This will be an online interactive case report. I will tweet this post, asking for comments and diagnoses in the comments below (preferable to twitter comments) and update with the answer and a discussion in 1-2 weeks.
A 68 year-old otherwise healthy male passenger was flying with his friend, a pilot, in a private plane from California to Montana for a fishing trip. Within an hour after takeoff, he became confused and diaphoretic and was hyperventilating, then he lost consciousness for approximately 20 minutes. The pilot applied supplemental oxygen and checked his passenger's arterial oxygenation via oximetry, finding it to be 95%. The flight was diverted for a medical emergency. During descent and landing, the passenger regained consciousness but remained confused. In the emergency department of a nearby hospital, he had normal vital signs, and had a non-focal neurological examination, but remained confused. Representative images from CTA of the chest are shown below (click on the images to expand). A head CT was normal excepting for some age related atrophy. What is the diagnosis?