Regional blockade at the plexus and peripheral nerve levels provides effective and reliable anesthesia and analgesia for upper and lower extremity procedures. However, success is highly dependent upon the precise localization of neural structures. Historically, this was accomplished through the elicitation of one or more paresthesias (1-4). The exclamation, "No paresthesia, no anesthesia" became the mantra of many (though not all) of our founding fathers. Although alternative methods of neural blockade such as the perivascular, nerve stimulator, and sheath approaches have been described, these techniques often fail to achieve the same success rate. This is especially true when bupivacaine, rather than lidocaine or mepivacaine is utilized (5,6). Horlocker et al (5) investigated the safety and success rates of several regional techniques in patients undergoing repeated axillary blockade. Success rates were significantly higher with the paresthesia technique (90%) compared to the nerve stimulator technique (83%; P=0.03) or transarterial injection (81%; P=0.008). Similarly, Schroeder and colleagues (6) reported that paresthesia techniques during axillary blockade result in significantly higher success rates compared to transarterial approaches (95% vs. 81%; P=0.036). Although elicitation of a paresthesia increased the success rate in these two studies, it did not increase the incidence of neurologic complications. In other words, improved outcome without increased risk.

Clinicians in opposition to paresthesia techniques often cite an increased risk of neurologic complications postoperatively. Although the intentional elicitation of a paresthesia may represent direct needle trauma and theoretically increase the risk of neurologic injury, there are no prospective, randomized clinical studies that are able to definitively support this hypothesis. In fact, several investigators have clearly established that paresthesia elicitation does not increase the risk of postoperative neurologic complications. Urban and Urquhart (7) performed a prospective investigation utilizing a variety of regional anesthetic approaches (transarterial, paresthesia, nerve stimulator) during brachial plexus blockade. The overall rate of neurologic complications was statistically higher with the axillary approach compared to the interscalene approach (19% vs 9%, respectively). Risk of postoperative neurologic dysfunction was not affected by type of local anesthetic, number of needle advances, duration of tourniquet inflation, or most importantly anesthetic technique.

The study by Selander et al (8) is often cited to support the premise, "no paresthesia, no dysesthesia." Patients were divided into two groups based on regional anesthetic technique- paresthesia or perivascular. Unintentional paresthesias were reported in 40% of patients in the perivascular group. Postoperative neurologic complications occurred in 8 of 290 (2.8%) patients where a paresthesia was intentionally sought compared to 2 of 243 (0.8%) patients undergoing a perivascular technique. Although the incidence of neurologic complications between the two groups was not statistically significant, all ten patients with persistent paresthesias had painful paresthesias elicited during performance of the block; in three cases the pain was enhanced during injection. In addition, 5 of 10 patients with nerve injury also had supplemental blocks performed at the level of the axilla, elbow or wrist. These results suggest that not all paresthesias are "equal". True pain during needle placement or injection of local anesthetic is not part of the paresthesia technique. Likewise, supplementary injection into a partially blocked plexus should be avoided (8).

Unfortunately, paresthesia critics often fail to recognize the potential complications of alternative techniques. For example, the blunt insulated needles used with a nerve stimulator approach cause discomfort when passing through tissue planes. In addition, the motor response associated with nerve stimulation is sometimes perceived as painful (9). More worrisome is that many advocates of the nerve stimulator approach argue that this technique may be performed on heavily sedated, anesthetized, or uncooperative patients since it provides exact needle localization without the elicitation of a paresthesia. A recent investigation by Choyce and colleagues (10) demonstrated that this may not be the case. During their technique, a non-insulated stimulating needle was advanced until a paresthesia was elicited. At this point, the current on a nerve stimulator was gradually increased until an associated motor response was obtained. Interestingly, after acquiring a paresthesia, nearly 25% of patients required a current >0.5 mA to manifest a motor response, with 42% needing currents as high as 3.3 mA.

These findings suggest an inconsistency of elicited motor responses, despite the needle presumably being near a nerve. Therefore, the illusion that nerve stimulation allows clinicians to approximate neural structures without the risk of mechanical trauma must be abandoned.

Transarterial approaches to the brachial plexus may also result in potentially devastating complications. Intravascular injection, vascular insufficiency secondary to vasospasm, hematoma or pseudoaneurysm formation, and axillary artery dissection have all been reported with varying frequencies. Hematoma and pseudoaneurysm expansion may result in pressure-induced neural ischemia and subsequent neurologic impairment in patients undergoing brachial plexus blockade (11,12). Pearce et al (13) demonstrated that transarterial injection may be associated with significantly more postoperative dysesthesias when compared to properly performed paresthesia techniques. The authors postulated that subclinical hematomas, not paresthesias, may be contributing to the development of transient neurologic symptoms postoperatively.

In summary, paresthesia techniques are a safe and effective means of performing peripheral nerve blockade. Their mastery requires a detailed knowledge of anatomy, technical aptitude, and astute clinical vigilance. Alternative techniques may be performed, but often at the expense of significantly lower success rates and prolonged onset times. Furthermore, it may be in fact difficult to perform peripheral nerve blockade without the elicitation of a paresthesia since unintentional paresthesias will occur in approximately 40% of patients, regardless of regional technique (7,14). Few clinicians attempting a transarterial approach would ignore the "gift" of a paresthesia, and use it to redirect the needle towards the artery. Why? For the same reason Willie Sutton robbed banks: "That's where the money is!"

References

1. Moore DC. Regional Anesthesia. Springfield, IL:Charles C. Thomas 1953:241.

2. Winchell SW, Wolfe R. The incidence of neuropathy following upper extremity nerve blocks. Reg Anesth 1985; 10:12-15.

3. Moore DC, Mulroy MF, Thompson GE. Peripheral nerve damage and regional anesthesia. Br J Anaesth 1994; 73:435-6.

4. Moore DC. "No paresthesias—No anesthesia," the nerve stimulator or neither? Reg Anesth 1997; 22:388-90.

5. Horlocker TT, Kufner RP, Bishop AT, et al. The risk of persistent paresthesia is not increased with repeated axillary block. Anesth Analg 1999; 88:382-7.

6. Schroeder LE, Horlocker TT, Schroeder DR. The efficacy of axillary block for surgical procedures about the elbow. Anesth Analg 1996; 83:747-51.

7. Urban MK, Urquhart B. Evaluation of brachial plexus anesthesia for upper extremity surgery. Reg Anesth 1994; 19:175-82.

8. Selander D, Edshage S, Wolff T. Paresthesiae or no paresthesiae? Acta Anaesth Scand 1979; 23:27-33.

9. McDonald SB, Thompson GE. See one, do one, teach one, have one. A novel variation of regional anesthesia training. Anesthesiology 1999;91:A1146

10. Choyce A, Chan VWS, Middleton WJ, et al. What is the relationship between paresthesia and nerve stimulation for axillary brachial plexus block? Reg Anesth Pain Med 2001; 26:100-04.

11. Groh GI, Gainor BJ, Jeffries JT, et al. Pseudoaneurysm of the axillary artery with median-nerve deficit after axillary block anesthesia. J Bone & Joint Surg 1990; 72:1407-8.

12. Ben-David B, Stahl S. Axillary block complicated by hematoma and radial nerve injury. Reg Anesth Pain Med 1999; 24:264-6.

13. Pearce H, Lindsay D, Leslie K. Axillary brachial plexus block in two hundred consecutive patients. Anaesth Intens Care 1996; 24:453-8.

14. Selander D. Catheter technique in axillary plexus block. Acta Anaesth Scand 1977; 21:324-9.