Ultrasound-guided Caudal Epidural Injection

To the Editor:—I read with interest the article “Ultrasound Guidance in Caudal Epidural Needle Placement” by Chen et al. However, 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. The 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, cannot 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. Toxic concentration of local anesthetic may occur on inadvertent injection into an epidural vein.