Does the sympathetic block outlast sensory block: a thermographie evaluation

A study to evaluate the duration of sympathetic and sensory block in the L₂ and L₅ dermatome distributions using thermography and pinprick was conducted. Twenty patients received epidural block using 2% lidocaine with epinephrine. Onset and duration of the sensory and sympathetic blocks were determined and compared statistically. There was no difference between the duration of sensory and sympathetic block over the L₂ dermatome, but sympathetic block was significantly longer than sensory block in the L₅ dermatome. This study demonstrates that the duration of sympathetic block can be either longer or shorter than sensory block in L₂ and L₅ dermatomes. This has important implications for interpretation of results of differential epidural studies in that one cannot predict the duration of sympathetic block based on duration of sensory block.