Terry Gilliam has opened up about the heartbreaking changes in his Monty Python collaborator Terry Jones since he was diagnosed with dementia in 2015.

Jones, 75, has been living with Primary Progressive Aphasia, a form of frontotemporal dementia (FTD) that impairs the sufferer's ability to speak and communicate, for almost three years and Gilliam, 76, admits it is painful to see his friend slowly lose sharp wit and ''argumentative'' nature to become a virtually unrecognisable person.

Gilliam insists the worst thing of all is knowing that the condition cannot be fixed.

In an interview with The Times newspaper, the 'Fear and Loathing in Las Vegas' filmmaker said: ''Terry was argumentative, Terry was trouble, Terry was all sorts of things. Terry was always so alive on every level and now he's not. He's quiet. It is extraordinary because Terry looks healthier, better groomed than he has ever looked, yet he is not in there any more. He's very pleasant and very nice but Terry was never like that. He's quiet. The worst thing is that there's nothing one can do it about it.''

The surviving members of Monty Python - also comprised of John Cleese, Michael Palin and Eric Idle - reunited for 10 live shows at The O2 arena in London in 2014, and it was then that the rest of the group could see Jones was having problems performing.

Jones' family - including his partner Anna Soderstrom - went public with his condition in September 2016 to raise awareness of FTD, which, unlike Alzheimer's disease, does not result in a loss of reasoning.

However, decision-making and speech are affected and sufferers often seem less caring.

Gilliam had his own health scare this year after finally completing work on his passion project 'The Man Who Killed Don Quixote', which stars Jonathan Pryce and Adam Driver, and which took him 25 years to complete.

The cinematic genius believed he was going to die when he had a major operation to cure a prostate condition and admits he was in a ''terrible state''.

The 'Jabberwocky' director said: ''By the end of it I was in terrible shape. I won't go into the whole thing, but I had a huge prostate operation when I got back to London. I did feel that I was going to die, not just that I was going to die, but that I was dying.''