Being told that your partner or parent has Alzheimer's disease is devastating. It's a very unusual person who, faced with this news, doesn't feel a huge sense of fear, loss, disbelief, ignorance and sometimes even anger.

One of the most upsetting aspects of the disease is the fact that it can only be diagnosed when the problem is well established, because the diagnosis is one of exclusion. There's no definite test and, at present, there's no method of screening to identify potential sufferers.

So what should you look for? It starts when the man, or woman, you have known so well changes in ways you cannot pinpoint. He may have become forgetful, suddenly neglecting to feed the cat or turn off the television, things he has been doing every night for 20 years. Maybe he's started asking the same questions over and over again, when you've only just given him the answer.

He may have become jealous, constantly asking you where you're going and what you're doing, even when you are just leaving the room for a minute. He may make up stories to cover up his memory loss. It's patchy. There are good and bad days. On the good days you can almost forget there's a problem.

I saw a patient recently whose husband has Alzheimer's. Unsurprisingly, she was having great difficulty coming to terms with this. But what she found even worse was his accusing her of having an affair. Nothing could have been further from the truth, but now, I think she secretly wishes that she had had an affair. Her future looks bleak - her plans for retirement have been frozen.

As a carer, you have to find the patience to cope with someone who has become irritable with you when you haven't done anything wrong, and be able to pacify him when he becomes frustrated with himself.

Sometimes, diagnosis is delayed because the sufferer's partner does not want to admit that there is something really wrong. Feelings of shame or guilt can delay the carer coming forward. How will others deal with you and your partner? You're not widowed, but you can't join in as a couple any more. Will you still have a social life when you can't even leave him for an hour, in case he sets fire to the sofa with his cigarette?

Establishing support for the carer is essential because many sink rather than swim. A study last year showed that of 700 carers who were looking after demented patients, a staggering 47 per cent were clinically depressed - in other words, they needed some sort of treatment themselves.

The first drug for the treatment of Alzheimer's , Donepezil hydrochloride, was recently licensed in the UK and there are more to follow. It has been indicated for the early treatment of the disease, when there are still nerve cells functioning. It may slow down the process, but it isn't a cure and it won't reverse the damage already done. So, if we give it to someone with moderate symptoms, we may prolong the time the carer has to cope with the sufferer at home, which could be unhelpful.

Alzheimer's affects one in a thousand people under the age of 65, and four to five per cent over this age. That means a lot of carers are hoping for a miracle. For now, Donepezil hydrochloride treatment is relatively untried and we still need to rely on the support services to help carers. With drugs of this type, the carer may have to accept an increased quantity of time with the patient rather than a improved quality of life.