Description of an ultrasound-guided technique for pudendal nerve block in sheep

To describe an ultrasound-guided technique for anaesthesia of the pudendal nerve with evaluation of the anatomical spread of a bupivacaine-methylene blue solution in sheep.

Prospective, experimental study.

A total of seven adult experimental sheep and one sheep cadaver.

The study had two phases. First, the anatomy and ultrasound imaging of the gluteal region, caudal to the region of tuber coxae was reviewed in one sheep cadaver. Second, in seven healthy sedated sheep, a linear transducer (8-13 MHz) was positioned on the lateral area of the hip to identify pudendal vessels at the level of the ischial spine ventral to the sacrotuberous ligament. The right and left pudendal nerves area were injected with 0.1 mL kg^-1 of a 1:1 solution of bupivacaine-methylene blue. The ultrasound image quality was considered as excellent, good, or poor. Following euthanasia, both pudendal areas were dissected and the staining of the nerves assessed, expressed as the percentage of success (%) and compared with the chi-square test.

The relevant ultrasound landmarks were visualized in both areas of all sheep, with an excellent visualization of the landmarks before and during injection in most cases (13/14). On dissection, methylene blue dispersion was observed surrounding the pudendal nerve, with at least 1 cm length in 13/14 cases (93%; the remainder partially stained). No evidence of dye was found around the sciatic nerve in any case nor was there any evidence of haematoma or faecal contamination within the punctured area.

Ultrasound-guided injection with 0.1 mL kg^-1 in close proximity to the pudendal nerve at the ischial spine level resulted in a high success rate of nerve staining. This procedure may be a suitable technique for pudendal nerve blockade providing analgesia for common reproductive or perineal surgical procedures in sheep.