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Correspondence |
Foothills Medical Centre, University of Calgary, Calgary, Canada, E-mail: beriaul{at}telusplanet.net
To the Editor:
We appreciated the case report by Morley-Forster et al. demonstrating the elegant utility of the Tsui test for the precise placement of a thoracic epidural blood patch.1 However, we are troubled by the unwarranted (in our opinion) caution the authors used to initiate their epidural entry at T 6–7 rather than begin at the T2–4 level where the indium cerebrospinal fluid flow (CSF) flow study demonstrated the CSF leak. Their stated reason for avoiding the T2–4 interspace was based on "a recent study (that) showed that the cervical and upper thoracic ligamentum flavum above T4 frequently fails to fuse in the midline."2 The concern arising from such defects is that without the definite loss of resistance afforded by the ligamentum flavum, the risk of dural puncture is higher. Does this admonition stand up to careful clinical scrutiny?
The author’s citation for this claim refers to the work from Lirk et al.2 In our opinion, the citation of Lirk’s anatomic studies of 52 embalmed cadavers must acknowledge, as Lirk did, "potential artifact resulting from the embalming or dissection process".2 Even if these cadaver studies did reflect midline ligamentum flavum gaps, these should not be of concern with a paramedian approach. Of note is the relative frequency of these potentially artifactual defects (partial or complete) at the different levels, with no complete gaps present at either T3/4 or T5/6, and only partial gaps present in eight cadavers at both T4/5 and eight at T5/6. At the T2/3 level, just two of 47 acceptable cadaveric specimens showed a complete gap in the midline ligamentum flavum. In similar previous lumbar epidural cadaver dissections reported by Lirk3 using the same methodology, the incidences of midline lumbar ligamentum flavum defects were reported as L1/2 (22.2%), L2/3 (11.4%), L3/4 (11.1%) and L4/5 (9.3%). This study also concluded that the lossof- resistance technique for midline epidural placement should be impaired, with attendant increased risk of dural puncture.
It is our belief that the possible risk of increased dural puncture derived from these cadaver studies is not substantiated by clinical experience in obstetric practice, where midline lumbar epidural approaches are routine. Nor is it substantiated in our cumulative nine-year experience of teaching residents and placing over 600 T4 thoracic epidural catheters (midline and paramedian) with only two dural punctures (unpublished data). Lirk acknowledged that "the clinical implications of our findings remain to be ascertained in studies identifying the frequency of entering the subarachnoid space without penetrating the ligamentum flavum in cervical epidural or high thoracic anesthesia." 2 We agree with this statement, and suggest that Morley-Forster et al.’s assertion that T4 epidural catheter placement, midline or paramedian is associated with a high risk of dural puncture, is not supported by sufficient clinical data.
Footnotes
Accepted for publication May 16, 2006.
References
1 Morley-Forster PK, Abotaiban A, Ganapathy S, Moulin DE, Leung A, Tsui B. Targeted thoracic epidural blood patch placed under electrical stimulation guidance (Tsui test). Can J Anesth 2006; 53: 375–9.
2 Lirk P, Kolbitsch C, Futz G, et al. Cervical and high thoracic ligamentum flavum frequently fails to fuse in the midline. Anesthesiology 2003; 99: 1387–90.[Medline]
3 Lirk P, Moriggl B, Colvin J, et al. The incidence of lumbar ligamentum flavum midline gaps. Anesth Analg 2004; 98: 1178–80.
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