New Delhi, Aug. 4 -- HOUSTON - The first warning sign arrives without pain. A patient might notice a shadow in one eye's peripheral vision, a gray patch that stays fixed no matter how many times they blink. Within hours, in the worst cases, the vision in that eye fades and does not recover. Ophthalmologists call it non-arteritic anterior ischemic optic neuropathy, known as NAION, a sudden blockage of blood flow to the optic nerve whose damage is, for most patients who experience it, irreversible.

Dr. Mirwat Sami, an eye specialist at Houston Oculofacial Plastic Surgery in Texas, has found herself discussing that condition more frequently in recent years. The reason is not that NAION itself has become more prevalent. It is that the popula...