Blog with interesting cases and/or problems related to anesthesia with discussion based on best evidence in the literature.

May 31, 2008

Sciatic Nerve block from Hell

A healthy (ASA I) 48 year old male presents for ORIF of ankle (bi malleolar fracture). Location is fast paced surgical hospital 1st case. Anesthetic Plan: placement of sciatic nerve catheter in OR before case begins, GA, and then saphenous nerve block post operatively in OR.

The posterior (Labat) approach was used. Sedation was provided with 150 mcg fentanyl and 2 mg of Versed in divided doses by an assistant. The stimulating current was initially set to 1 mA. Initial twitch was strong and the current was reduced to 0.2 mA. A dorsiflexion twitch was still present, therefore the 17 G stimuplex needle was withdrawn slightly. Now at 0.3 mA, a very slight twitch was visible and after aspiration 30 mL of 0.5% ropivacaine was injected containing 1:400k epinepherine. The patient did not complain of severe pain during injection. Because of time constraints the patient was immediately placed supine and GA was induced w/ Propofol and an LMA was placed without difficulty. The lateral aspect of the ankle was reduced and plated first. No additional fentanyl was required during this portion of the case. During the placement of screws into the medial malleolar fracture, an additional 100 mcg of fentanyl was required. At the end of surgery, the LMA was removed, and 0.5% Ropivacaine was injected at just deep to the sartorius muscle for 7 mL and then an additional 5 mL was injected just medial to the tibial tuberosity.

In the PACU the patient complained of pain and was given an additional 100 mcg of fentanyl for pain. The ankle was tightly wrapped and braced, but sensory exam using ice revealed a dense block on the lateral leg just below the knee. The patient was discharged soon after to home.

At 9 pm on the day of surgery, the patient was contacted by phone for follow up. The patient complained of urinary retention that lasted until aproximately 7:30pm (from time of discharge at 12 pm). Although the patient was able to urinate, he stated that it was very difficult and that he was experiencing severe numbness of his entire right lower extremity, right buttocks and his entire penis. The patient described a very dense block in the sciatic nerve distribution, however, he was experiencing 8 to 10 out of 10 pain at the surgical incision site on the lateral portion of the ankle. The patient had numbness that went down to just below the mid portion of the lateral leg and then stopped. The patient self d/c'd the continuous infusion of bupivacaine 0.25% at 7:30pm.

On POD 1, the patient still complained of numbness to his buttocks and groin area, and entire right upper leg. He felt that the numbness was less dense however. He did not experience any further problems with urinary retention, and his penis had nearly returned to normal.

On POD 2, the patient described a significant improvement. He described a circumferential area of slight tingling and dysesthesia of the upper leg. He estimated that it was about 90% back to normal in terms of sensation. The patient did not complain of any motor deficits.

Using the labat approach for sciatic block for ankle surgery may not be the best approach. I choose this approach for a few reasons. 1) Less local anesthetic is needed at this level adequate block, 2) time of onset is faster according to one study, 3) more reliable block and 4) I can place a sciatic block more quickly using the labat or posterior approach when compared to a lateral popliteal or posterior popliteal approach. Nevertheless, this may not be the ideal block for ankle surgery for a patient destined to return home given that a more proximal sciatic nerve block makes ambulation more difficult and may result in urinary retention. In this patient it is likely that an intraneuronal injection occurred. With an intraneuronal injection, local anesthetic was not able to spread around the nerve to the same degree as it would with an injection outside of the nerve sheath (epinuerium). The local anesthetic then tracked proximally and distally. Clinically this resulted in urinary retention and a dense penile block. The duration of anesthesia was also atypical (lasting nearly 48 hrs). This is expected with an intraneuronal injection of local anesthetic. The patient seemed to develop only anesthesia to the common peroneal nerve componenent of the sciatic nerve. During the case, I injected after seeing dorsiflexion. Although, there is not much published information dealing with anesthesia of a single component of the sciatic nerve, this seems to represent a case of a common peroneal nerve block, sparing almost completely the tibal componenet. The skin overlying the ankle is served by the sural nerve which is a continuation of the tibial nerve. This portion was not numb according to the patient despite a dense block at other locations of the leg. Therefore, when performing a sciatic nerve block, consider whether the surgical site will include the common peroneal nerve or the tibial nerve. For ankle surgery, accepting plantarflexion would allow blockade of the tibial component and thus result in higher rate of success. The common peroneal nerve serves the lateral leg just below the knee and the dorsal aspect of the foot, and stimulation of this component results in dorsiflexion.

This patient complained a circumferential numbness around the thigh. It is likely that some of the numbness in the femoral nerve distribution was a result of the tourniquet used during surgery for aproximately 2 hours at 300 mmHg. Indeed, nerve injuries from tourniquet use, especially at times greater than 2 hours, are well documented. Paralysis from tourniquet use has been estimated to occur with an incidence of 1:8000. Less severe and subclinical neuropraxias likely develop with far greater frequency. Recommendations include maintaining pressure at no greater than 150 mmHg greater than the systolic blood pressure and deflation after no more than 90 to 120 min. Nerve damage from tourniquet use may also occur despite following these guidelines. For example, a healthy patient suffered severe conduction block of both sensory and motor fibers localized to the margins of a tourniquet after hand surgery. The tourniquet was at 300 mmHg for 45 minutes, well within the current recommendations for tourniquet use.
Peripheral nerve injury is classified into three degrees of severity:
A) Neuropraxia-myelin damage
B) axonotmesis-loss of axonal continuity with intact endoneurium.
C) Neurotmesis-loss of axonal and endoneurial continuity.

This patient suffered a neuropraxia-nerve dysfunction that lasted longer than clinically expected from the local anesthetic block alone. In this case, the nerve axons and fascicles remained intact, and injury was likely a result of pressure induced ischemia. Although, neuropraxia may result in months of nerve dysfunction, this patient was essentially back to normal after three days. In order to cause more permanent neurologic injury, an injection must disrupt the nerve fascicles causing a disruption of axons.

In this case all steps to avoid neurologic damage were followed: the patient was lightly sedated, injection only occurred at >0.2 mA, injecting against excessive resistance was avoided, and a blunt tuohy styled needle was used. Nevertheless, following these recommendations religiously will not prevent all intraneuronal injections and severe nerve damage can still ocurr, although the likelyhood is decreased tremendously. Recently, in the journal Anesthesiology, Tsui and colleagues published their measurements using electrical impedance (EI). This was done in a pig model, but is probably relavent to humans. In their study, the EI increased by greater than 50% from baseline when the needle entered a nerve. I did not measure EI in this case and it would have been interesting to see whether this parameter would have been an early clue that an intraneuronal injection was about to occurr. I am currently measuring EI in all my nerve blocks to see if this might be clinically useful.

Options available to the clinician facing a possible nerve injury after peripheral nerve block

If a patient complains of pain, weakness and/or paresthesias after surgery, consideration should be given to all possible causes, as opposed to assuming the cause to be the nerve block. If a patient still has significant complaints 1 to 2 days after surgery which do not appear to be resolving, consideration of nerve conduction studies should be given. Nerve conduction studies will only detect injury to myelinated nerves and so will not be useful in patients' whose only complaint is pain. If the study is normal, a neuropraxia is likely and expectant management is indicated. If abnormal, then electromyography (EMG) studies can be used to localize the lesion. It takes 2 to 4 week after injury for EMG to detect injury.

Fortunately nerve injury following peripheral nerve blockade is extremely rare. The commonly sited incidenc is 0.4%. However, vigilance and impeccable technique is still required to avoid what could be a devestating injury.

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