A simple eye test may one day be able to detect Alzheimer’s disease decades 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, the New York Post reports.

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

It also opens the door to an inexpensive screening test to identify 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. Scientists believe the protein is one of the hallmarks of Alzheimer's.

Neurosurgeon Maya Koronyo-Hamaoui, of Cedars-Sinai Medical Center, Los Angeles, said the research showed: “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.