A leading dementia doctor has revealed the four subtle signs of Alzheimer's that can strike decades before a diagnosis.
Dr Daniel Amen, who is based out of California and runs a number of clinics across the US, says the crippling disease 'actually starts in the brain decades before you have any symptoms.'
But the first - and most common - early warning sign, the doctor says, is your memory becoming increasingly worse.
He explains: 'Eighty percent of people who say their memory is worse than it was 10 years ago have an 80 percent chance it will continue to get worse.'
The National Institute of Aging says memory problems associated with the early stages of Alzheimer's include 'a decline in non-memory aspects of cognition, such as finding the right word, trouble understanding visual images and spatial relationships, and impaired reasoning or judgment.'
It adds: 'As the disease progresses, symptoms become more severe and include increased confusion and behavior changes.'
Dr Amen's second early alarm bell is 'poor judgement and impulsivity'.
He says this is because the frontal lobes - the largest lobes of the brain located directly behind the forehead responsible for behavior and emotions - are 'decreasing in activity' due to the disease, which is 'sort of like your brain going offline'.
Only around 1 in 50 people with Alzheimer's disease find their frontal lobes are affected early on.
When this happens, the condition is known as 'frontal variant Alzheimer's disease' (fvAD).
The third unnerving symptom Dr Amen talks about involves developing a shorter attention span and being distracted more easily.
He says of this characteristic: '[It is] not like ADHD, that you've had it your whole life, but it seems to be accelerating.'
Touching on why this happens Ruth Drew, the director of client and information services for the Alzheimer's Association, explains: 'Alzheimer's disease attacks the brain, and it tends to start in hippocampus where new memory and new learning are stored so anything that disrupts that could certainly make it more difficult for a person to hold on to a train of thought.
'It is different for each person. Certainly by the middle stages [of the disease], focus is definitely going to be impacted.'
Moving to his fourth indicator, Dr Amen warns viewers in a TikTok to watch out for 'low mood'.
The Alzheimer's Society says it is hard to know what causes depression in dementia.
It adds: 'For many people, the challenges of living with dementia can lead to feelings of deep sadness or hopelessness.
'In addition, the diseases that cause dementia may damage parts of the brain involved in emotions and behavior.'
After detailing four main warning signs, Dr Amen runs through some of the things that can increase the likelihood of your developing the disease.
He reveals: 'Depression doubles the risk of Alzheimer's disease in women and quadruples the risk in men.
'Being overweight or obese [increases the risk]. As your weight goes up, the size and function of your brain goes down.
'That's why I'm trim. I do not want to purposely do anything that damages my brain.'
He also points out a few more conditions that could lead to poor brain health, including erectile dysfunction - 'because if you have blood flow problems anywhere, it means they're everywhere' - chronic insomnia, and sleep apnea.
Roughly 7million Americans currently live with Alzheimer's and those cases are expected to grow to nearly 13million by 2050, in part due to an aging population, according to the Alzheimer's Association.
Roughly one in five women get the disease, and one in ten men.
In 2022, Marvel actor Chris Hemsworth, 40, found out he was likely to develop the disease after going through genetic testing.
He's become a public figure in the fight against dementia.
He openly shares details about his health and wellness routines and encourages others to attend regular doctors visits.
Despite how common the disease is, there's not much known about what causes the dementia or how it progresses, but ongoing scientific research hopes to bring a breakthrough.