A differential block refers to the clinical phenomenon that nerve fibers with different functions have different sensitivities to local anesthetic blockade. This is well proven in both neuraxial and peripheral nerve blocks. Classically, it was taught that small diameter axons (ex. C-fibers) are more susceptible than larger diameters, but this is not true – there are probably multiple reasons for differential blockade (ex. length of each nerve in thecal space, depth of nerve fiber, distribution of Na+ and K+ channels on each nerve, etc.).
Despite the elusive nature of the cause, with regard to neuraxial blocks sympathetic nerve fibers are blocked by the lowest concentration of local anesthetic followed by nerve fibers responsible for pain, touch and finally motor function.
This relative sensitivity of certain nerve fibers is displayed by a spatial separation (i.e. the sympathetic block will be approximately 2-4 dermatomes beyond the motor block, the pain/touch will be 2-3 dermatomes beyond the motor block). The presumed etiology is as the local anesthetic gets further from injection site it is present in a lower concentration and sympathetic nerve fibers do not require the same concentration to be blocked as do motor nerve fibers (as stated above).
Differential Block (a NORMAL phenomenon)
- Sympathetics: most sensitive to local anesthetic agents (2-4 levels beyond motor)
- Pain/touch: moderately sensitive (2-3 levels beyond motor)
- Motor fibers: least sensitive