Symptoms can include headache, dizziness, sensitivity to light and blurred vision, all of which can occur even when MRI and CT scans are normal, said Steven Galetta, a neuro-opthalmologist at the NYU Langone Medical Center in New York.

Rove’s comment that when Clinton appeared in public after recuperating “she’s wearing glasses that are only for people who have traumatic brain injury,” referred to a lens or lenses that the former first lady wore for a short period after her concussion and a subsequent blood clot that developed in her brain. Known as a Fresnel prism, the device helps treat double vision, especially for close-up viewing, experts said.

Special lines in the prism bend light seen by one eye and align it with the image seen by the other, according to Galetta and a representative of the company that distributes the lenses. The prism can be in the form of a clear plastic overlay that is pasted to the lens of eyeglasses in cases, such as Clinton’s, when the double vision is expected to be temporary, or can be ground into the lens for longer-term use, according to Laura Balcer, director of the Concussion Center at NYU Langone medical center. When built into the eyeglass lens itself, the prism is not visible, Balcer said.