the exact question asked. Different assessment techniques can yield results varying by 2-6 dermatomes 5, 6.

Recommended Assessment Techniques

1. Pinprick (Sharp Sensation)

• Use a Neurotip® or similar sharp stimulus
• Ask: "When do you FIRST feel sharp sensation?" (not "when is it the same as normal")
• This represents the most clinically relevant level for surgical anesthesia 5

2. Light Touch

• Critical distinction: "First touch level" vs. "touch same as control"
• The "first touch level" averages 2 dermatomes lower than "touch same as control" (range 0-6 dermatomes) 5
• For cesarean section, a block to T5 using "first touch" is standard, but studies using "touch same as control" showed inadequate anesthesia 5

3. Pressure Palpator (Novel Method)

• A 650g pressure palpator provides non-invasive assessment without breaking skin integrity
• Consistently assesses block 0.5-1.2 dermatomes higher than pinprick 6
• Pitfall: If preceded by pinprick testing, the pressure palpator shows falsely elevated thresholds due to increased sensitization 6

4. Cold Sensation

• Use ice or cold stimulus
• Correlates with C fiber function recovery 7

Temporal Assessment Protocol

Timing matters for accurate assessment:

• 3 minutes post-injection: Earliest reliable assessment point; SBL ≥T8 at 3 minutes predicts hypotension with 82% sensitivity and 88% specificity 3
• 5-minute intervals: Standard for dermatomal level documentation
• 10-minute intervals: For quantitative current perception threshold testing 7
• 15 minutes: Typical time to maintain lateral position before turning supine 2

Differential Block Physiology

Sequential return of sensation follows a predictable pattern as different nerve fiber types recover:

1. Touch returns first (A-beta fibers, 2000 Hz stimulation)
2. Pinprick returns second (A-delta fibers, 250 Hz stimulation)
3. Cold sensation returns last (C fibers, 5 Hz stimulation)

Surgical anesthesia is lost when A-beta fiber function returns to baseline, even though A-delta and C fiber blockade persists 7. This explains why patients may report "feeling something" before actually experiencing pain.

Practical Assessment Algorithm

For segmental spinal anesthesia monitoring:

1. At 3 minutes: Assess with pinprick bilaterally to predict hemodynamic stability (target <T8 for reduced hypotension risk) 3
2. Every 5 minutes until stable: Document highest sensory level bilaterally using pinprick, asking "when do you FIRST feel sharp?"
3. Before surgical incision: Confirm adequate level (typically T8-T10 for lower limb, T4-T6 for lower abdominal surgery)
4. Intraoperatively: Reassess if patient reports discomfort
5. Document both dependent and non-dependent sides separately when attempting unilateral block

Common Pitfalls to Avoid

• Don't use "touch same as control" as your endpoint—this overestimates block height by 2+ dermatomes 5
• Don't assess with pressure palpator after pinprick—this creates false elevation 6
• Don't assume symmetry—always check both sides, especially with segmental/unilateral techniques 2
• Don't rely on motor block alone—sensory assessment is essential for surgical readiness
• Don't turn patient supine too early—maintain lateral position with flexion for full 15 minutes to optimize segmental distribution 2