Anatomical observation of phrenic nerve branch variations at the cervicothoracic junction and upper mediastinum: two case reports

BACKGROUND: The authors report phrenic nerve branch and communication variations at the cervicothoracic junction and upper mediastinum through adult cadaveric dissection, analyze their local morphological characteristics, and discuss their anatomical significance in conjunction with previous literature. MATERIALS AND METHODS: During adult cadaveric dissection, two cases of phrenic nerve anatomical variations were discovered. RESULTS: Case 1 showed the left phrenic nerve composed of upper and lower branches that straddled the transverse cervical artery at the C4 level before reuniting and entering the thoracic cavity, with subsequent mediastinal course and terminal distribution consistent with standard anatomical descriptions. Case 2 showed early branching of the left phrenic nerve within the upper mediastinum, with its superior and inferior branches connected by communicating branches before joining the vagus nerve, while the main phrenic nerve trunk continued along its typical path to terminate at the diaphragm. Both variations were primarily limited to proximal branches or communicating structures, without significantly altering the main trunk course of the phrenic nerve within the mediastinum. CONCLUSIONS: This study reports two types of branch and communication variations of the phrenic nerve at the cervicothoracic junction and upper mediastinum. However, the main trunk course within the mediastinum and terminal distribution at the diaphragm remained relatively constant overall. Understanding these variations helps enrich phrenic nerve anatomical data and provides anatomical basis for identification and protection of the phrenic nerve during related cervical, mediastinal, and minimally invasive surgeries.