The cleidoatlanticus muscle: a potential pitfall for the practice of ultrasound guided interscalene brachial plexus block.

INTRODUCTION: Image interpretation during ultrasound guided interscalene nerve blocks might be difficult and misleading due to rare anatomical variations of the sternocleidomastoid muscle. CASE REPORT: During a routine dissection, we found a cleidoatlanticus muscle, which originated lateral to the regular cleidomastoid portion of the sternocleidomastoid muscle. This muscle separated from the sternocleidomastoid muscle at level of the dorsally crossing omohyoid muscle to fuse with the levator scapulae muscle's origin at the transverse process of the atlas.
CONCLUSION: As this muscle crosses at a level, where ultrasound guided interscalene blocks are performed, this unusual structure might lead to misinterpretation of the confusing ultrasound image, resulting in misguided needle positioning and consecutive inefficiency of the block technique.