DEMENTIA isn鈥檛 inevitable. The human brain can stay sharp well past 100 years of life. Yes, getting older slows us down: parts of the brain associated with memory and executive function shrink, myelin sheaths around our neurons start to erode, slowing down signalling, and arteries narrow diminishing blood supply. But those things mainly affect speed: when healthy older people are given extra time to perform cognitive tasks, the results are on par with younger folks.
In contrast, dementia alters the cognitive playing field. As well as affecting memory, it causes issues with understanding or expressing oneself in language, problems with sensory perception, and disturbances in executive function that can undermine day-to-day independence.
Advertisement
Read more: Defying dementia
As we hunt down cures for dementia, one of today鈥檚 most feared illnesses, there are ways to fend off symptoms for longer
Dementia also . 鈥淧eople sometimes use dementia and Alzheimer鈥檚 disease interchangeably. But that isn鈥檛 correct,鈥� says , director of Behavioral and Social Research at the US National Institute on Aging (see 鈥淭he different kinds of dementia鈥�, below).
Genes play an important part in many kinds of dementia. If you have a parent or sibling with it, you are more likely to develop it yourself. More than 20 different gene variants are now known to influence susceptibility, (see 鈥�Should you test your genes?鈥�).
The various conditions give rise to similar symptoms by different means. Vascular dementia, for instance, can result when cardiovascular disease or a stroke limits blood supply and damages brain tissue.
Alzheimer鈥檚 disease, the most common cause of dementia, is characterised by a build-up of hard plaques of beta-amyloid protein between brain cells, and tangles of tau protein within them. The , the leading idea for how these plaques drive cognitive decline, suggests that a build-up of plaques causes inflammation in the brain, which spurs development of tau, which disables and then kills brain cells, resulting in memory loss, confusion and other symptoms.
3 out of 4
The number of people in the UK who don鈥檛 realise they can reduce their risk of dementia
This hypothesis is supported by research in families with early-onset Alzheimer鈥檚, which strikes before age 65; many have gene variants that interfere with the ability to clear amyloid. That is also the mechanism by which a certain variant of the APOE gene that codes for apolipoprotein E 鈥� a protein that binds to and affects the clearing of beta-amyloid 鈥� can increase risk of Alzheimer鈥檚.
Yet, despite the dominance of the amyloid hypothesis, the absolute cause of Alzheimer鈥檚 is far from being agreed on. To begin with, autopsies reveal that many people die with a significant amount of amyloid in their brain without ever showing dementia-like symptoms. And promising amyloid-clearing drugs have failed spectacularly in clinical trials involving people with advanced disease, (see 鈥�Will we find a cure?鈥�).
Still, most researchers in the field remain convinced that beta-amyloid is central to the Alzheimer鈥檚 tale, even if it doesn鈥檛 tell the whole story, says , programme director for Alzheimer鈥檚 disease clinical trials at the US National Institute on Aging. There is also evidence that it may be a kind of 鈥�diabetes of the brain鈥�, Ryan says, where the ability to use glucose, our brain鈥檚 essential fuel, is impaired. With a condition as complicated as Alzheimer鈥檚, as with other forms of dementia, many different factors probably contribute.
Why is Alzheimer鈥檚 on the rise?
ALZHEIMER鈥橲 disease strikes after our reproductive years, so the established thinking is that there has been no evolutionary pressure to weed it out, and its prevalence has risen simply because we are living longer.
Things might not be that simple. The 鈥�grandmother hypothesis鈥� has it that helping raise your grandchildren boosts the chances your genes will be passed on. So any gene that lets you do this by fending off Alzheimer鈥檚 provides an evolutionary edge.
suggested such genes offer protection by enhancing the hormone oestrogen鈥檚 anti-inflammatory activity in the brain. Now Molly Fox of the University of California, Los Angeles, has . Her team calculated lifetime oestrogen exposure for 81 women over 70 by looking at onset of puberty, pregnancies, age at menopause and other factors. Each extra month with oestrogen was associated with a 0.5 per cent decrease in Alzheimer鈥檚 risk. That suggests changing exposure to the hormone 鈥� due to having fewer children, among other things 鈥� may be linked to the rise in Alzheimer鈥檚.
This article appeared in print under the headline 鈥淲hat causes dementia?鈥�
