What is the sequence of nerve fiber blockade during spinal anesthesia?

Sequence of Nerve Fiber Blockade During Spinal Anesthesia

During spinal anesthesia, nerve fibers are blocked in a specific sequence: preganglionic sympathetic B fibers first, followed by small myelinated fibers (Aδ and Aγ), then large myelinated fibers (Aα and Aβ), with unmyelinated C fibers being most resistant to blockade.

Classical Order of Blockade

The traditional understanding of spinal blockade follows this craniocaudal segmental sequence 1:

• Sympathetic function (vasoconstriction) - blocked first
• Temperature discrimination - blocked second
• Pinprick pain sensation - blocked third
• Touch and pressure - blocked fourth
• Motor function - blocked last

Fiber-Specific Susceptibility

Research demonstrates that susceptibility to local anesthetics does not strictly follow the simple "size principle" that smaller fibers are always blocked first 2:

• Small myelinated fibers (Aδ sensory and Aγ motor) show the lowest threshold concentrations (0.03% lidocaine) and are blocked earliest 2
• Large myelinated motor fibers (Aα) require slightly higher concentrations (0.05% lidocaine) 2
• Large myelinated sensory fibers (Aαβ) are more resistant (0.07% lidocaine threshold) 2
• Unmyelinated C fibers are paradoxically the most resistant (0.09-0.1% lidocaine threshold), contrary to traditional teaching 2, 3

Mechanisms Underlying Differential Block

Two key principles explain the differential blockade pattern 1:

• Three-node block principle: Conduction can bypass two consecutive blocked nodes but not three, making small fibers with short internodal distances more susceptible 1
• Decremental conduction: Longer fiber segments bathed in weak anesthetic concentrations can block through cumulative effects, with the required concentration varying inversely with the number of nodes exposed 1

Clinical Manifestation Sequence

The practical clinical sequence observed is 1:

1. Sympathetic blockade - warmth, vasodilation (typically 2-6 dermatomes higher than sensory block)
2. Loss of temperature sensation
3. Loss of pinprick sensation (defines the "level" of anesthesia)
4. Loss of proprioception
5. Motor blockade (typically 2-4 dermatomes lower than sensory block)

Monitoring Motor Block Recovery

Motor function should be assessed using the Bromage scale, with inability to perform straight-leg raise at 4 hours post-block requiring immediate evaluation 4, 5:

• Score 1: Complete block (unable to move feet or knees)
• Score 2: Able to move feet only
• Score 3: Can flex knees with free foot movement
• Score 4: Full movement restored 4

Important Clinical Caveats

• Motor deficits appearing first may indicate Aγ fiber blockade affecting muscle spindle function before pain sensation is lost 2
• Faster-conducting C fibers (>1 m/s) are more susceptible than slower C fibers, creating variability in pain blockade 2
• The spatial differential sequence in spinal anesthesia resembles the temporal sequence seen in peripheral nerve blocks 1
• Use-dependent blockade contributes minimally to spinal anesthesia compared to tonic blockade 2

Safety Considerations

Epidural hematoma can cause irreversible neurological damage if not evacuated within 8-12 hours 4, 6. Warning signs include:

• Persistent or increasing motor block beyond expected duration 4
• Back pain with progressive weakness 6
• Bladder or bowel dysfunction 6

MRI of the spine is the preferred imaging modality for suspected complications 4.