My dad has severe swelling in the oesophagus, could it be related to his sleep apnea machine? He's on steroids now but we need to know the cause.

This is probably... Not directly related to the CPAP machine. How was the diagnosis of swollen esophagus made? He should have an egd performed by a gastroenterologist who would be better equipped to answer your question. The treatment with steroids could suggest eosinophilic esophagitis.