What is the order of sensory block in spinal anesthesia?

Order of Sensory Block in Spinal Anesthesia

In spinal anesthesia, sensory modalities are blocked in the following sequential order: sympathetic function (vasomotor tone) is lost first, followed by temperature discrimination, then pinprick pain sensation, and finally light touch, with motor function being the last to be affected. 1

Sequential Blockade Pattern

The differential blockade occurs in a predictable craniocaudal segmental sequence:

• Sympathetic blockade extends 1-2 spinal segments above the sensory levels of light touch and pinprick 2
• Temperature discrimination (cold sensation) extends above pinprick levels 2
• Pinprick sensation is blocked at intermediate levels and extends more cephalad than light touch 2
• Light touch represents a lower level of sensory blockade 2
• Motor function is the last modality to be blocked 1

Mechanism of Differential Block

This differential pattern occurs because of fundamental neurophysiological principles 1:

• Fiber size and internodal length determine susceptibility to blockade - smaller fibers with shorter internodes (sympathetic and pain fibers) block more readily than larger fibers with longer internodes (motor fibers) 1
• Three-node block principle: Conduction can leap two consecutive blocked nodes but not three, making small fibers more vulnerable 1
• Decremental conduction: In subarachnoid blockade, increasingly concentrated anesthetic bathes increasing lengths of fibers in the craniocaudal succession of spinal nerve roots, producing block in progressively larger fibers 1

Clinical Monitoring Implications

During establishment and recovery of spinal anesthesia:

• Pinprick levels more accurately reflect sympathetic block levels than light touch levels 2
• Temperature discrimination provides the most cephalad assessment of sensory blockade 2
• Motor block assessment should use straight-leg raising as a screening tool, with inability to perform this maneuver at 4 hours post-block requiring immediate anesthesiologist evaluation 3
• Recovery occurs in reverse order: Motor function returns first, followed by light touch, pinprick, temperature, and finally sympathetic function 3

Important Clinical Pitfalls

• Do not rely solely on light touch for assessing block height, as it underestimates the extent of sympathetic blockade by 1-2 segments 2
• Zones of differential block develop within 5 minutes of local anesthetic injection and persist throughout the block duration 2
• Expected recovery times are 2-3 hours for intrathecal bupivacaine and 3-5 hours for epidural bupivacaine/lidocaine, though variability exists 3
• Prolonged blocks beyond 24-48 hours should never be attributed solely to expected anesthetic effects and require urgent investigation 3