Scientists have made a surprising discovery about the eye and its blood supply that may help to improve the diagnosis and treatment of stroke.
In a paper that was recently published in the journal Neurology, they describe how they found that a contrast agent that is given to stroke survivors in order to highlight brain abnormalities can also leak into the eyes.
Senior study author Dr. Richard Leigh, who is an assistant clinical investigator from the National Institute of Neurological Disorders and Stroke — which is one of the National Institutes of Health (NIH) — states that he and his colleagues were "kind of astounded" by the discovery, and that "it's a very unrecognized phenomenon."
"It raises the question," he continues, "of whether there is something we can observe in the eye that would help clinicians evaluate the severity of a stroke and guide us on how best to help patients."
A stroke occurs when a part of the brain loses its life-giving blood supply, either due to a blockage (ischemic stroke) or a rupture (hemorrhagic stroke) in a blood vessel. Most strokes are ischemic.
Stroke used to be the fourth leading cause of death in the United States, but — thanks to medical improvements — it is now the fifth. Prompt medical care following a stroke can not only save lives, but it also improves quality of life for survivors.
More than 795,000 people every year have a stroke in the U.S., where the total annual cost of healthcare, drugs, and missed work days associated with the condition comes to around $34 billion.
Blood-brain barrier and blood-ocular barrier
When people are admitted to hospital following a stroke, they will usually undergo an MRI scan to assess the damage to the brain. Often, this involves having an injection of a contrast agent called gadolinium, which is a harmless substance that travels to the brain and lights up any abnormal areas on the scan.
In healthy people, the blood-brain barrier usually stops the contrast agent from getting into the brain tissue. It stays in the bloodstream and is eliminated through the kidneys.
The blood-brain barrier, a thin layer of highly active tissue that lines the tiny blood vessels that feed the various parts of the brain, prevents potentially harmful substances from crossing over from the bloodstream.
But a stroke can damage the tiny blood vessels and cause leakage in the blood-brain barrier, which allows some gadolinium to leak into the brain tissue. This shows up as bright spots on MRI scans.
Between the eyes and the bloodstream, there is a similar barrier called the blood-ocular barrier. There is evidence from previous studies that some eye diseases can disrupt the blood-ocular barrier.
Stroke can disrupt blood-ocular barrier
The NIH researchers discovered that a stroke can also disrupt the blood-ocular barrier and allow gadolinium to leak into the eye.
Evidence of this was visible in the glowing eyes on the MRI scans of some of the stroke survivors who took part in their study.
They suggest that gadolinium leakage in the eyes could be used to help assess the severity of strokes and decide on the best treatment.
For their study, the scientists compared hospital MRI scans of 167 stroke survivors that were done before and after they were injected with gadolinium.
The participants all underwent three scans altogether: the first was before receiving the contrast agent, the second was 2 hours after receiving it, and the third was 24 hours afterwards.
As gadolinium is transparent, the only way that it could be detected was on the MRI scans. The participants' vision was unaffected by it.
Gadolinium leakage and stroke severity
The results showed that gadolinium had leaked into the eyes of 66 percent of stroke survivors at the 2-hour scan and 75 percent at the 24-hour scan.
The team observed gadolinium leakage in both the people who received a clot-busting treatment known as tissue plasminogen activator and those who did not.
At the 2-hour scan, gadolinium was mostly present in the aqueous chamber, which is at the front of the eye. At the 24-hour scan, it was more commonly spotted in the vitreous chamber at the back of the eye. It was less common for gadolinium to be present in both chambers at the 2-hour scan.
The participants who showed gadolinium present in the vitreous chamber at the 24-hour scan tended to be older and had a history of high blood pressure.
They were also more likely to have signs of lesions in the brain's white matter that are linked with cognitive decline and aging. In MRI scans, these lesions show up as bright spots of "white matter hyperintensities."
The minority of individuals who had gadolinium in both eye chambers at the 2-hour scan were mainly those whose strokes had affected a larger area of the brain and had impaired more of the blood-brain barrier.
The researchers suggest that their findings could pave the way for developing a diagnostic for stroke severity without the need for an MRI. Instead, survivors could be given a substance that collects in the eyes in the same way that gadolinium does.
"It is much easier for us to look inside somebody's eye than to look into somebody's brain. So if the eye truly is a window to the brain, we can use one to learn about the other."
Dr. Richard Leigh
The researchers found no link between the extent of gadolinium leakage and the amount of disability that participants experienced after their strokes.
They also concluded that further studies should examine whether or not gadolinium leakage occurs in healthy individuals.