An unusual anatomical variant of the left phrenic nerve encircling the transverse cervical artery.

During educational dissection of cadavers, we encountered anatomical variability of the left phrenic nerve (PN). In this cadaver, nerve fibres from C3 and C4 descended and crossed behind the transverse cervical artery (TCA), a branch of the thyrocervical trunk, at the level of the anterior scalene muscle. On the other hand, nerve fibres from C5 descended obliquely above the TCA and then joined the fibres from C3-C4 on the medial side of the anterior scalene muscle to form the PN. To our knowledge, the encircling of the TCA by the left PN in the neck has not yet been reported and may pose a potential risk for nerve compression during movement of the neck. We discuss several types of anatomical variants of the PN and the associated risk during thorax and neck dissection procedures.