Elevation of the hemidiaphragm is a radiologic finding seen in conditions such as diaphragmatic eventration, lobar lung collapse, subphrenic mass or abscess and phrenic nerve paralysis. In diaphragmatic paralysis, the entire hemidiaphragm is typically affected. The atrophic changes of the left hemidiaphragm, combined with the history of ipsilateral radical neck dissection strongly suggests phrenic nerve paralysis.

Phrenic nerve palsy occurs in about 10% of neck dissection cases. Symptoms include dyspnea, cough, chest pain and an increased risk of atelectasis. Phrenic nerve arises from the ventral rami of the C3, C4 and C5 nerve roots, part of the cervical plexus. It descends on the anterior surface of the anterior scalene muscle, so it can be affected in neck dissection.