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 one hour after ultrasound-guided injection of either methylene blue or blue tissue marker in rat cadavers. Study design Prospective, randomized, cross-over design. Animals Twelve 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 one 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 and clinical relevance The length of sciatic nerve staining increased one hour after injection of bupivacaine with methylene blue, but not after injection with tissue marker. This finding shows that methylene blue can lead to time-dependent increase in length of nerve staining.