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Interscalene Brachial Plexus Block: Shoulder Paresthesia versus Deltoid Motor Response: Revisiting the Anatomy to Settle the Controversy

Winnie, Alon P., MD

doi: 10.1097/00000539-200212000-00068
LETTERS TO THE EDITOR: Letters & Announcements

Department of Anesthesiology

Northwestern University Medical School

Chicago, IL

In Response:

I appreciate this opportunity to respond to the letter submitted by Drs. Sukhani and Candido, as it gives me the opportunity to correct a misconception concerning the performance of an interscalene brachial plexus block and the anatomy upon which it is based, a misconception that has existed in the literature for almost 20 years.

First of all, contrary to the introductory statement by Sukhani and Candido, in the original description of the interscalene technique of brachial plexus block, Winnie did not emphasize (or even suggest) “that only a paresthesia distal to the shoulder was acceptable to ensure successful block.” What he did state was that “usually, however, a very short distance under the skin the fascia is penetrated, a nerve root is encountered and the appearance of paresthesias to the arm and/or shoulder verifies that the needle lies in the interscalene space.”(1). The discrepancy between these two statements is the result of the fact that 10 years after publishing the original article, when writing my textbook, Plexus Anesthesia: Volume I: Perivascular Techniques of Brachial Plexus Block, the following sentence was erroneously (and unfortunately) transposed from the chapter on Subclavian Perivascular Technique of Brachial Plexus Block (2) to the chapter on Interscalene Perivascular Technique of Brachial Plexus Block (3): “Only a paresthesia below the level of the shoulder is acceptable, since a paresthesia to the shoulder could result from stimulation of the suprascapular nerve inside or outside the sheath.”(3). That statement applies only to the subclavian perivascular technique, as is clearly stated in the text concerning the paresthesia obtained in performing the subclavian perivascular technique which states “It is important before beginning a subclavian perivascular block to instruct the patient to say ‘stop’ when he feels an ‘electric shock’ to the arm and to tell the anesthetist verbally (not by pointing) where he feels it. The reason this is important is that a paresthesia to the shoulder is unacceptable, since it indicates that the needle has contacted the suprascapular nerve, and it is impossible then to tell whether this nerve has been contacted inside or outside the sheath of the brachial plexus. Therefore, only a paresthesia that radiates to any point below the shoulder is acceptable” [when performing a subclavian perivascular block](2). The accompanying illustration shows in great detail that the suprascapular nerve is a branch of the superior trunk, and once it leaves the sheath, it may be contacted by the advancing needle, giving the anesthetist the impression that the paresthesia indicates contact with the superior trunk inside the sheath rather than contact with the suprascapular nerve outside the sheath.

The important anatomical (and clinical) fact is that a properly performed interscalene brachial plexus block is carried out at the level of the roots of brachial plexus, not the trunks; so that regardless of whether the technique is carried out using a paresthesia or a nerve stimulator, contact with or stimulation of any of the roots must indicate that the needle is within the sheath, or more properly, within the fascia surrounding the roots of the plexus. Thus, I must disagree with the statement by Sukhani and Candido concerning the interscalene technique that “a shoulder paresthesia results from stimulation of the suprascapular nerve, which may be within or outside the fascial sheath, while a deltoid motor response results from stimulation of axillary nerve roots, which lie within the fascial sheath.” As stated above, the suprascapular nerve arises at the level of the trunks, so since the interscalene block is carried out at the level of the roots, a shoulder paresthesia or a deltoid motor response are both indicative of the fact that the needle is within the sheath in the interscalene space. This was demonstrated clinically 10 years ago by Roch et al. (4) who carried out interscalene blocks on 45 patients for shoulder surgery, obtaining paresthesias to the shoulder in 20 patients (45%) and distal to the shoulder in 25 patients (55%). Not surprisingly, they obtained surgical anesthesia adequate for shoulder surgery without supplementation in 100% of the patients. In a similar study, carried out 2 years ago (alluded to by Sukhani and Candido). Silverstein et al. (5) performed 160 interscalene blocks, also for shoulder surgery, but unlike Roch, using a nerve stimulator, evoking a biceps twitch in 61 patients (38%), a deltoid twitch in 54 (34%) and a simultaneous twitch in both muscles in 45 patients (28%). While there was one failed motor block in the deltoid group, surgical anesthesia was equally effective in all three groups, with no statistical (or clinical) differences between them. Knowing that the interscalene technique is carried out at the level of the roots of the plexus, the results of the Roch and Silverstein studies are predictable, but both their studies were performed because then “current texts of regional anesthesia” recommended that shoulder paresthesias not be accepted as an indication that the needle tip is near the plexus (6–8) or that using a nerve stimulator technique, a motor response (twitch) in either the biceps or more distal muscle group is required to indicate correct needle placement (9,10). While I have always personally favored the paresthesia technique, Silverstein et al. indicate that a significant advantage of the nerve stimulator is the information gained by stimulation of the phrenic nerve (located anterior to the brachial plexus, causing a diaphragmatic twitch), or the spinal accessory nerve (posterior to the brachial plexus, producing a trapezius twitch). Obviously, stimulation of either of these nerves should alert the anesthesiologist to redirect the needle accordingly.

On the other hand, since a subclavian perivascular block is carried out at the level of the trunks, and since the suprascapular nerve arises from the superior trunk, it is obvious that if one evokes a paresthesia to the shoulder or illicits a twitch response to a nerve stimulator of the muscle innervated by C5, C6, as correctly stated in my text (2), it is difficult, if not impossible, to tell whether the needle is stimulating the suprascapular nerve outside the sheath or the superior trunk within it. Therefore, I still believe that with this technique a more distal paresthesia or twitch response is mandatory.

To summarize, in view of all of the above, I must disagree with the conclusion of Sukhani and Candido that in performing an interscalene block one must seek a paresthesia distal to the shoulder, but I certainly agree with their conclusion that when performing an interscalene block using a peripheral nerve stimulator, a deltoid motor response is an appropriate end-point.

Alon P. Winnie, MD

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1. Winnie AP. Interscalene brachial plexus block. Anesth Analg 1970; 49: 455–66.
2. Winnie AP. Plexus anesthesia. Volume 1: Perivascular techniques of brachial plexus block. Philadelphia: WB Saunders, 1984: 157.
3. Winnie AP. Plexus Anesthesia: Volume 1: Perivascular techniques of brachial plexus block. Philadelphia: WB Saunders, 1984: 176.
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© 2002 International Anesthesia Research Society