Photo: PA
The 60-year-old writer invited television cameras to follow the "dark path" of his condition over the course of a year for a BBC Two documentary.
Viewers will see him struggling to write his Discworld books and trying out experimental "cures" as he attempts to stave off the symptoms of the degenerative brain disease.
Pratchett was diagnosed with early onset Alzheimer's in December 2007 and donated $1 million to fund research into the condition.
In the two-part documentary, Terry Pratchett: Living With Alzheimer's, the author says: "I've decided I'm not going to go down without a fight. For me, this really is a matter of life and death. This damn disease is not going to go away, it's only going to get worse. There is a war being fought out there and maybe it's time to go and join the troops."
He suffers from posterial cortical atrophy (PCA), a rare form of the disease which affects the part of the brain that deals with vision. He has difficulty with spelling and reading, and it takes him several minutes to perform simple functions such as knotting a tie.
In one uncomfortable scene, Pratchett is forced to halt a book reading when he loses the ability to read the words on the page. He acknowledges: "What may look like a nightmare from the outside is just one day at a time for me."
Among the unproven treatments he tests out is a bizarre-looking helmet invented by a GP, Dr Gordon Dougal, who claims it can halt degeneration by firing infrared light at brain cells. Pratchett admits: "Like anyone who suffers from an incurable disease, I will grab hold of even the slightest promise of hope. There is a kind of empowerment in knowing that you are doing something."
Throughout all, Pratchett maintains his black humour and is determined not to be seen as a victim. "I don't particularly want to be 'Terry Pratchett, the famous Alzheimer's sufferer'. Everywhere I go, people expect me to be some kind of a wreck and it's beginning to get on my nerves."
:: Terry Pratchett - Living With Alzheimer's is on Wednesday at 9pm, BBC Two
Terry Pratchett in 2005 
The ''Discworld'' author Sir Terry Pratchett is to appear in a BBC documentary covering a year of his struggle with the brain disorder Alzheimer's disease.

Pratchett, 60, asked the BBC to spend a year documenting any decline in his condition and seeing him experiment with various supposed cures. One alleged cure involved firing infrared light at his brain via a helmet invented by a general practitioner doctor. Pratchett also examines the serious research into dementia and its cure.

The author has donated £500,000 to the Alzheimer's Research Trust and his campaigning has raised another £200,000. Pratchett has posterior cortical atrophy, a rare form of the disease which affects vision, causes him to forget words and makes dressing confusing. He has written 36 ''Discworld'' novels amongst other works and was knighted in the 2009 New Years Honours.

Posterior cortical atrophy is also known as Benson's syndrome. It is the progressive shrinking of the back of the brain. It is a form of Alzheimer's disease, and the wider disease affects some 26.6 million people worldwide.

The documentary - ''Terry Pratchett: Living with Alzheimer's'' - will air on BBC Two on February 4 at 21:00.