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*Department of Anesthesiology and Pain Medicine, University of Alberta, Edmonton, Alberta, Canada; and Department of Pediatric Oncology, Stollery Children's Hospital, Edmonton, Alberta, Canada
Address correspondence and reprint requests to Ban C.H. Tsui, MSC, MD, FRCP(C), Department of Anesthesiology and Pain Medicine, University of Alberta Hospitals, 3B2.32 Walter Mackenzie Health Science Centre, 8440–112 Street, Edmonton, Alberta, Canada T6G 2B7. Address e-mail to btsui{at}ualberta.ca.
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Abstract |
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Introduction |
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Although reports of perioperative nerve injuries in pediatric patients receiving thoracic epidural anesthesia are rare (11), failure to recognize a misplaced thoracic epidural needle or catheter in the intrathecal space followed by the injection of the usual epidural dose could result in a total spinal block or serious neurological sequelae. In adults, inadvertent intrathecal or intraneural needle placement may possibly be identified by patient reports of paraesthesia or pain on injection. In children, this is not possible because of the need for deep sedation or general anesthesia before neuraxial block placement. Therefore, the development of an objective means that enables early warning of misplaced subarachnoid needles is desirable.
Direct intrathecal placement of thoracic epidural needles or catheters in patients would be unethical because of the risk of spinal cord injury at these higher vertebral levels. On the other hand, diagnostic and therapeutic lumbar punctures require intrathecal needle placement at lower vertebral levels where there is a minimal risk of spinal cord injury. By using pediatric patients scheduled for lumbar punctures, we can mimic the scenario of an epidural needle that has inadvertently entered the intrathecal space. In this model, insulated lumbar puncture needles can permit the determination of the electrical current necessary to elicit a motor response to stimulation in the intrathecal space. Existing data suggest that a much lower threshold current will be needed to elicit motor responses from a needle in the intrathecal region than in the epidural space (4–6,12). The purpose of this study is to establish the usual threshold current range in the intrathecal space.
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Methods |
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Patients were administered IV propofol for sedation during the procedure. Patients were then turned to the lateral position and the appropriate dermatome for lumbar puncture (L4-5) was identified using bony landmarks. All lumbar punctures were performed by a single nurse practitioner. A stylet from a 24-gauge Sprotte needle (Pajunk, Dyna Medical Corp, London, Ontario, Canada) was fitted into a 24-gauge Pajunck unipolar needle with a Sprotte tip and connecting cable. After sterile preparation, the skin was punctured using either an 18-gauge or a 20-gauge introducer needle. Through the introducer needle, the modified unipolar Pajunck needle (with Sprotte tip and stylet) was advanced into the intrathecal space as suggested by the presence of a "pop." A separate investigator connected a peripheral nerve stimulator (MaxiStimTM Model ST5; Life-Tech, Inc., Stafford, TX) to the insulated needle. The negative lead of the nerve stimulator was attached to the metal connector of the needle. The positive lead was attached to a surface electrocardiogram (ECG) electrode placed remotely on the patient's trunk. The nerve stimulator frequency was set to 1 Hz with pulse width 0.2 ms. The output current was gradually increased from 0 mA until a twitch response was visible at the corresponding myotome or until a maximum current of 20 mA was attained. The needle hub was checked for cerebrospinal fluid (CSF). If CSF was not present, the nerve stimulator was turned off and the needle was advanced until CSF was observed. At this point, the threshold current was then retested. After the data were recorded, the nerve stimulator was removed and the procedure was completed in a standard fashion.
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Results |
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In 4 cases, CSF was obtained on the first attempt. In the remaining 16 children, the nurse practitioner believed that the needle had entered the intrathecal space based on her subjective observation of a dural "pop," but no CSF was observed in the needle hub. Needle stimulation testing was conducted at this level. Seven of these patients failed to show any muscle twitch despite stimulation with a maximum current of 20 mA. The remaining nine subjects exhibited muscle twitches at an average current of 5.2 ± 2.4 mA. These nine needles were then further advanced an average of 3 mm until CSF was present. Retesting elicited a motor response at an average current of approximately 0.6 ± 0.3 mA.
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Discussion |
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1 mA indicates with a 100% positive predictive value that a stimulating epidural needle is within the intrathecal space. Conversely, a perceived dural pop using our custom stimulating needle had only had a positive predictive value of 20%. However, this is not surprising, as it is likely that the perceived pop indicated passage of our custom insulated needles through the ligamentum flavum. Thus, needle advancement was halted at a point before advancement of the needle tip through the dura. In clinical practice, it is the presence of CSF and not the observation of a pop that is the standard for confirming entry into the intrathecal space. The seemingly infrequent success may also have been influence by the nurse practitioner's inexperience with the custom insulated 24-gauge Sprotte needles, as she was accustomed to using only 22-gauge Quincke needles. In addition, none of the nine needles in the patients who had an initial twitch with a current >1 mA were in the intrathecal space, indicating a 100% negative predictive value. A previous study in pediatric patients demonstrated that a mean current of 11.1 ± 3.1 mA (range, 6–17 mA) was necessary to elicit a motor response in the epidural space via an insulated needle (Fig. 1) (9). Thus, the results from our current investigation are in keeping with this previous work. In addition, we have confirmed the hypothesis that there is a significant difference between the Tsui test current thresholds for the needles placed in the intrathecal and epidural spaces (P < 0.01). It would have been ideal to determine the epidural and intrathecal electrical threshold values in each patient. Reliable confirmation of epidural needle tip location uses the technique of LOR, which is difficult unless performed using a large-bore Tuohy needle. As the patients in this study were having deliberate lumbar punctures performed, it would have been unethical to have used large-bore needles. Further, in our previous study (9) that determined the threshold current in the epidural space of pediatric patients using LOR via 18-gauge insulated Tuohy needles, it would have been equally unacceptable to have deliberately advanced an 18-gauge Tuohy needle into the intrathecal space before catheter placement for epidural analgesia. Therefore, we chose to perform two separate studies using accepted clinical techniques. In the first, we determined the threshold current in the epidural space after LOR and in the second we measured the threshold current in the intrathecal space in patients undergoing intended intrathecal puncture. This study design allowed us to determine the threshold stimulation currents for these two spaces without jeopardizing patient safety.
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In one child in our study, the muscle twitches were observed at a higher myotome (L2) than intended (L4-5). This is consistent with other work demonstrating the inaccuracy of using surface anatomy to identify a spinal level. This should serve as a reminder that clinicians should carefully select the lowest possible interspace for intrathecal injection to reduce the risk of neurological damage from misinterpretation of the vertebral level (13).
Electrical stimulation has been applied to neural structures for neurophysiologic evaluation or pain control for many years (14). Spinal cord stimulation has been suggested to be an effective and safe means of controlling pain on a long-term basis (14–18). Complications associated with spinal cord stimulation have been reported to be very infrequent. A stimulating electrode placed in the intrathecal space has also been used with minimal complications (18). In our reported clinical trials, none of the studied patients experienced any discomfort or obvious side effects from the stimulation test (1–9). It is anticipated that the risk of brief (<30 seconds) intermittent stimulation as used in this test would be even less than the risk of chronic (months to years) epidural/spinal stimulation as used in long-term pain management. Because a motor response was expected to be elicited at a very low current (<1 mA) in the intrathecal space, the current output was carefully increased from zero and immediately stopped once motor activity was visible. Thus, the nerve stimulator used in the test must have a sufficiently sensitive output control to allow a gradual increase in current. Furthermore the duration of stimulation should be brief (<minutes).
In conclusion, this study established that the mean threshold current Tsui test criteria for an insulated needle in the intrathecal space is <1 mA. This is lower than the established criteria for an insulated needle in the epidural space (6–17 mA) (9). We would like to stress that electrical stimulation is not intended to replace LOR but rather is to be used as an adjunct to confirm optimal epidural needle placement. Further studies will be required to determine if this technique can minimize the risk of spinal cord or nerve injury during epidural catheterization in anesthetized children. Readers are cautioned that as a result of the small number of patients studied, the milliamperage current settings for insulated needles are intended as guidelines and may require adjustment as experience increases.
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This article has been cited by other articles:
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  B. C. H. Tsui Epidural stimulation test criteria. Anesth. Analg., September 1, 2006; 103(3): 775 - 776. [Full Text] [PDF]
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B. C. H. Tsui Is It Time to Perform All Thoracic Epidural Placements Under Fluoroscopy? Anesth. Analg., May 1, 2006; 102(5): 1586 - 1586. [Full Text] [PDF]
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 B. C.H. Tsui Verifying spinal needle location in the presence of a "dry tap". Can J Anesth, April 1, 2006; 53(4): 424 - 425. [Full Text] [PDF]
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B. C. H. Tsui and K. Armstrong Can Direct Spinal Cord Injury Occur Without Paresthesia? A Report of Delayed Spinal Cord Injury After Epidural Placement in an Awake Patient Anesth. Analg., October 1, 2005; 101(4): 1212 - 1214. [Abstract] [Full Text] [PDF]
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B. C. H. Tsui, D. Emery, R. R. E. Uwiera, and B. Finucane The Use of Electrical Stimulation to Monitor Epidural Needle Advancement in a Porcine Model Anesth. Analg., June 1, 2005; 100(6): 1611 - 1613. [Abstract] [Full Text] [PDF]
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