Technically, Republican strategist Karl Rove was correct when he suggested that former Secretary of State Hillary Clinton suffered a "traumatic brain injury" after a fall in 2012. A concussion is the most common form of mild traumatic brain injury, according to experts in the field. It is caused by a blow to the head or an action that bounces the brain around in the skull, such as whiplash from a car accident.

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.

On Tuesday, Rove distanced himself from a provocative report in Monday's New York Post, saying he does not believe -- as the newspaper asserted he had said -- that Clinton suffered "brain damage" when she fell and sustained a head injury in December 2012. The fall was attributed to dehydration from a stomach virus, and Clinton subsequently developed a blood clot, which was treated.

"Of course she doesn't have brain damage," Rove said in an interview with The Washington Post. But Rove added that it is apparent that Clinton suffered "a serious health episode" and said that if she runs for president in 2016, "she is going to have to be forthcoming" about the details of where, how and when it happened. Rove claimed that Clinton spent 30 days in the hospital, according to the report, but actually it was three.

Nick Merrill, Clinton's press secretary, said in an e-mail that "even this morning, Karl Rove is still all over the map and is continuing to get the facts wrong. But he doesn’t care, because all he wants to do is inject the issue into the echo chamber, and he's succeeding. It’s flagrant and thinly veiled...What he’s doing is its own form of sickness. But she is 100%, period."

Galetta said most concussion victims recover quickly, but some can experience symptoms a year later.

Galetta and Balcer, emphasizing that they could not speak about Clinton's case specifically, said that either the concussion or the blood clot could have caused blurred vision. In both cases, they said, a patient would typically suffer blurred vision in both eyes. Single-eye blurred vision is more common from injuries to the eye itself, rather than the brain, they said.