I read with interest the article “Ultrasound Guidance in Caudal Epidural Needle Placement” by Chen et al.  1However, I have a couple of points to raise. The author’s assertion that the application of ultrasonography to locate the sacral hiatus for caudal epidural injections has not been described is inaccurate.2The footprint property of the transducer is not mentioned in the article. The linear array transducer cannot be used in all patients. In obese patients, it is sometimes necessary to use a curvilinear array transducer with lower frequency ranges to achieve a sonographic image of reasonable quality. Similarly, in very thin individuals, a transducer with a smaller footprint is more appropriate.

I agree with the authors regarding the advantage of using ultrasonography for caudal epidural needle placement. The article mentions the fact that ultrasound cannot provide us with the image information as to the depth of the inserted needle as the only disadvantage. It makes no mention of the most important limitation of this method, i.e. , inadvertent intravascular injection. Inadvertent intravascular injection, which has been reported to occur in 5–9% of these procedures,3cannot be avoided with this technique. This is important because aspiration or return of blood is neither sensitive nor specific for intravascular positioning of the needle.4Toxic concentration of local anesthetic may occur on inadvertent injection into an epidural vein.

Hull and East Yorkshire Hospitals NHS Trust, Hull, United Kingdom. regi.pereira@virgin.net

Chen CPC, Tang SFT, Hsu TC, Tsai WC, Liu HP, Chen MJL, Date E, Lew HL: Ultrasound guidance in caudal epidural needle placement. Anesthesiology 2004; 101:181–4
Klocke R, Jenkinson T, Glew D: Sonographically guided caudal epidural steroid injections. J Ultrasound Med 2003; 22:1229–32
White AH, Derby R, Wynne G: Epidural injections for the treatment of low-back pain. Spine 1980; 5:78–86
Renfrew DL, Moore TE, Kathol MH, el-Khoury G, Lemke JH, Walker CW: Correct placement of epidural steroid injections: Fluoroscopic guidance and contrast administration. AJNR Am J Neuroradiol 1991; 12:1003–7