Motor response evaluation during brachial plexus block anesthesia: An ultrasonography aided study.

BACKGROUND: The sparing of ulnar nerve often leads to the failure of the upper limb blockade. It has been claimed that local anesthetic injection at the site of stimulator evoked finger flexion response is associated with highest success rate of a successful block. The lower trunk stimulation of plexus should yield similar results as this trunk contributes significantly for median and ulnar nerves of hand and forearm.
MATERIALS AND METHODS: The patients with intact anatomical upper limb structures underwent (a) combined ultrasound (USG) assisted nerve stimulator evoked motor response evaluation or in (b) open brachial plexus trunk stimulation evaluations. The individual patient's lower trunk stimulation motor responses were documented.
RESULTS: When combined the results of both USG and open plexus groups, wrist flexion was seen in 52%, finger flexion in 61% and forearm pronation or twitches of anterior compartment in 48% of total subjects studied. These responses were noted either separately or in combinations.
CONCLUSION: We conclude other than finger flexion, the wrist flexion the forearm twitches, pronation and wrist adduction may be used for lower trunk blockade and thus for higher accuracy.

INTRODUCTION

While the ultrasound (USG) has been widely accepted as the guidance modality of choice in administering the regional anesthetic drug to the brachial plexus, this may not be available at all centers.[1] The alternative, nerve stimulator assisted blocks has disadvantages of its own like increased time requirement, varied responses on stimulation, and unsuccessful blockade. The upper limb is mainly innervated by radial, median and ulnar nerves. However, the sparing of ulnar nerve often leads to the failure of the blockade. Very few reports are available on detailed techniques of nerve stimulation motor response techniques.[2] It is claimed that local anesthetic injection at the site of stimulator evoked finger flexion response is associated with highest success rate of a successful block.[3] We believe, therefore, that lower trunk stimulation of plexus should yield similar results as this trunk contributes significantly for median and ulnar nerves of hand and forearm. In this direction, we attempted evaluating the lower trunk evoked motor response using nerve stimulator, with USG assistance at the background during supraclavicular brachial plexus block. Indeed, this may help the anesthesiologists to know and understand the target response to be elicited for the best results, when nerve stimulation is used alone in a supraclavicular block.

MATERIALS AND METHODS

The data are provided from the series of patients underwent elective or emergency surgical procedures of upper limb under USG guided supraclavicular brachial plexus block (July 2014 to March 2015). The patients who were included in the study were almost with intact anatomical upper limb structures and underwent combined USG assisted nerve stimulator motor response evaluation. Patients underwent arteriovenous fistula creation for dialysis (14), single finger contracture release (1), minor injuries of the forearm (9), cross finger flap (2) were included for the study. Three patients had open brachial plexus repair with contralateral cervical roots transposition, and thus direct identification of roots (C8, T1) was possible in the intact limb and thus included for the study.

After proper placement of the patients in supine position, under aseptic precautions and use of lignocaine jelly, the USG evaluation (linear, high frequency probe, 6–13 MHz, Fujifilm, SonoSite, USA) of the brachial plexus was done next to subclavian artery in supraclavicular area. A separate attempt was made to identify the upper, middle and lower trunks of brachial plexus during the evaluation. The roots at interscalene level were traced downward to the supraclavicular fossa whenever there was difficulty encountered while identifying the trunks [Figure 1]. With inline approach, the nerve stimulator needle was passed to stimulate the lower trunks, specifically. The current needed was 0.4–0.7 mA for stimulation of lower trunks. A deeper penetration of the needle or a change of direction of the needle or USG probe was performed for reconfirmation of lower trunks was done, whenever necessary. The individual patient's lower trunk stimulation motor responses were documented. Any pain or discomfort during the needle stimulation warranted to terminate the evaluation. Patients were excluded from the study whenever unable to identify, stimulate, evaluate, re-evaluate or suspicious of lower trunk. After the identification of plexus, patients were further preceded with administration of local anesthetic solutions as applicable and current practice recommendations for the surgical procedure. A second confirmation was done during the drug injection, through a better attained fluid acoustic window, whenever possible.

Figure 1

The trunks in ultrasound evaluation (UT = Upper trunk, MT = Middle trunk, LT = Lower trunk)

RESULTS

Ultrasonographic aided nerve stimulation was carried out in total of 26 patients, and three patients underwent open brachial plexus roots stimulation under direct visualization (n = 29).

Among the patients who underwent USG stimulation, the lower trunk identification and stimulation was possible in 77% (n = 20) of individuals. Among them, 90% (n = 18) and 45% (n = 9) had upper and middle trunk identification was possible, respectively, and these were stimulated to obtain motor responses.

When lower trunk was stimulated, the observed motor responses were as follows. Wrist flexion was seen in 45% (n = 9), finger flexion in 55% (n = 11), forearm twitches including pronation were seen in 45% (n = 9). Smaller number of subjects had thumb movement (5%, n = 1), finger adduction or abduction (10%, n = 2), elbow flexion (5%, n = 1), anterior chest muscle twitches (10%, n = 2) patients.

When combined the results of both USG and open plexus groups, wrist flexion was seen in 52% (n = 12), finger flexion in 61% (n = 14) and forearm pronation or twitches of anterior compartment in 48% (n = 10) of individuals. These responses were noted either separately or in combinations; the data are briefed in console chart [Consort Chart 1].

Consort Chart 1

Motor response distribution among individuals

No patient had any type of complications during the evaluation.

DISCUSSION

Identification of the plexus using the nerve stimulator is technically challenging and time-consuming. Improper localization could end up with an incomplete anesthesia or adverse neurovascular injury.[4] De Andrés et al. have described the nerve stimulator response at various levels of the brachial plexus but not based on USG evaluations.[2] Anesthesia textbooks barely describe intricacies of nerve stimulator techniques, probably owing to the archaic nature of the method itself. More recent USG and combined nerve stimulation blocks have overshadowed the older methods. Majority of developing countries still depend on either blind or purely nerve stimulator brachial block techniques and have never achieved the high standards of accuracy provided by USG control. For routine clinical practice, the complexity of distribution of motor component to individual muscles through various roots and different joint movements caused by different muscle's actions makes anesthesiologist a composite task in understanding and application of nerve stimulation.

For anesthetizing the forearm and hand, it is obligatory to block median, ulnar and radial nerves and its distribution. However, the median dominates the supply over other two, and it is considered as the principal nerve of the hand.[5] In supraclavicular approach, ulnar sparing is well-known in patients who were anesthetized with local anesthetic even in best of the hands (for its T1 distribution). Haleem et al. claimed 100% success rate brachial block anesthesia when finger flexion as motor response was recorded with a nerve stimulator.[3] This implies the intrinsic muscles and flexors, which are largely supplied by C8-T1 component of plexus to be blocked for higher success. Anatomically, C8-T1 components join to form lower trunk of brachial plexus, share the median and ulnar nerve distributions as they descend down. If one attempts to obtain evoked motor response of lower trunk using nerve stimulator and injecting local anesthetic around it, undoubtedly increases the efficacy. In clinical practice, it is not always possible to obtain the very similar (mirror) stimulation evoked motor responses in every individual owing to different reasons. First, anatomically the different part of brachial plexus might get stimulated with a nerve stimulator. That is, at different divisions or different trunk stimulations. Second, the “dominance” of root supply on muscles may result in “unlooked-for” response.

For better understanding, the “dominance” of responses is explained with an example [Figure 2]. This evidence-based data are described in Gray's anatomy text book and explained for locating nerve root lesions, previously.[6] As we believe, the very similar principles can be applied for nerve stimulation for eliciting motor response. Wrist flexion can be caused by C6,7 (cervical) of flexor carpi radialis (FCR) or palmaris longus (PL) of C7,8 or flexor carpi ulnaris (FCU) of C7,8 and T1 (thoracic). FCR and PL receive their motor contribution as equal distributions from C6,7 and C7,8, respectively. However, it is not so with FCU; it receives the major dominant contribution from C8. When lower trunk is stimulated (0.5 mA current) for an anesthetic block, it results in C8, and T1 stimulation and surely not the C7. As shown, the dominant supply of FCU is C8; its stimulation is the key for the elicited wrist flexor response [Figure 2]. Thus, reciprocally, if wrist flexion is observed through FCU, the dominance of C8 is considered for lower trunk. However, one may argue the role of T1 for its dominance for lower trunk for a given stimulation. Essentially, its dominant response would have manifested elsewhere; e.g., at fingers. Similarly, each root and trunk can be explained for its dominance depending on the muscle response; the details for most common responses are shown in Table 1.

Figure 2

The example for dominance of roots on muscles

Table 1

Most common responses noticed while isolated root stimulation based on dominant responses

Prior to labeling responses as successful, one require to identify which of the observed muscle twitches during nerve stimulation suggest failures. Nerve stimulation can result in the stimulation of trunk in majority of patients or the root stimulation, depending on which the desired responses should be targeted. For simplicity, as described in Figure 3, the ipsilateral diaphragmatic contraction implies the needle direction is too medial, and thus a change in the direction is considered. In contradiction to this, trapezius twitches alarm the possibility of too lateral entry.[2] The most common response for an upper trunk stimulation observed is of the shoulder girdle, implies C5-axillary nerve or from C6 response of musculocutaneous nerve, the later manifests as elbow flexion. During the further course stimulation, the likely observed next response is of the middle trunk; the extensor responses at different levels. These responses at 0.5 mA current are not preferred; drug injection will cause inadequate anesthesia. Further advancement or placement of the needle medially and deeper (avoiding arterial puncture) may help in lower trunk or its roots identification. Isolated C8 response is observed mainly as wrist flexion and isolated T1 as finger flexion, adduction or abduction [Table 1]. The “dominance” of root supply for various muscles explains the possible responses; the observed responses should be used for identifying the roots or trunks. As earlier described, the median nerve is the principal nerve of forearm and hand, the C8- injection will result in successful block.[5] The ulnar nerve with “dominant motor, T1” shares the supply of hand; if identified, should be prioritized in identifying over C8-median component for drug injection.

Figure 3

Schematic representation of the possible nerve trunk fibers and responses which can get stimulated during nerve stimulation based supraclavicular block. The green colors represent “positive” responses and other colors represent “negative” responses

In majority of patients evaluated, the response observed were flexion of fingers or wrist and forearm twitches when lower trunk was specifically stimulated. Pronation of hand also can be considered significant positive response for identifying lower trunk. Therefore, the lower trunk stimulation motor response may be largely included finger or wrist flexion or pronation of the hand. However, anterior forearm twitches, wrist adduction too can’t be ignored which were recorded in minor subjects. Since the success rate is associated with finger flexion as motor response,[3] it goes with lower trunk stimulated responses and thus when one attempts supraclavicular blocks, it is essential that lower trunk should be targeted. Alternatively, in any stage of nerve stimulation, the positive responses should be considered in the priority manner for a successful block. These dominant positive responses should be identified in “cadre” manner from the center of the circle outward of Figure 3, when a prior one is unable to achieve. The USG evaluation was further supported by three cases of open roots stimulation of patients recorded during brachial plexus repair [Video 1].

At this juncture, the possible errors and limitations need to be considered. We used USG evaluation for identifying the trunks in our study; for higher accuracy, the trunks were routed higher-up to roots level. Further, the stimulated trunks were confirmed after injecting the drug; the trunks will disperse and seen ultrasonographically in a better fluid acoustic window. However, this methodology could not be followed for primary trunk identification since a prior local anesthetic injection-induced negative nerve stimulation. Alternatively, we presume saline injections could have improved as well as reduce the errors of primary identification of trunks and roots. In a small, but considerable number of patients the discrimination of trunks was not possible; especially between upper and middle ones. Stimulation of various divisions of trunks (anterior or posterior) could not be accurately made out; for the fear of arterial puncture and thus may contribute to operator errors. Further, the site of needle insertion was different in our series of patients, and this will not be analogous in nerve stimulator alone performed blocks.

CONCLUSION

The primary aim of nerve stimulation is to achieve higher accuracy with intentional reduction in dose, the total volume of injectable local anesthetic, to obtain longer duration of the block and effectiveness (dense block).[78] The utility of nerve stimulator technique is still valid, even in the era of evolving USG techniques especially in developing countries owing to latter's higher cost and need for expertise despite the fact that its combination favor more success and realistic. One-hundred times less expensive peripheral nerve stimulator equipment may provide nearly cent percent accuracy using our response-based approach if one understands what responses give best results for injection of local anesthetic. We conclude other than finger flexion, the wrist flexion, the forearm twitches, pronation and wrist adductions may be alternatively used for lower trunk blockade and thus for higher accuracy.