Alzheimer's disease is entering the world of what Americans call "gee-whiz" medicine (high tech, high cost) but it is difficult to know yet whether the added cost results in improved accuracy.  A question frequently asked in my clinics is whether mildly impaired memory in a person in their seventies or eighties is or is not going to progress to the full disabling and ultimately fatal condition we know as Alzheimer's disease. If 20 per cent of people can carry heavy beta amyloid loads in their brains without detectable dementia then perhaps these tests may mislead too many people to be useful in an everyday clinical setting. If two out of every ten people you tell are on their way to developing Alzheimer's disease turn out not to be then the reputation of this approach would soon be in tatters.  

If plaques and tangles are indeed the root cause of Alzheimer's disease then the fact that some people have severe symptoms yet very few tangles, while others have no symptoms but a lot of them, needs to be investigated.  

In the US a neurologist, David Snowdon, studied ageing and the onset of dementia in 678 Roman Catholic nuns. The population has been followed since the late 1980s and as the nuns died their brains have  been examined. On joining the order in their early twenties all the nuns provided autobiographical sketches. These little essays were categorised according to what is sometimes called "idea density", which includes concepts such as linguistic complexity, vivacity and fluency. Roughly 80 per cent of those nuns whose essays were found to be low in idea density went on to develop dementia in old age, while only 10 per cent of those who were rich in this quality developed the disease. Another longitudinal study being carried out in Chicago has recently published findings linking the density of a person's social network to their vulnerability to plaques and tangles. At post mortem it was found that the individuals with dense social networks were less severely demented than their counterparts with smaller social networks even though they had the same tangle density in their brains.

 My team has studied a group of people with established dementia living in nursing homes and compared their personality characteristics with those of their unaffected siblings. The striking difference between the two groups was that those with Alzheimer's disease were much more likely to be solitary, anxious or dependent in terms of lifelong personality traits. This evidence seems to support the existence of a personality-based quality that makes some individuals more, or less, susceptible to the onslaught of plaques and tangles. In balancing the equation between disease (plaques and tangles) and susceptibility (personality) this second ingredient is the one that requires more weight, clinical attention and research.