A simple eye test could spot Alzheimer’s disease 20 years before symptoms develop, according to new research.

A study of patients with the devastating form of dementia found they had more than twice as much of a telltale brain protein in their retinas.

Scientists believe this begins to gather decades before symptoms develop, which offers a window for early treatment, when drugs and lifestyle changes are likelier to work.

It opens the door to an inexpensive screening program that would flag those most at risk, who would then undergo more extensive scanning.

The non-invasive technique uses the fluorescence of curcumin, the main chemical in the curry spice turmeric, to light up amyloid deposits at the back of the eye.

It’s long been thought there is a link between the amount of this rogue protein in the eye and amyloid in the brain. The retina is formed from the same tissue as the brain when a baby is developing in the womb.

Neurosurgeon Professor Maya Koronyo-Hamaoui, of Cedars-Sinai Medical Center, Los Angeles, said: “Analysis of retinal amyloid index (RAI) scores showed a 2.1-fold increase in Alzheimer’s disease patients.”

Curcumin is taken as a pill and crosses the blood-brain and blood-retina barrier, then binds to the amyloid plaques.

The chemical is naturally fluorescent, so the toxic proteins light up and are then captured by the RAI imaging device.

Koronyo-Hamaoui said previous research has suggested amyloid plaques appear in the retinas of Alzheimer’s patients in the early stages of the disease.

In addition, her experiments have shown they develop at least two months prior to their presence in the hippocampus, the brain’s memory center, and cortices of mice.

Two months of mouse life is the equivalent of about 20 years in the life of a human.

Koronyo-Hamaoui said: “These data may suggest amyloid accumulation in the retina is an early event in Alzheimer’s.”

The study published in JCI Insight involved Alzheimer’s patients both alive and dead. Post-mortems on 23 people who had died from the disease showed there were large amounts of amyloid in the retinas of all of them.

Koronyo-Hamaoui and colleagues then compared 10 living Alzheimer’s patients in their 70s and 80s with six healthy controls.

They found the Alzheimer’s patients had more than twice as much retinal amyloid even though they were in the mild to moderate stages.

Koronyo-Hamaoui said: “This study demonstrates the feasibility to noninvasively detect and quantify amyloid deposits in the retinas of live human subjects.”

“In a proof-of-concept trial, the mean RAI score in Alzheimer’s patients was elevated compared with that of the healthy controls.”

There are about 5 million people living with dementia in the US. Most are diagnosed too late to do anything about it.

Koronyo-Hamaoui said the retina is a central nervous system tissue readily accessible for direct imaging at high resolution and low cost.

It could “define at-risk populations for further clinical evaluation with gold-standard brain imaging,” she said.

Koronyo-Hamaoui added: “Alzheimer’s disease is an invariably fatal neurodegenerative disorder and the leading cause of senile dementia worldwide.”

“There is no effective treatment and limited functionality for early unequivocal diagnosis.”

“Cerebral amyloid accumulation may occur as early as 20 years before the onset of clinical dementia.”

“With the development of promising disease-modifying therapies, intervention during this phase when damage to neuronal tissue is minimal would offer significantly increased treatment efficacy.

“Existing brain amyloid imaging tools are invaluable for research and diagnosis, yet they present challenges for screening large-scale populations in clinical settings and predicting disease progression.”

“In part, this is due to high costs, limited availability, and exposure to radioactive isotopes.”

There is no cure for Alzheimer’s and existing drugs can only ease symptoms. The condition is diagnosed by memory tests and occasionally brain scans.

The disease can only be confirmed by a post-mortem examination, which reveals the presence of the harmful amyloid plaques in the brain.