Like many of us, I was shocked by the news of the 82-year-old man with Alzheimer's disease left at a hospital by his wife, who then flew back to Spain. Ken Baker had been an RAF fighter pilot, a top racing driver, a father of four and a successful businessman. Then Alzheimer's struck, and last month he was found alone, abandoned at an Essex accident and emergency department. His wife said she was acting out of desperation; her own health had been affected enormously. His family described their incredulity, anger and disgust and wondered what sort of person could do such a thing.

This is such a sad case, as many involving Alzheimer's are. Every case is different, though all are heartbreaking. My own experience of caring for my wonderful mother, who had Alzheimer's for four and a half years, is not something I would wish on anybody, although I would do it all over again. I adored my mother and wanted to be close to her during this terrible illness, but it is a harrowing ordeal that takes its toll on the carer. The situation is often an insidious one - as your relative declines into dementia, your own health can deteriorate.

Mum came from the island of Guernsey. When she was 17 she travelled to England and married my father. We lived in Essex. When he died 20 years ago, she remarried and moved to London and then East Sussex, where I still live. Mum would often say how lucky we were to live between the South Downs and the sea. We both belonged to rambling clubs and even when mum was in her 70s we climbed Snowdon together - she was a very fit lady. She had a mischievous sense of humour and was great company. Every year we would go on holiday together - she was more fun than someone half her age.

It was after returning from a holiday in the Isle of Wight where we had been cliff walking that we found out mum had the illness. Our lives were to change for ever.

She gradually became more absent-minded - forgetting keys, things like that - as many of us do when we get older. But it was more than that. Once we met a long-term friend of mother's in the street. We had a chat, then said goodbye, and my mother turned to me and said: "Darling, who was that?"

There were many moments like that. She was an avid reader. She loved books all her life; she treasured them and looked after them. But I had taken a library book to her once to try to help her get her brain back. When I went in days later, she was there methodically tearing the pages into strips. I knew then that this was something dreadful.

At first it was very, very frightening. You are with someone you love and know so well, and it's almost as though they are becoming another person. Usually when I walked in to see her, she managed somehow to get her faculties together, but one time I looked around the door and she was looking at the ceiling, crying out to her dead brothers, calling to them for help. It was unbearable.

Then it became dangerous. She would muddle up the gas cooker and the fridge. Once I visited her and she had singed her eyebrows.

Sometimes what I was doing felt like an impossible strain on me emotionally and physically. I would often convince myself that I could cope, but then paid the price later. Going through a painful caring role, I found my normal flexibility and ability to cope reduced drastically. I found myself in an isolated, all-consuming role.

I was constantly ill myself. I had colds, flu, bronchitis. My immune system was completely wrecked; lots of carers suffer from depression. Alzheimer's is such a terrible illness, it's enough to drive the most calm, rational person right to the edge. It's crueller than any physical illness.

Greater than the worry of looking after my mother was the complete lack of support. I don't have family who live near enough to help me, and have no partner or children. There were problems almost daily and I felt I had nowhere to go for help. That's why I wanted to write a book that would help people in a similar situation to myself.

A very difficult time for most carers is when they have to decide whether to put their relative into a care home. In the end, the decision was made for me, because although I almost begged for my mother to be allowed to stay with me, the social services said she was their responsibility and her interests were their priority. At the time I was upset, but in retrospect I realise they gave me good advice - she had deteriorated to a point where I couldn't cope.

I guess most people hope that their relative will best be cared for by professionals when they can no longer cope themselves - however, it is imperative to find the right home. There are some good ones and some very bad ones, so do your research. Many people suffer guilt when they leave a loved one in care, and may even feel slightly relieved at their relative's eventual death - apparently this is quite normal.

In the later stages of dementia, my mother had deteriorated physically and mentally as far as is possible for a human being. She was doubly incontinent and unable to move. She then went into hospital with pneumonia and caught the MRSA bug. At this stage I was desperate - I couldn't bear to see her suffer any more in this living-death state. I asked the doctor if he could do something to help my mother. "I know what you mean, Marion, but legally we can't do anything," he said. "Do you have diamorphine in your office?" I asked. "I can give it to her." He put his hands over his ears and said, "I didn't hear you say that." And he walked away. I doubt very much whether I would have actually had the nerve to do it, but it does show the desperation one can feel in these situations.

I had the great privilege to be with my mother when she died. By chance I had left work early and caught the last 20 minutes with her. I put the cot side down and lay on the bed, and her last breath was over my face. You would imagine an ill, elderly lady would have bad-smelling breath, but it was sweet. She smelt like Mum. It was wonderful.

· Have You Seen My Daughter?, Marion Langton's account of caring for her mother, can be bought via www.alzheimerscarersbook.com