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My elderly uncle is in hospital with a urine infection. His capacity for normal behaviour and thought is greatly reduced and I鈥檓 told the same happens when other older people have the same kind of infection. Why is this? If I (a 43-year-old) picked up a urine infection it might sting, but wouldn鈥檛 lead to mental incapability.

鈥� A sudden alteration in normal behaviour and acute confusion can occur in people who have sepsis as a result of a urinary tract infection. This acute neurological dysfunction is termed 鈥溾�� (SAE) and is more common in elderly patients.

SAE is probably caused by a diffuse neurological dysfunction as a result of the body鈥檚 inflammatory response to an infection, as opposed to direct infection of the central nervous system. As we age, our immune system becomes less effective at fighting infections. This means older people contract infections more often and with greater severity, increasing their susceptibility to SAE.

Elderly patients often also have underlying chronic neurological disorders, such as dementia, which can increase the probability of acute confusion when sepsis is present.

There is no specific treatment for SAE. As a result, the approach tends to focus on dealing with infection and supportive measures, such as management of any organ failure, prevention of metabolic disturbances and avoidance of neurotoxic drugs.

Dr Matt Rowland, John Radcliffe Hospital, University of Oxford, UK

鈥� There are a few reasons why the writer鈥檚 elderly uncle may react differently to a younger relative. First, his prostate gland will have enlarged with age and may be partially blocking the bladder outlet. So it probably doesn鈥檛 completely empty each time he passes urine. This stagnation means an infection is not flushed out, as it would be in a younger man.

Then there is 鈥渃ognitive reserve鈥�. Young people have tremendous redundancy built into their organs, but a man may have lost 10 per cent or more of his brain cells by the age of 85, and hence confusion is much more likely to appear with any general illness.

鈥淧eople may have lost 10 per cent of their brain cells by the time they reach age 85鈥�

The vigour of the adult immune system gradually fades with age. An older man isn鈥檛 able to fight the infection as strongly, and less inflammation occurs with the infection, so there鈥檚 less pain. As the infection lodges in a younger person鈥檚 bladder, they would feel at least some lower abdominal pain, but it wouldn鈥檛 lead to delirium and confusion. As it reaches the kidneys, you鈥檇 have quite incapacitating lower back pain, on one or both sides. And although it鈥檚 unlikely, if the infection then spreads into your bloodstream with huge showers of bacteria, you might well feel quite delirious and have blood poisoning (septicaemia). But your uncle might not get any of these symptoms and merely become totally confused, while his organs suffer in silence.

Dr James Wakely, Boxted, Colchester, Essex, UK

鈥� The association between urinary infections and impaired mental capacity may be the other way around. Having cared for a 90-year-old with similar medical history, I can vouch that there are several aspects of behaviour and brain function that may predispose them to urinary tract infection.

For example, older people can suffer a loss of taste, so they consume too much salt. This is exacerbated by a failure to drink enough partly due to loss of sensitivity to thirst. Also an older person may be reluctant to get up for the toilet too often, especially at night in the dark, if they have arthritis, or impaired balance.

Food preferences may shift away from healthy foods such as fresh fruit and vegetables, partly because of reduced ability to taste, and partly because preparing meals takes more cognitive skill than, making sandwiches, say. If personal hygiene is lacking and dehydration is present (also a risk factor for high blood pressure and mini strokes), mental impairment may well be accompanied by kidney and bladder ailments.

Dr Hillary J. Shaw, Newport, Shropshire, UK

鈥� Delirium can affect older brains when someone is fighting an infection. The reasons are neatly explained on the .

Avis Powell, Penn, Buckinghamshire, UK

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