Posterior Ischemic Optic Neuropathy (PION) - Cardiovascular Cause

A 58-year-old man undergoes a 9-hour prone spine fusion with an estimated blood loss of 2.5 L and intraoperative mean arterial pressure maintained at 55–60 mmHg. On awakening, he reports painless, severe bilateral vision loss. Visual acuity is hand motion in both eyes, and bilateral relative afferent pupillary defects are noted. Fundoscopy reveals the optic disc marked **A** to be pink, sharp, and without swelling. Which of the following best explains why the optic disc appears normal in the acute phase despite severe vision loss?