Picture the following: you go to sit where there is no seat and end up sprawled on the floor, in a less than dignified pose. You become disorientated in a friend's garden, which is barely bigger than a quilt, and can't find your way out. You make a beeline for the front of a long, snaking queue, incurring mass fury. You go to a Michelin restaurant, suitably dolled up - and you butter your plate instead of your bread. Welcome to PCA .

No, I had never heard of it either. Nor, unfortunately, had my GP.

PCA stands for Posterior Cortical Atrophy. For an author, as I am, it is pretty inconvenient, as it renders you virtually illiterate. It is the same form of Alzheimer's which afflicted Terry Pratchett, and usually kidnaps you in your 50s or 60s. It is described as rare. I have my doubts. Why should it be? I believe it is simply undiagnosed or misdiagnosed; not surprising, considering the weird raft of apparently disparate symptoms.

Unlike the more usual Alzheimer's, which affects the frontal lobes, governing cognitive function and memory, PCA attacks the back of the brain, causing it to wither.This area is responsible for vision; thus, spatial awareness, sequential awareness, orientation, perception, motor skills and, as mentioned, literacy, are all affected. If I had to define PCA, I might liken it to being both profoundly dyspraxic and dyslexic - only worse. Trust me to have something obscure nobody has heard of.

There was a time when my brain and I were on intimate terms; cosy bedfellows privy to one another's innermost secrets. Now the brain gremlins and I are at war. PCA takes away the free spirit in you. It chips away at confidence, deprives you of independence. The second you walk out through your front door, you feel vulnerable and exposed. I was eventually diagnosed two years ago, but I believe the first signs crept up on me several years earlier. Perhaps I should start with my driving. Oh dear, my driving! I'd loved driving, but increasingly it became a source of dread. I could see perfectly, yet I could no longer judge distances, or where to position my car in relation to the road. I couldn't work out where or how to pitch my line of vision. Signs leapt out at me, filling me with panic. Busy junctions were a nightmare. I was forever clipping the curb, or, worse, the wing mirror of an on-coming car. I would arrive at my destination wrung-out. I used landmarks, counting them as I passed them unscathed. Reversing down our own driveway became a hit and miss exercise, leaving zigzagging imprints on the lawn.

For the last two years that I drove, I rarely had a passenger, other than my German Shepherd, and even he made it clear he didn't like my driving. Nobody knew the extent of my terror: that I regarded my car as a killing machine and had to brace myself whenever I climbed into it, or that I dreaded being responsible for a fatal accident.

Around the same time, I developed an inexplicable fear of steps and stairs. Winding ones, crumbling ones, short ones your feet overhung. They existed solely for me to break my neck. I could ascend them - no problem; going down was another matter. Perilous as a rope-bridge, they swayed before me, as I clung to the rail and made my tentative descent. As for escalators: horrible things. I couldn't get the timing right and would lurch onto them, getting my feet sandwiched between treads.

Another incident: It was four years ago. My mother was dying. She had Parkinson's and dementia. In an effort to engage her, I offered to read to her from a novel of my own. A spark of interest lit her eyes. Sitting close, I began to read. Stumbled. Stopped. Started again. The same thing happened. I couldn't comprehend it; I continued for a few sentences, from memory, but had to abandon it. My mother had fallen asleep anyway.

''You hardly seem to read nowadays,'' remarked my husband casually, one day. And, although I protested with some force, I knew it was true. What he didn't realise was that reading had ceased to be enjoyable, and, instead, had become a strain. Letters and words were witches dancing before me; I would lose my place then resume on the wrong line.

It was the same thing with playing the piano: overnight it seemed to have deserted me. In fact there had been a lapse of several years, but I assumed, like riding a bike, it was something I would retain. I positioned a Mozart sonata on the piano, and, expectant, leant forward. Crotchets and quavers leaped out at me. I couldn't read a note. You're just tired, my husband, the kindest man on the planet, tried to console me. I knew otherwise. But what was happening? Off I went to my GP.

''I think I have a kind of dementia,'' I said.

''I think you're suffering from anxiety,'' he countered.

''I was born anxious,'' I quipped. ''This is different.''

He recommended tranquilisers, which I declined. I returned home. I felt weary, inadequate alone. And continued to bluff.

Every day, the Brain-Gremlins plotted a different prank to play on me. They had a warped sense of humour: a third of the way through writing my new novel, I started losing my way round the computer. My expletives, as I struggled with every word, are best left to the imagination.

By now I was grappling with simple tasks - laying the table; putting lids on jars; failing to help my five-year-old granddaughter with her Lego; entangling myself in Sellotape, like a mummy, while wrapping a parcel; finding it hard to read the time on the large faced kitchen clock, using the phone…. I was forever mislaying things and failed to see what was under my nose.

The supermarket became a place to dread, as I searched for items in vain - only to be told it was staring me in the face. When it came to paying, my hand trembled as I rummaged in my bag for my wallet, then held up everyone as I attempted to fit my card into the slot; and I felt waves of irritation from staff and customers alike.

The pretence went on as I floundered with daily life, trying not to let myself down, too ashamed to confide. But I was exhausted by now. Enough was enough. I made an appointment with the optician, and requested a Macular Degeneration test - knowing it would reveal nothing wrong with my eyes. The tests confirmed this, and the ophthalmologist agreed with me that my brain seemed unable to process signals. Armed with a letter from him, I returned to my GP. Finally - still reluctant - he referred me to a neurologist. The scan showed substantial atrophy to the back part of the brain - ie, PCA.

I cannot express my relief upon learning that diagnosis. I could stop pretending and be open. My very supportive husband is no longer impatient when I don't see what is in front of me, and he ferries me about, now that I no longer drive; if I lose my bearings or get lost he comes to the rescue. I can, to some extent, capitulate to the illness, and can enlist help when I need, if, as is sometimes inevitable, I do something batty, I simply admit the truth. There should be no shame attached to Alzheimer's. Naturally I have occasional low moments - I wish I could just pick up a book or newspaper - but I try to focus on what I can do rather than what I can't. I run a tiny choir, I go for walks with my German Shepherd; though I can't draw horses anymore, I can paint abstracts full of colour. Though I no longer read music or recognise notes, I improvise, letting my fingers go where they want; and it is unexpectedly liberating, engrossing me for hours at a stretch, and I forget there is anything wrong with me.

Maybe it is an elaborate trick. And five years, from its inception, I have finished my novel. Writing The Lupo Stick was accompanied by ever more colourful language as I painfully negotiated my way round my laptop. I hope that it will not be my last book.

Viewed objectively, PCA is a fascinating disease. Life with it is certainly different, but, without wishing to minimise it, it doesn't have to be insurmountable. I am fortunate in my family, but for those less so, charity-run organisations are to hand.

As for the future? At some stage my PCA will, as it invariably does, merge with the more usual Alzheimer's; but I rarely dwell on the situation: it is what it is. From day to day, nobody knows what lies in wait for them.

For more information about PCA, contact Dr Chris Butler at Oxford University on chris.butler@ndcn.ox.ac.uk or Alzheimer's Research UK.