Alzheimer’s and the eye – imaging device for screening
Polarimetry device could provide early diagnosis
A novel imaging device using polarised light can detect and measure amyloid deposits on the retina and could become an ophthalmic tool to help screen for Alzheimer’s disease (AD).
Details of the device were presented by Melanie Campbell PhD, Professor of Physics and Astronomy, and Professor in Optometry and Vision Science at the University of Waterloo, Ontario, Canada, who was speaking at the 2016 Alzheimer’s Association International Conference in Toronto, Canada.
Amyloid beta protein deposits in the brain are a known diagnostic biomarker for AD and can accumulate many years before the first symptoms occur. Currently, their presence is measured by PET scans of the brain.
However, amyloid deposits also occur on the retina, an extension of the brain. Researchers hypothesise that amyloid protein is synthesised by neural cells within the eye, in the same way as in the brain in AD, appearing in both the retina and the vitreous. There is also evidence for fluid exchange between the cerebral spinal fluid which contains amyloid and the vitreous.
The imaging device uses polarised light, polarimetry, and could provide a “low-cost, non-invasive, comfortable and potentially widely available test for early detection of amyloid”, which could be done in the offices of ophthalmologists, Dr Campbell said.
She presented results of a series of proof-of-concept scans which were conducted on human and canine retinas. Scans were conducted on a series of post-mortem retinas from the Eye Bank of Canada (20 from people who had AD and 22 from healthy controls), as well on living and post-mortem canine retinas.
Human samples were excluded if there was a history of neurologic disease or any sort of cognitive impairment, apart from AD. In addition, the retinas from five canines were imaged in vivo using amyloid fluorescence and optical coherence tomography for comparison.
The retinas were mounted flat for the post-mortem study, meaning whole retinas were scanned. There has been some debate in the Alzheimer’s community as to whether amyloids appear on the retina, since not everyone finds them, she said.
This could be due to sparse sampling of the retina using thin sections. Amyloid deposits in canine retinas had very similar properties to the deposits imaged in human retinas. They were close to nerve cells and interacted the same way with polarised light.
The study found that the number of amyloid deposits present in the human retinas varied widely. Sometimes the researchers found just a few deposits, while other samples had many more.
However, as with brain scans, the presence of amyloids does not always correlate with active disease. Deposits in the canines differed before and following cognitive dysfunction, potentially indicating disease progression.
The device can also measure the size of amyloid deposits and variety of interactions with polarised light, an advantage over ophthalmic fluorescence imaging, she said. Fluorescence requires the use of a dye marker, whereas polarimetry does not. Polarimetry also demonstrated very high sensitivity and specificity, she said.
The next step will be testing the device clinically on patients with AD.
Melanie Campbell: firstname.lastname@example.org