A simple eye test can quickly detect the early signs of a rare type of dementia, a new study found.
The thinning of the outer retina indicates the patient has frontotemporal dementia, an uncommon type of dementia that mainly affects the front and sides of the brain.
It is hard to diagnose because it does not initially cause memory problems and like other types of dementia, it tends to develop slowly and get gradually worse over several years.
Now scientists at the University of Pennsylvania School of Medicine have found eye changes may signal frontotemporal dementia, also known as frontotemporal lobe degeneration (FTD).
FTD affects around 16,300 people or two percent of all dementia cases and signs include personality and behavior changes and problems with language, mental abilities and memory.
Sufferers may also experience physical problems, such as slow or stiff movements, loss of bladder or bowel control, muscle weakness or difficulty swallowing.
There is no single test and it is among the most common causes of midlife dementia.
But it is often misdiagnosed as Alzheimer’s or vice versa.
Currently diagnosis is based on an assessment of symptoms and mental abilities, blood tests to rule out conditions with similar symptoms, brain scans and testing spinal fluid to rule out Alzheimer’s.
The University of Pennsylvania’s simple eye exam and retinal imaging test improves the accuracy of a diagnosis by detecting a thinning of the outer retina – the layers with the photoreceptors through which we see.
The retina is potentially affected by neurodegenerative disorders because it is a projection of the brain.
Previous studies suggested those with Alzheimer’s and motor neurone disease also have thinning of the retina in a different part of the retina.
Lead author Assistant Professor of Ophthalmology Dr Benjamin Kim said: ‘Our finding of outer retina thinning in this carefully designed study suggests that specific brain pathologies may be mirrored by specific retinal abnormalities.’
Senior author Professor of Neurology Dr Murray Grossman added: ‘As we enter an era of disease-modifying treatments for neurodegenerative disorders, it is essential for us to have tools that can identify the specific pathologies accumulating in the brain so that we can administer the appropriate treatments to patients who are likely to benefit.’
The study looked at 38 FTD patients and 44 who did not have any neurodegenerative disease.
The FTD patients were carefully characterised with clinical exams, cerebrospinal fluid biomarkers to exclude Alzheimer’s Disease, and genetic testing.
Researchers then employed an eye-imaging technology called spectral-domain optical coherence tomography (SD-OCT), which uses a safe light beam to image tissue with micron-level resolution. SD-OCT imaging is inexpensive, non-invasive, and quick.
Measurements of the retinal layers showed that the outer retinas of the FTD patients were thinner than those in the control subjects.
This relative thinning of outer retinas was caused by a thinning of two specific portions of the outer retina, the outer nuclear layer (ONL) and ellipsoid zone (EZ).
The ONL of FTD patients was about 10 percent thinner than controls, and this ONL thinning was the primary source of the outer retina thinning.
The degree of retinal thinning among FTD patients also had a significant tendency to be worse when the patients’ scores on a standard cognition test were lower.
Previous studies found a loss of optic nerve fibres and associated thinning of the inner retina in a few other neurodegenerative disorders including Alzheimer’s, motor neurone, and Lewy-body dementia.
The results suggested FTD manifests in a different way in the structures of the retina, and this difference, detectable with a retinal imaging test, might help doctors distinguish one disorder from another.
And FTD is not a single disorder but rather a grouping of distinct disorders so retina imaging can distinguish the different sub types.
Some FTD subtypes involve the abnormal accumulation, in affected brain areas, of thread-like aggregates of a protein called tau.
Other FTD subtypes feature abnormal aggregates of a protein called TDP-43.
They observed that the outer retinal thinning seemed to occur chiefly in the probable-tau pathology group.
They grouped the participants into probable-tau, probable-TDP-43, and unknown pathology categories and found the outer retinal thinning seemed to occur chiefly in the probable-tau pathology group
Prof Kim concluded: ‘Prior studies have suggested that tau is expressed in photoreceptor cells, and so we hypothesised that patients with tau brain pathology may have photoreceptor abnormalities.
‘It was exciting to acquire data that appear to confirm our hypothesis.’