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Delayed Onset of Motor Block in the Hand following Interscalene Block
February 22, 2008
Jack Vander Beek
I’d like to present an interesting case which gives hints to the actions of local anesthetics in regional blocks. It involves a young man about 19 years old who came to surgery for repair of a rotator cuff tear and who received an interscalene block with bupivacaine.
This young man was very calm and even enthusiastic about the block and the interscalene block procedure under ultrasound guidance went well. Bupivacaine 0.375% with epinephrine, 25 mls, was used. The local anesthetic volume was divided as injected; between the brachial plexus sheath and the anterior scalene, within the sheath, and between the sheath and the middle scalene. The onset of the block was fairly rapid, and within a few minutes he could not raise his arm. He could however still move his fingers and even produce a forceful grip although he reported some numbness in his thumb and forefinger. He was reassured that it was common to be able to still be able to move his hand and fingers.
A few minutes later the patient was rolled back into the operating room where he was given a general anesthetic and the surgical procedure was performed without incident and took a little over an hour. He required very little general anesthetic during the procedure and after transfer to recovery room he awakened very quickly and reported no pain at all in his surgical shoulder and arm and he could still move his hand and fingers. He spent approximately an hour in the recovery room and was transferred back to the outpatient unit to complete preparations for his discharge to home.
After approximately 2 more hours he noted he could no longer move his hand and fingers and had no grip. His concerned mother notified the nurse who, in turn, called the anesthesiologist who was still working on scheduled cases in his room. The anesthesiologist reassured the unit nurse that this happened sometimes and that it would “wake up” when the block wore off. The patient was not upset or concerned by this extension of the block, he was in fact happy that his shoulder did not hurt and was anxious to be discharged.
The patient was discharged and the block lasted through the night and completely dissipated the following day with no sequela.
Okay, big deal, right? Maybe not big deal, but interesting deal. Most conventional wisdom holds that a block, regional or other, caused by local anesthetic is “set” after a given amount of time. For bupivacaine, 40-50 minutes is usually given as the time in which the molecules have found their musical chairs and are comfortably seated for the duration. AND YET, here is a case where the block progressed for at least 4 hours after placement.
Like a Sponge
The direction of onset is not surprising. Proximal to distal progression makes sense since nerves that will distribute to more proximal end organs will exit the brachial plexus nerve bundle earlier and therefore need to be arranged more superficially to make exit easier. Since they are more superficial the local anesthetic solution will come in contact with those early departure axons more quickly because of their exposure. Conversely the nerves in the bundle that will be the last to depart the bundle will be those located most centrally in the nerve bundle. This is the most efficient evolutionary arrangement available.
When looking at nerve groups (bundles and fascicles) exiting the cord for distribution down an extremity, we would expect to see the following characteristics.
- The closer to the spinal cord, the larger the overall bundle.
- The closer to the cord more axon and glial tissue and the less connective tissue (accounts for more hollow appearance on ultrasound)
- As distance from the cord increases, more axon is leaving the bundle to connect to end organs, less nervous tissue (axon & glial) is present, more organizing and supporting connective tissue is present.
- As distance increases overall diameters will begin to shrink.
Therefore; the closer to the cord the farther it is, in terms of diffusion, from the outside of the nerve group to the inside. So if you are surrounding a bundle, for instance the trunks of the brachial plexus, with local anesthetic, the diffusion of the local from the outside to the inside will result in a stabilization of the axonal membranes in the most proximal distributions to the most distal, in that order. The most distal (most internal) axons will only become blocked when and if the anesthetic molecules reach them. In my opinion the efficiency of the diffusion of the local through the substance of the radius of the bundle will depend upon;
- The beginning concentration of the local anesthetic solution being used. (density of the molecules available, therefore the gradient of the solute at the leading edge of the solution injected)
- Total volume and distribution of volume of the local anesthetic injected for the block. (higher volumes can delay redistribution away from the site by vascular absorption and normal circulation of the interstitial fluid)
- The pH of the nervous tissue and interstices of the area being injected for the block (slight variations in the area toward the more acidic will cause more the local anesthetic molecules to become ionized and thereby unavailable for crossing the lipid membrane and participating in the block. This is usually seen clinically as a delayed onset of a block. Tissue acidosis can be caused by conditions such as trauma, nearby infection, or peripheral micro-vascular disease.)
- The physical size (radius) of the target nerve.
- The distribution of the local solution around the circumference of the nerve bundle. Local solution which is placed only on one side of the circumference of a nerve will take longer to completely diffuse through a nerve than solution placed in more than one place or even complete around the nerve. (How much this really effects block efficiency in real life is probably negligible.)
- The number and character of barriers that the local molecules must cross on their passage to the center of the nerve. As discussed above, while more distal nerve bundles are smaller in diameter they will contain more connective tissue packaging that resists diffusion.
- Block efficiency is greatly influenced by the proximity of the local solution injection to the target nerve bundle but this only sets up the conditions described above for the passive phase of diffusion into the nerve.
In an empty sink basin, put a drop of black ink on a wet sponge, the ink molecules will gradually diffuse to the center of the sponge. Whether the ink color will be noticeable at the center will depend on how big the drop, how dark the ink is, and how thick the sponge is. If you place the sponge in a slow moving stream of water and apply the ink you have a situation closer to the regional nerve block.
The brachial plexus model provides the ideal example for tailoring the placement of the local to the site of the desired effect.
It is really is true that wrist & hand pain is best addressed from a brachial plexus block at the axillary approach (or even below), mid-arm & elbow pain to mid-upper arm is best handled at the supraclavicular level of the brachial plexus, and pain of the shoulder & clavicle to the sternal border can be covered from the interscalene approach.
Sure, a lot of times the hand is numb after an interscalene, but sometimes it’s not and it’s always numb after the axillary level block. So why tempt fate if you don’t have to. Using these 3 approaches to the brachial plexus any of the situations can be handled with confidence and when the unusual need arises, such as a cardiac patient with no neck, and bad bad COPD comes to you for the total shoulder arthroplasty, you will know that a high supraclavicular approach with less than 20 mls stands a good chance of covering the pain while missing the phrenic (possibly) and that an axillary approach is probably not going to get you there.
So, Maybe it’s the Placebo Effect! (WoooOOOoooOOO ← scary other-worldly sound)
So applying the scenario described above to the problem child of blocks; the Sciatic.
The sciatic is a large caliber nerve and yet many times we make the mistake of expecting it to block like a small nerve bundle. I don’t know about your situation but at our house there is an ongoing “discussion”, especially with a few orthopedic surgeons as to whether a sciatic block is necessary or desirable for pain control after total knee arthroplasties. The logic goes that most people don’t have posterior pain after the operation and you only need to observe patients in recovery to understand this. Okay, well I guess some patients do well with a femoral only but a lot require quite a bit of narcotic for something after femoral-only block.
The femoral-only viewpoint was bolstered by observations that patients with failed sciatic blocks (quite common at our operating room in the earlier days) were completely comfortable. The qualification for inclusion to the failed-sciatic club was that the patient could move and feel their feet. We were expecting every sub-gluteal sciatic to diffuse to the center of this large nerve and thereby block the foot. The group lament went like this; “Well, it doesn’t look like your sciatic block worked, but luckily, the patient didn’t need it anyway. Look how comfortable he is!”
Using the model of the above 19 year old guy’s brachial plexus, all of these comfortable post-op total knee patients who could move their feet, actually had very good sciatic blocks even though they could move their feet. It is, in fact, remarkable that so many people with sciatic blocks had blocks that progressed all of the way through to their feet. It is certainly not necessary to block every fiber in the sciatic in order to have a comfortable post-op total knee patient and in fact a numb foot is not appreciated by physical therapy the following morning when they are trying to ambulate the patient.
The Conclusions
The block progression isn’t over ‘til it’s over. While some, maybe most, blocks are “set” after 30-40 minutes, it’s a continuum— some require less time, some require more.
The block may be therapeutically effective without being complete. The aim should be comfort. Remember that the majority of the time we want a pain block, we’ll take full sensory, but we need not demand motor.
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