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Correspondence |
University of Alberta Hospital, Edmonton, Canada, E-mail: btsui{at}ualberta.ca
To the Editor:
It is with great interest that I read the letter by Ramachandran and Ponnusamy describing successful spinal anesthesia after multiple attempts with a distinct "give", but no free flow of cerebrospinal fluid (CSF).1 The authors of this article should be commended for their cautious management of this infrequent clinical situation. I agree with the authors’ statement that "common sense dictates that the procedure be repeated, but if the outcome remains the same, and the patient refuses general anesthesia the options are limited." I would also like to point out other options that may be considered. Although radiological imaging with contrast may be impractical and cumbersome in some settings, this diagnostic approach would verify exact needle position prior to injection of the local anesthetic. The recently introduced epidural stimulation test can also provide objective assessment of needle location, without cumbersome equipment.
Electrical stimulation is a useful and reliable ‘real-time’ technique to confirm epidural insulated needle placement.2–5 Not only do muscle twitches elicited by low current stimulation detail the position of the insulated needle within the epidural space, but the threshold current for motor responses can also be used to predict intrathecal placement.2–5 Insulated needles correctly situated within the epidural space should result in muscle twitches being elicited with a current well above 1 mA. However, whenever a motor response is detected at currents below or around 1 mA, intrathecal, or subdural placement should be suspected. Alternatively, the needle may be resting against a nerve. Under very low current stimulations, intrathecal needle placement will likely produce a bilateral motor response. Subdural placement may elicit multiple patchy segmental responses; whereas needles placed against a nerve root will elicit a unilateral single level motor response. In addition, paresthesiae are likely to occur if the spinal needle is placed directly against a nerve root in an awake patient. The advantage of real-time stimulation testing is that needle location can be assessed objectively, even in the absence of CSF flow, before injecting a loading dose of local anesthetic.
The clinical response to the epidural stimulation test using an insulated needle, in the absence of free flow of CSF, has not been reported. This scenario might reproduce the local motor responses associated with intrathecal placement of an insulated needle at low current (< 1 mA). I would caution that experience with the technique is based upon results using an insulated needle or catheter, whereas commonly used spinal needles are uninsulated. Thus, even if a confirmation test had been utilized, the authors’ assertion that the (spinal anesthesia) procedure be repeated, still holds true.1
Footnotes
Accepted for publication December 17, 2005.
References
1 Ramachandran K, Ponnusamy N. Dry tap and spinal anesthesia (Letter). Can J Anesth 2005; 52: 1104–5.
2 Tsui BC, Tarkkila P, Gupta S, Kearney R. Confirmation of caudal needle placement using nerve stimulation. Anesthesiology 1999; 91: 374–8.[Medline]
3 Tsui BC, Wagner A, Cave D, Seal R. Threshold current for an insulated epidural needle in pediatric patients. Anesth Analg 2004; 99: 694–6.
4 Tsui BC, Wagner AM, Cunningham K, Perry S, Desai S, Seal R. Threshold current of an insulated needle in the intrathecal space in pediatric patients. Anesth Analg 2005; 100: 662–5.
5 Tsui BC, Wagner AM, Cunningham K, Perry S, Desai S, Seal R. Can continuous low current electrical stimulation distinguish insulated needle position in the epidural and intrathecal spaces in pediatric patients? Paediatr Anaesth 2005; 15: 959–63.[Medline]
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