Relationship between evoked motor response and sensory paresthesia in interscalene brachial plexus block

Background and objectives: This study sought to define the relationship between a paresthesia and a motor response (MR) to electrical nerve stimulation using a peripheral nerve stimulator (PNS) during interscalene block. We sought to determine if at a low amperage (less than or equal to1.0 mA) a MR would precede a paresthesia. Methods: Twenty-two interscalene blocks were performed using insulated needles and a PNS. A MR was obtained at 0.5 mA and then the PNS was turned off. The needle was further advanced until a paresthesia was elicited. The PNS was again turned on, the needle held immobile, and the amperage increased in 0.1 mA increments up to 0.5 mA, or an MR obtained, whichever occurred first. If no MR was obtained, the needle was withdrawn at 0.5 mA in the same direction as it entered until MR was again observed. Results: A MR was obtained at 0.5 mA in all the patients. After the PNS was turned off and the needle further advanced, a paresthesia was elicited in 21 patients. When the PNS was turned on again, a MR was produced at 0 to 0.5 mA in 13 patients. In a subset of 8 patients without a second MR to stimulation up to 0.5 mA, the needle was withdrawn at that amperage. A MR was subsequently obtained during withdrawal in each patient in this subset. Conclusions: MR preceded paresthesia in every patient. The most likely explanation for this observation is that MR can be achieved at a small distance from the nerve, whereas elicitation of mechanical paresthesia requires either nerve contact or more intimate location of the needle's tip relative to the nerve. Another possible explanation is that motor fibers are located in a more superficial position and are therefore encountered first. Motor and sensory responses are separate and discrete phenomena.