Frontotemporal dementia is one of the less common forms of dementia, striking patients as young as 40. In fact, it is known as the dementia that starts to show itself in much younger patients, according to Medical Daily. 

But according to studies published in August the eyes undergo changes that flag the onset of the disease, making it easier to detect in all patients, including those in their 40s.

The term covers a range of specific conditions. It is sometimes called Pick's disease, or frontal lobe dementia. FTD is caused when nerve cells in the frontal or temporal lobes of the brain die and the pathways that connect them change. Roughly 60 percent of cases occur in people between 45 and 64, but can happen at 40 or younger, The Association for Frontotemporal Degeneration reported. FTD only accounts for 10 to 15 percent of all demetia cases.

This damage to the brain causes the typical symptoms of frontotemporal dementia, which include changes in personality and behavior, and difficulties with language, according to the United Kingdom's Alzheimer's Society. 

But before all of that happens, scientists say the eyes undergo a benign change that signal the impending onset of the disease. In people with a family history of frontotemporal dementia, their retinas get thinner, a valuable signal for researchers. 

But most people who acquire the disease have no family history of dementia, according to Medical Daily.

When the brain's frontal and temporal lobes begin to shrink as FTD progresses, the retina, comprised of light-sensitive neurons in the eye, connects directly to the brain through the optic nerve.

"This finding suggests that the retina acts as a type of 'window to the brain,'" said lead investigator Dr. Li Gan in a statement. "Retinal degeneration was detectable in mutation carriers prior to the onset of cognitive symptoms, establishing retinal thinning as one of the earliest observable signs of familial [FTD]."

Using mice as models, the scientists investigated the underlying neuron changes that prelude the disease by observing the mouse retinas.

"With these findings, we now not only know that retinal thinning can act as a pre-symptomatic marker of dementia, but we've also gained an understanding into the underlying mechanisms of frontotemporal dementia that could potentially lead to novel therapeutic targets," Gan said.