Evaluation of methylene blue and tissue marker for assessing sciatic nerve staining one hour after ultrasound-guided injection in rat cadavers.

OBJECTIVE: To compare the length of sciatic nerve staining immediately or 1 hour after ultrasound-guided injection of either methylene blue or blue tissue marker in rat cadavers. STUDY DESIGN: Prospective, randomized, crossover design. ANIMALS: A group of 12 adult female frozen-thawed Wistar rat cadavers. METHODS: Rats were assigned to two treatment groups: ultrasound-guided injections of the sciatic nerve with bupivacaine combined with either methylene blue (MB, n = 6 rats) or blue tissue marker (TM, n = 6 rats). For each rat, the sciatic nerve of each pelvic limb was injected with the same dye, and gross anatomical dissections were performed immediately after injection (T0) or 1 hour later (T1). The sciatic nerves were classified as stained when dye was visually detected along the external surface of the nerve, and the length of staining was measured using a caliper. Data for each group were analyzed using a one-tailed Wilcoxon test. RESULTS: In the MB group, the median (interquartile range) length of sciatic nerve staining at T1 [20 (16.8 to 22.3) mm] was greater than at T0 [16 (15.5 to 16) mm] (p = 0.031), with a median difference of -4.0 (-6.3 to -2.3) mm between T0 and T1. In the TM group, sciatic nerve staining did not differ significantly between T0 [4 (4 to 5.3) mm] and T1 [5 (4 to 5.3) mm] (p = 0.312), and the median difference was 0.5 (-0.3 to 1) mm. CONCLUSIONS: The length of sciatic nerve staining increased 1 hour after injection of bupivacaine with methylene blue, but not after injection with tissue marker. CLINICAL RELEVANCE: This finding shows that methylene blue can lead to a time-dependent increase in length of nerve staining.