Ultrasound imaging for transversus abdominis blocks

The transversus abdominis block has been recently described, using a surface anatomical technique [1]. We find this block extremely useful but employ ultrasound to assist correct needle placement and local anaesthetic delivery. With the patient supine, the probe is placed immediately superior to the iliac crest, facing medially. The 22G spinal needle is inserted in an antero-posterior direction, so that it is seen along the long axis of the probe (that is, visualised ‘in plane’ with the probe). Both the needle and relevant anatomy, including parietal peritoneum, are visualised. The transversus abdominis plane can be observed distending with local anaesthetic during injection. Our patients are comfortable following abdominal surgery, and often have an area of discernable anaesthesia, confirmed by lack of cold or pinprick sensation. O’Donnell et al. identify Petit’s area as their landmark, just superior to the iliac crest, and defined as a triangular region between the posterior border of the external oblique muscle, and the anterior border of the latissimus dorsi. The abdominal wall in this area, therefore, is composed of internal oblique and transversus abdominis muscles only [2]. Our ultrasound imaging has shown some variability in this area. In most patients, only two muscle layers are seen. However, sometimes four distinct abdominal wall muscle layers are seen, presumably caused by the external oblique and latissimus dorsi muscles overlapping. This has been described in a few individuals [3]. Petit’s area is, therefore, absent in these individuals. We recommend that ultrasound imaging assists accurate location of the transversus abdominis plane compared with a technique that relies on anatomical landmarks with acknowledged variability.