Macroscopic dye distribution after classical versus extrafascial interscalene block: an exploratory comparative cadaveric pilot study

Background: Extrafascial injection has been proposed as a phrenic nerve-sparing alternative for interscalene brachial plexus block (ISB), but the anatomical basis for the reported reduction in phrenic nerve involvement remains incompletely characterized. We compared macroscopic dye distribution patterns between classical ISB and extrafascial injection in paired cadaveric specimens. Methods: In five formalin-fixed cadavers, bilateral ultrasound-guided injections with indigo carmine (20 ml per side) were performed. A classical ISB and extrafascial injection were applied to contralateral sides. Three blinded anatomists independently assessed staining intensity (deep, light, none) in real time during dissection. Results: Both techniques produced deep staining of C5-6 roots, upper trunk, and suprascapular nerve in 5/5 specimens. The phrenic nerve was stained in all 10 specimens (5/5 in each group); deep staining occurred in 4/5 (80%) classical ISB specimens and 0/5 extrafascial specimens, though all 5 extrafascial specimens showed only light staining. Spread to infraclavicular cords was limited with both techniques. Conclusions: Dye reached the C5-6 roots, upper trunk, suprascapular nerve, and phrenic nerve in all 5 specimens of both groups. Phrenic nerve staining intensity was lower with the extrafascial technique than with classical ISB in 4 of 5 paired cadavers, identical in 1 of 5, and never higher. The functional and clinical implications of this intensity difference cannot be determined from the present anatomical model and require prospective validation.