Anesthetizing the Phantom: Peripheral Nerve Stimulation of a Nonexistent Extremity

A 57-yr-old woman (American Society of Anesthesiologists physical status III) was scheduled for drainage of an abscess on the stump of her right below-the-knee amputation. Her medical history was significant for insulin-dependent diabetes, coronary artery disease, peripheral vascular disease, gastroesophageal reflux, and renal insufficiency. She had undergone her initial right below-the-knee amputation 6 yr earlier. Two months after her right below-the-knee amputation, she underwent a left below-the-knee amputation. She had constant bilateral phantom sensations that her feet were still present. The sensations were vivid and consisted of an entire foot on each side. They were not associated with perceived altered length, size, or location. She also had constant phantom pain that she described as sharp, stabbing, and throbbing. On a verbal pain scale (0 = no pain and 10 = the worst pain imaginable) she rated the phantom pain as 6. Physical examination was remarkable for a right amputation 4 inches below the knee with a 5-cm ulcer on the lateral aspect of the stump. Her daily oral medications included 20 mg omeprazole, 100 mg metoprolol, 80 mg furosemide, 325 mg aspirin, 300 mg gabapentin twice daily, and 10 mg cyclobenzaprine three times daily, as well as 4 U each morning and 6 U each evening of subcutaneous neutral protamine Hagedorn insulin.

A femoral and sciatic nerve block was planned. The procedure was explained to the patient, and she was encouraged to report any change in sensation or movement in her phantom extremity during the block placement. Standard monitoring was placed, and the patient was sedated using 100 μg fentanyl and 2 mg midazolam IV. A classic sciatic nerve block as described by Labat 5was performed using a nerve stimulator connected to a 21-gauge, 100-mm, short-bevel insulated needle (B. Braun Medical, Bethlehem, PA). The nerve stimulator was set at 1.5 mA. The initial insertion of the needle produced a hamstring motor response. The needle was redirected medially and the patient reported movement in her phantom foot. When asked to be more specific about the movement, she first said “It’s just moving;” after a few seconds she said “Its moving like this,” at which point she began tapping her index finger in a flexing motion in coordination with the 2-Hz signal of the nerve stimulator. The current was reduced to 0.5 mA, and 20 ml ropivacaine, 0.5%, with 1:400,000 epinephrine was injected. The patient was placed in the supine position and, using a nerve stimulator, a femoral nerve block using mepivacaine 1% with 1:400,000 epinephrine was performed to cover the saphenous nerve distribution. Three minutes after the sciatic block, the patient reported a decrement in her phantom pain to 3 (out of 10). When asked about her extremity 3 min later she said, “It’s just gone. There’s no pain.” When asked the same question 3 min after that she said, “No, I take that back. There’s no pain and my foot is numb.” She reported no changes in her contralateral phantom or pain. The patient subsequently underwent abscess drainage with no postoperative complications. The sensation returned in her phantom foot approximately 24 h after block placement. There was also return of her usual phantom pain at about the same time.

This case demonstrates the successful use of a nerve stimulator technique to elicit the sensation of movement in a phantom extremity. It further illustrates that the perception of movement can be replicated by a proximal stimulus without the presence of an extremity or peripheral cues 6 yr after an amputation. Although spinal, epidural, and axillary blocks have been previously reported as effective at blocking phantom limbs, to our knowledge this is the first report using a sciatic block to produce these results. Given the prevalence of lower limb amputations and the inherent difficulties in providing lower extremity blocks in this population, this technique, if reproducible, may help facilitate their performance.

Peripheral nerve stimulation has been a useful adjunct when performing regional anesthesia. Conceptually, by preferentially stimulating A-δ nerves with a low current and a 0.1-ms pulse width-square waveform, the needle can be located in close proximity to a peripheral nerve, facilitating success. In an amputee, not having a distal extremity may make this technique less desirable or impossible because of the lack of a definitive endpoint. Having an alternative method for placing a peripheral block such as the one described in this case could be helpful for anesthesia or perioperative pain management.

Our ability to evoke the sensation of movement in a phantom extremity using a stimulus for motor response illustrates a complex process of neural integration beyond simple sensory fiber or motor fiber nerve stimulation. Recent investigations have demonstrated that phantom limb sensations and phantom pain are complex phenomena that may be attributed to neural reorganization at multiple levels, including the spinal cord, brainstem, thalamus, and cortex, as well as factors in the periphery. 6The role of the spinal nerves and peripheral nerves have been demonstrated by several investigators who have shown that performing a spinal anesthetic, 1an epidural, 2or a brachial plexus block 3,4can eliminate the sensation of a phantom extremity and temporarily eliminate phantom limb pain, as well as induce phantom sensation and pain. 7Therefore, it is not unexpected that a sciatic nerve block could produce similar results in the lower extremity.

The exact mechanisms of this complex neural interaction are still unclear. However, this observation suggests that our knowledge about peripheral nerve stimulators for regional anesthesia, although practical, still has many areas that need further investigation. Urmey and Stanton 8highlighted part of this void and controversy when they demonstrated that it was possible to consistently elicit a paresthesia without simultaneously evoking a motor response while performing interscalene blocks.

Several factors encouraged us to attempt a nerve stimulator technique in this patient. The patient had severe coexisting disease and preexisting limb pain that we believed would benefit from the anesthetic and postoperative analgesic benefits of peripheral nerve blocks. The patient was also especially cooperative, articulate concerning her perceptions, and enthusiastic about the technique. This enabled us to describe the technique and prompt her for information about her phantom. We believe this aspect of patient education in this circumstance was essential for success.