Nuances to Ultrasound-Guided Subgluteal Sciatic Nerve Block Part 1

Full Commentary on “Ultrasound-Guided Sciatic Nerve Block in Overweight and Obese Patients”             

by Abdallah et al, Regional Anesthesia & Pain Medicine Vol 38, Number 6, Nov-Dec 2013, p547-552

 

There are a number of points made in this article that I find very interesting and that I believe are worth discussing further. Some of my comments are in praise of the intent of the article, some comments are extensions of important points that were brought out by this article and some are differences in opinion (or at least clarifications of concepts) that I would like to make. The first point that catches my attention is the fact that the article emphasizes that with the utilization of ultrasound to perform a nerve block, it can be performed more quickly with equal effectiveness and with an added bonus of potentially decreasing the incidence of multiple complications. I also like that both arms of the study were performed only with ultrasound though this should only be attempted once a considerable degree of skill and experience is attained. I would recommend for most folks the use of nerve stimulation in addition to ultrasound. Nerve stimulation should be used as an alarm and to help confirm the position of the needle within the correct tissue plane (as described below) or to positively identify the contacted structure as the sciatic nerve.

Unknowingly, I incorporated several years ago the “Subgluteal Space Technique” that was described by Karmakar et al. My rationale was one of practical importance: speed & patient comfort. I recognized after the fact that I may be decreasing the incidence of complications by avoiding direct nerve contact. At the time that I started using an infragluteal/subgluteal approach to the sciatic nerve, I was routinely performing this block for Total Knee Arthroplasty (TKA). I believed at that time that it was important to perform the sciatic block high above the popliteal space because a lower block might risk missing some of the fibers supplying the posterior knee. I have found since that time that it is not necessary to block the sciatic nerve this high, and a popliteal approach is sufficient (see The ‘Right’ Way for more on this).

Identifying the sonoanatomy in the infragluteal region is relatively simple (Click HERE to read further Tips on identifying sonoanatomy for Subgluteal Sciatic Nerve Blocks), however it always bothered me (not as much as it bothered the patient I imagine!) to pull back some distance to readjust the needle more than once when my out-of-plane needle angle did not intercept the sciatic nerve several centimeters deep to the skin as I had intended. The relatively thin single injection needles will bend, and sometimes I would just pick a poor trajectory. Beyond this, sometimes it is difficult to get the sciatic nerve to twitch, multiple passes take extra time, and it is not always possible to recognize the needle tip with an out-of-plane technique (and I am using a relatively sharp 30 degree beveled needle). I used an in-plane technique for a while to address some of these concerns, but trajectory is an even bigger issue with that approach, and it entails traversing a great deal of real estate that is difficult to easily make numb.[nonmember]

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[ismember]Instead of retracting and redirecting my needle when I did not intersect the sciatic nerve, I began to use hydrolocation to my advantage. If I am off-course, I wait to feel the ‘click’ of penetrating the epimysium of the GM, and then I inject a few ml’s of local and watch the spread. I look for spread in a linear fashion between the two muscle layers. Now, I see this like doing a really deep TAP block! Even if the needle is a few centimeters away from the sciatic nerve, if you put the local between the GM and QF, the technique is a success. If you are too shallow or too deep, just pull back or advance the needle until the space opens for you. I utilize this exact same approach to the anterior sciatic nerve, and you can even use the nerve stimulator to directly stimulate the GM and QF muscles to determine your relative location if absolutely needed. You will see the GM twitch, then stop, and then you will see twitches deep to this. The patient can also inform to you that they feel the twitch ‘in a different place now’ (see ANTERIOR SCIATIC). Usually, the sciatic nerve becomes less anisotropic (it is more easily seen even at slightly different angles) after several ml’s of local are injected if the needle is properly placed between the GM and QF. With this technique, I am generally assured of completing the nerve block with a single pass, and I am less concerned about damaging the nerve through direct contact or from penetrating it inadvertently. Ultimately, I look for the nerve and select a path near it or I aim for the space in an ‘unlikely’ place for the nerve to rest. If I am unable to see the nerve at all, I go ahead and attach the nerve stimulator.

Another topic that this article raises is the choice of local anesthetic to use for single injection and continuous nerve blocks. A typical choice is to use 0.5% ropivacaine or bupivacaine for the primary block (to more fully block surgical stimulation) and 0.2% ropivacaine (or 0.125% bupivacaine) for an infusion because the rest and dynamic pain afterward is less plus it minimizes muscle weakness. Single injection nerve blocks are typically done with a more concentrated local anesthetic simply to introduce more drug mass to extend analgesia as long as possible. It is important to minimize the mass of local anesthetic required whenever possible without sacrificing patient comfort or creating the need for excess opioid use.

Because the design of this study (and possibly their common practice) utilizes a spinal for the primary anesthetic for TKA’s, this obviates the need to bolus the femoral catheter prior to surgery and should allow them to just initiate an infusion of dilute local anesthetic in the PACU or to add a small bolus then begin an infusion. It is not clear to me the advantage that was gained in the study design (or in their common practice if this is what they commonly do) to bolus the femoral catheter prior to surgery (especially if it was done as it appears to be described in the article and occurred after the spinal is performed). Even if I were bolusing the catheter through the needle after the catheter was placed because of some study design issue, I would prefer to evaluate the onset of the femoral nerve block which would need to be accomplished prior to the spinal anesthetic.  I say this especially since they could not have used ultrasound to evaluate local anesthetic spread to confirm the correct needle position for the primary nerve block, and details of how the catheter position is confirmed was not elucidated in the study. Granted, they are experts, and a femoral nerve catheter placement is a relatively simple technique. Baring some logistic concerns, I do not understand why they did not just wait until the PACU to bolus the catheter when they started the infusion especially since they only used 0.2% ropivacaine. In any event, they were able to use a lower mass of drug by bolusing with 0.2% instead of the more common 0.5% bupivacaine. Do you see how a thoughtful plan can help to minimize exposure to error and injury as well as optimize outcomes? This topic may or may not be a major issue in your mind, but consider the situation of a bilateral TKA and the potential mass of drug necessary to accomplish bilateral femoral and bilateral sciatic nerve blocks!

In this article, the authors ended up using a dilute solution for the femoral nerve block, and they chose for this experiment to use a concentrated solution for the sciatic nerve block. It is interesting to me that they chose a mixture of lidocaine 2% and bupivacaine 0.5% as there is evidence that the ultimate outcome from a mixture of these drugs is a slower onset than lidocaine and a decreased length of analgesia compared with bupivacaine. It would certainly  improve the likelihood than their technique would register successful data points when the two individual nerves are tested at the various collection times. This may have been chosen simply for the proof of concept for the experiment. As I said previously, it is a good strategy to use a more concentrated solution for most single injections. A continuous catheter can be held, extended, modulated or bloused with dilute or concentrated local anesthetic at any time, but usually you will want to have a single injection last as long as it possibly can because when it is done, it is done.

Usually, that is. In the specific case of TKA’s, it is important to provide optimal analgesia without compromising physical therapy. As a brief side comment, I am not particularly concerned with quadriceps function or weakness from the femoral nerve infusion interfering with physical therapy (and I will make the argument from experience that if done properly, it will OPTIMIZE physical therapy goals!), and the goals and regimen at many hospitals is different. The thing that will definitely interfere with physical therapy is common peroneal palsy and foot drop. If the patient cannot dorsiflex their foot as they step forward, they will be prone to tripping over their foot and falling. In fact, if they still have proprioceptive loss in their foot, they may be more prone to a fall.

I have found that 0.5% ropivacaine for the sciatic nerve will provide a dense block for surgery and overnight pain relief, but the motor block will end abruptly. Many times, however, bupivacaine will have an extended ‘motor tail’ and result in common peroneal palsy through the next morning. It may even happen with reduced volumes or when using only 0.25% bupivacaine. You can see this in another article in this same issue of Regional Anesthesia and Pain Medicine in the article by Nader et al page 492-502 where the authors had successful infragluteal sciatic nerve blocks with as little as 10 ml of local anesthetic placed under the CIEL (common investing extraneural layer), but 10% of bupivacaine patients (vs 0% of ropivacaine patients) were unable to meet ambulation goals for physical therapy the next day. This is more common with surgical cases done at the end of the day that have less time to wear off before morning physical therapy. It also makes surgeons nervous to see a patient the next morning that can’t dorsiflex their foot with their concerns about potential common peroneal nerve damage that they could have caused from the surgical procedure. It is an unfortunate trade-off that exists for some lower extremity surgeries where we need to end the nerve block in order to facilitate recovery.

Here’s the point about the local anesthetic choice for the sciatic nerve. This was an experiment to test the theory that a new approach to the subgluteal sciatic nerve block can be performed faster and with equal success even with obese patients. Read from another angle, one might conclude that the authors are suggesting that you ought to consider using this technique with the same local anesthetic solution. I am not sure that is what they are suggesting, and it could lead to issues for you the next day if you apply this specifically to TKA’s.

I need to make one further comment. Remember that I mentioned that the mixture they used may have a delayed onset and a quicker offset? I do not use a combination of 2% lidocaine and 0.5% bupivacaine, and I have never used it in a 1:2 mixture. I cannot tell you from experience how long you should expect to have the motor block last from this particular combination. What I will tell you is that a less concentrated option should still provide excellent analgesia overnight and will not interfere significantly with day of surgery or post-operative day 1 physical therapy. I would recommend ropivacaine over bupivacaine here if possible. The patient is going to feel a ‘step up’ in pain in the back of the knee at some point because the sciatic nerve block can’t be used throughout recovery. There is no way to time it perfectly, but don’t let your choice of local anesthetic (volume and concentration) regularly interfere with the recovery regimen at your facility. Again, do you see how a thoughtful plan can help to minimize exposure to error and injury as well as optimize outcomes?

Stay tuned for Part 2 of this series when I will discuss further points related to this study with further implications for your practice such as the relevance of their definition of nerve block ‘success’, the choice to maintain the infusion for 48 hours without holding it for physical therapy, the relative amount of opioids their patients received, and evaluating knee pain after TKA. [/ismember]

Not sure you want to tackle the anterior sciatic or subgluteal sciatic nerve block for your TKA’s? Click HERE to learn how to do a popliteal sciatic nerve block that will be just as effective and much easier to accomplish!