Eye test can pick up Alzheimer's, study claims
Summary
"Alzheimer's disease can be spotted through simple eye test," reports the Daily Telegraph.
A new study has found that people with Alzheimer's had fewer blood vessels and less blood flow in the retina (back of their eye).
The Alzheimer-linked eye changes were detected by an eye test that uses a scanning technique called Octa (optical coherence tomography angiography). It can show up blood vessels in the retina that are finer than the width of a human hair.
The media has described the eye test as an easy new way to detect early Alzheimer's.
And it's easy to see why they would jump to this conclusion as it can be very difficult to definitely diagnose Alzheimer's, especially in the early stages.
The reality is, however, that this is very early research. It's too soon to say that it will lead to a simple test for Alzheimer's.
The research doesn't tell us whether the retinal changes occurred before or after Alzheimer's set in. And we don't know that the changes are unique to Alzheimer's. They might also be seen in people with other types of dementia, medical conditions like diabetes, or other eye conditions.
Where did the story come from?
This study was conducted by researchers from Duke University in North Carolina, US and published in the journal of the American Academy of Opthalmology.
It was funded by grants from the National Institutes of Health; Research to Prevent Blindness, Inc, New York; and the Karen L. Wrenn Alzheimer's Disease Award, Durham, North Carolina.
The media was over-optimistic in its reporting of this story by suggesting it may usher in a new diagnostic eye test for Alzheimer's. The Mail and Sun even suggest that the eye test could spot changes before symptoms appear, which is wrong and not supported by this research.
What kind of research was this?
This was a cross-sectional study that compared the eyes of people with and without Alzheimer's at a single point in time.
This type of study is useful for showing characteristics of a population, in this case that people with Alzheimer's have fewer blood vessels and less blood flow in the retina of their eyes.
But it doesn't follow people over time so we can't say whether the retinal changes occurred before or after they were diagnosed with Alzheimer's.
What did the research involve?
The study compared the retinas of 39 people with Alzheimer's, 37 with mild cognitive impairment and 133 healthy people with normally functioning brains.
It recruited people from a memory clinic in North Carolina. They were all adults aged over 50 (with an average age of 71) who had been diagnosed with either Alzheimer's disease or mild cognitive impairment (MCI).
All the diagnoses were made by experienced specialists using standard and accepted criteria. Healthy age-matched controls were recruited from the community.
The researchers excluded various people from the study, including people with non-Alzheimer's dementia, diabetes, high blood pressure, neurological conditions like multiple sclerosis, glaucoma, age-related macular degeneration or poor vision.
They took scans looking at the tiny blood vessels in different parts of the retina, and then compared blood vessel density between the groups.
What were the basic results?
The researchers give quite detailed results of the blood vessel measurements seen at different parts of the retina.
Essentially people with Alzheimer's had:
- fewer blood vessels
- reduced blood flow
compared with both healthy controls and people with MCI.
There was no difference in blood vessels between MCI and healthy controls.
Also, the layer of nerve fibres that surround the optic nerve where it attaches to the retina was thinner in both those with Alzheimer's and with MCI compared with the control group.
How did the researchers interpret the results?
The researchers conclude that people with Alzheimer's had fewer blood vessels and less blood flow in the retina than healthy controls and those with MCI.
They suggest that these small changes in the tiny blood vessels of the retina may mirror small blood vessel changes seen in the brain.
More research is needed to tell whether this eye test could pick up progression of MCI to Alzheimer's.
Conclusion
These differences in the retinal blood vessels between people with and without Alzheimer's will be of interest to doctors in the field and beyond, and is worthy of further research. But it's far too soon to hail this as a "simple eye test" to detect Alzheimer's.
There are several limitations to this research.
Firstly it's a cross-sectional study. This kind of study measures changes at a particular point in time. So we don't know which came first, the retinal changes or the onset of Alzheimer's.
It would be valuable to follow over time people with Alzheimer's and with MCI - where the blood vessel changes aren't currently seen - to see whether things progress or change.
We don't know that these changes are unique to Alzheimer's. People with other types of dementia, medical conditions like diabetes or high blood pressure, or different types of eye disease were all excluded from this study.
Notably the researchers wondered if the blood vessel changes could mirror those occurring in the brain in Alzheimer's. But vascular dementia is characterised by small blood vessel changes in the brain, even more so than Alzheimer's. If these changes aren't specific to Alzheimer's it is questionable whether they could have value in diagnosis.
Blood vessel changes were also seen only in people with Alzheimer's, not those with earlier stage MCI. If the eye changes don't help earlier diagnosis, and are only seen when dementia is more advanced and would be diagnosed clinically, again there could be questionable value to the test.
Finally it’s worth noting that although eye scans may be seen as a 'simple test', the researchers noted that many people with advanced Alzheimer’s were 'easily fatigued by imaging'. So although non-invasive, long eye examinations could still be difficult for some people.
