CHAPTER 35 Inguinal field block
Sonoanatomy
Perform a systematic anatomical survey from the iliac crest to the lower abdomen. The abdominal wall is scanned about 5 cm cranial to the anterior superior iliac spine. A sagittal oblique transducer orientation is used (Fig. 35.4). At this point, all three muscle layers of the abdominal wall can easily be identified by ultrasound and facilitate orientation (Fig. 35.5). The peritoneum and bowel are seen deeper to these (Fig. 35.5). The nerves appear as hypoechoic fascicular structures with hyperechoic rims sandwiched between the layers of muscle (Fig. 35.6). Trace the course of the nerves from above ASIS and distally towards the inguinal region. The iliohypogastric and ilioinguinal nerves consistently lie between the internal oblique and transversus abdominis muscles here. The recommended injection site for landmark-based approaches is situated medial to the anterior superior iliac spine. At this site, both nerves are often penetrating the internal oblique muscle. Performing a ‘blind’ technique here may result in difficulty for the injected local anesthetic to reach both nerves if they are not lying in the same compartment. This is a possible explanation for the high failure rates of 20–30%. It is more likely to reach both nerves with local anesthetic using the landmark-based approach where the nerves are lying in the same layer of the abdominal wall. Small vessels are frequently seen to accompany nerves within the plane.
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