Soliciting a motor response with a nerve stimulator has become increasingly popular in the performance of peripheral nerve blocks, and the physics and physiology of this technique have been reviewed in detail (1). There has been the implication that “the use of a nerve stimulator … should minimize the possibility of neuropathy by avoiding actual physical contact with a nerve (2).” This theoretical advantage has been extrapolated to imply a greater comfort in performing regional techniques on heavily sedated or anesthetized adult patients (3). The hypothesis that the nerve stimulator will prevent nerve injury by always identifying a nerve before making direct needle contact has been challenged by a report of four patients who suffered permanent spinal cord damage with neurologic findings subsequent to the performance of interscalene blocks with a nerve stimulator in the presence of general anesthesia (4).
We report a series of four consecutive patients in whom a paresthesia response (presumed nerve contact) was obtained before a motor stimulation during performance of interscalene brachial plexus anesthesia with a nerve stimulator. In all four of these cases, a Braun Stimuplex stimulator was attached to a 1.5-in., 22-gauge Stimuplex needle (B. Braun Medical, Bethlehem, PA) with an appropriate electrical ground.
A 42-yr-old woman was scheduled for a right acromioplasty. After sedation with 2 mg of midazolam and 50 μg of fentanyl, an interscalene block was attempted with the nerve stimulator set to deliver 1.2 mA of current. The needle was introduced at the level of the cricoid cartilage and directed towards the transverse process of the C6 vertebrae. On the first pass of the needle, there was direct muscle stimulation in the neck area. The needle was redirected towards the tubercle, and the patient reported a nonpulsing electric shock sensation in her right thumb without any evidence of muscular movement in the shoulder or upper extremity. A test dose of 1 mL of 1.5% mepivacaine did not recreate the paresthesia sensation. Thirty milliliters of the drug was then injected through the needle. Within 10 min, the patient had an onset of surgical anesthesia in the distribution of the brachial plexus. The surgery was completed uneventfully, and she recovered without sequelae.
A 57-yr-old, 122-kg man was scheduled for a left shoulder hemi-arthroplasty. An interscalene block was initiated with the stimulator guidance set to deliver 1.2 mA. Three initial passes of the needle did not produce motor or sensory responses. The needle was redirected posteriorly 1 cm, and the patient immediately described a nonpulsing electric shock sensation in his left elbow. A test dose of 1 mL of 1.5% mepivacaine did not reproduce the paresthesia. Thirty milliliters of the drug was injected, and the patient had an onset of surgical anesthesia within 10 min in the left arm. Because of the extent of surgery, supplemental general anesthesia was performed after fiberoptic intubation and using isoflurane and 0.5% inspired concentration plus 60% nitrous oxide. After a 2 h procedure, he had total analgesia of the left arm in the postanesthesia care unit (PACU). He recovered uneventfully and was discharged from the hospital two days later.
A 32-yr-old, healthy, 92-kg man was scheduled for right shoulder acromioclavicular joint reconstruction. Interscalene block was attempted with the stimulator delivering 1.2 mA. Upon insertion of the needle, the patient reported a nonpulsing paresthesia in the middle finger with no motor response. Injection of a 1 mL test dose did not recreate the paresthesia. Thirty milliliters of local anesthetic was injected. Twenty minutes later, adequate surgical anesthesia on the arm was noted. The operation proceeded uneventfully, and the patient was pain free in the PACU and recovered full neurologic function before discharge two days later. After this anesthetic, the Braun nerve stimulator (B. Braun Medical) was taken to the biomedical laboratory where its output was confirmed to be at the rated voltage and amperage.
A 41-yr-old, 82-kg, healthy man was scheduled for an open reduction internal fixation of a right shoulder fracture. An interscalene block was performed using a second Braun nerve stimulator. On the second introduction of the needle, the patient reported a nonpulsing paresthesia in the right upper arm without motor response. Again, a 1 mL test dose was injected and did not recreate the paresthesia. After 30 mL of local anesthetic, the patient had onset within 10 min of satisfactory anesthesia of the upper extremity. The shoulder was reduced without requiring surgical incision. The patient was pain free in the PACU and had return of normal neurologic function before discharge from the hospital the next day.
Choyce et al. (5) examined the relationship between paresthesia and motor response. They elicited motor response to a nerve stimulator after obtaining a paresthesia in all of 53 patients undergoing axillary block. Twelve patients required a current more than 0.5 mA, and four required currents more than 1 mA to elicit a motor response. Their findings suggest that as many as 10% of patients might not obtain a motor response with the typical 1 mA (seeking) current during nerve stimulator techniques. Their use of an uninsulated needle would require a more intense current to produce a motor response, and thus their findings are not as relevant to standard nerve stimulator technique. Urmey (6) has claimed a similar experience and concluded that nerve contact without a motor response is possible and might represent a risk to a sedated patient.
These investigators introduced a needle without a stimulating current before nerve contact. Our case reports involved the opposite technique (presence of a current before paresthesia), but we confirmed that there is a subset of patients who do not obtain motor response to a nerve stimulator before eliciting paresthesia, even with the use of standard high seeking currents with an insulated needle. The true frequency of this event cannot be extrapolated from our experience, but the observation of four consecutive patients in a single day demonstrating this phenomenon suggests that it is not a rare or infrequent event. It is not clear whether we would have obtained a motor response if we had used a higher seeking current, but conventional teaching suggests that 1 mA should be sufficient to identify localization of the nerve in normal patients. Our experience supports the concern of others (6–8) that reliance on a nerve stimulator for performance of a peripheral nerve block does not eliminate the potential for nerve injury (9), and in the presence of heavy sedation or general anesthesia may represent a significant risk to adult patients.
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