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Spinal cord transmission of afferent neuronal activity despite surgical conduction anesthesia

Maisonneuve-Rosemont Hospital, Montreal, Canada, E-mail: joanne.guay{at}umontreal.ca

To the Editor:

It remains unclear as to what is the most reliable method for testing the anesthetic level after intrathecal injection of local anesthetics. Although transcutaneous electrical stimulation (TENS; 5 sec, 60 mA, 50 Hz, 0.25 msec square-wave electrical impulse) has been proposed as an equivalent to surgical incision, a recent study in patients undergoing Cesarean delivery demonstrated that TENS could be felt by 30% of patients despite an adequate surgical block.^1,2 This observation attests to spinal cord transmission of afferent neuronal activity despite adequate conduction anesthesia. We recently witnessed evidence of this phenomenon in a clinical setting.

A 56-yr-old male patient presented for elective repair of a pathologic fracture of his left acetabulum under spinal anesthesia. He had previously been treated for multiple myeloma, and three weeks prior to surgery, he developed a herpes zoster lesion at his left T₄ dermatome. Medications at the time of admission included gabapentin 800 mg tid, and a transdermal fentanyl patch (25 µg·hr^–1). Preoperatively, the subjective pain rating related to the zoster lesion was 4/10 on a verbal ten-point scale. A spinal block was performed at the L₃-L₄ interspace using isobaric bupivacaine 15 mg and fentanyl 15 µg. The patient was transferred thereafter from a sitting to a supine position. A propofol infusion was started at 100 µg·kg^–1·min^–1.

After verifying an absent response to skin pinching, the surgeon incised the skin overlying the left iliac crest (T₁₂-L₁ dermatome). The patient immediately complained of a burning type pain which he clearly localized to the left T₄ dermatome, and quantified at 7/10. He reported having no pain at the level of the incision. The patient underwent general anesthesia with a laryngeal mask airway. Anesthetic maintenance was provided with 1% sevoflurane and 50% nitrous oxide for surgery which proceeded uneventfully and lasted 120 min.

Following emergence, the patient had complete motor block of both legs, and residual sensory levels at T₉ and T_11, as evaluated by both ice and pinprick. He rated his herpetic pain at 4/10; and this decreased to 1/10 in the postanesthesia care unit where he denied any pain at the surgical site. After complete regression of the spinal block, the patient was treated with sc hydromorphone and resumed gabapentin and the fentanyl patch. There was no further exacerbation of his herpalgia during the hospital stay.

This case demonstrates a clinical scenario where a surgical stimulus in an anesthetized dermatome evoked a referred sensory response in a proximal non-anesthetized dermatome. This phenomenon is in accordance with the demonstration by Zaidi et al. that an adequate surgical block does not abolish all afferent sensory pathways.² Pain is the most common complaint of patients suffering from herpes zoster, and these patients may experience pain in response to nonnoxious stimuli, where even the slightest pressure may elicit pain. We hypothesize referred pain/ hyperalgesia from the surgical incision to the inflamed/infected neural tissue (T₄) was probably elicited by a reflex arc activation similar to that described in referred muscle pain resulting from algogenic conditions in viscera or other deep somatic structures. Whether or not this phenomenon could have been prevented by preoperative application of lidocaine patches, a higher spinal block, or a simple intercostal or paravertebral block at the left T₄ level, remains speculative.³

Footnotes

Accepted for publication May 16, 2006.

References

1 Petersen-Felix S, Zbinden AM, Fischer M, Thomson DA. Isoflurane minimum alveolar concentration decreases during anesthesia and surgery. Anesthesiology 1993; 79: 959–65.[Medline]

2 Zaidi AS, Russell IF. Is transcutaneous electrical stimulation a realistic surrogate for genuine surgical stimulation during spinal anesthesia for cesarean delivery? Anesth Analg 2005; 100: 1477–81.[Abstract/Free Full Text]

3 Rowbotham MC, Davies PS, Verkempinck C, Galer BS. Lidocaine patch: double-blind controlled study of a new treatment method for post-herpetic neuralgia. Pain 1996; 65: 39–44.[Medline]

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