It must be one of the hardest things of all for an artist, to be losing her sight. But the Hon Mrs Sophie Montgomery, 43-year-old wife of Northern Irish newspaper magnate David Montgomery, and only daughter of Lord and Lady Birdwood, concentrates on the smaller, daily drawbacks. "One of the annoying things is that people will acknowledge me and I won't respond," she says. "They think they've been snubbed but it's simply because I haven't seen them. People put their hand out to shake mine and I don't see it. One just feels a bit vulnerable, you know?"

She adds that she is now going public on her inherited complaint, retinitis pigmentosa (RP), which leads to blindness, as it saves embarrassment and raises awareness of this debilitating condition. "I don't want people thinking I'm drunk or rude," explains Sophie. "But the days of going out on my own at night, to parties or whatever, are numbered. I get disoriented if the light levels are low. I bump into people, knock trays over, fall down stairs."

As well as the cost in terms of her social life and career, there is the cost to her as a mother. She has four children, three daughters — Olivia (18), Constance (16) and Claudia (12) — by her first marriage to the Earl of Woolton, and six-year-old William with Montgomery. "One day I won't be able to see my children's faces and I'll be unable to see my grandchildren." she acknowledges sadly.

In fact, she took the risk that her fourth pregnancy would make her sight deteriorate since RP speeds up with pregnancy. But as she says, "It was a life-affirming thing. I knew what I was potentially risking but I really wanted a baby with David." And was her sight affected? "Yes, it was."

Sophie knew there was a 50:50 chance she would suffer the same fate as her mother. "I had terrible night vision, and always wondered whether I might have it. I was aware that every year there was something else mummy found harder, but she was amazingly brave and was a history and English teacher until she was 50." Now 70, Lady Birdwood was registered blind four years ago.

Two years after marrying David, Sophie noticed something was wrong: "People were coming up to me and I wasn't seeing them ..." Typically, RP causes a symptomatic roller-coaster, rather than a consistent decline. Sophie had her peripheral vision tested and the result was "completely abnormal".

She copes but as her eyesight worsens — she holds her arms out two feet apart to indicate the limits of her detailed vision — daily life becomes increasingly fraught. Her son William's miniature cars litter the floor of her West London kitchen, causing problems. "Willie leaves his toys around and I'm always treading on them. He gets really angry. But I get cross with the children because they all leave things on the stairs and I fall over."

Sophie must be undergoing an emotional rollercoaster too but uses the usual diversionary tactics, including dry humour. She tells herself she's lucky to have developed the condition late, as the onset of RP usually occurs between the ages of 10 and 30. But, of course, it's depressing and she admits to having down days. Sophie becomes emotional when discussing her mother's courage. "She's a very good role model," she says "The kids know they may develop RP but because she handles it so well, it alleviates a lot of fear for them. She gets frustrated but she's dignified."

Talking to Louette Harding in You magazine, she adds: "I'm worried that the children are affected and I've noticed signs in a couple of them — so have they." Sophie reassures them with the fact that there's been something of a breakthrough with gene therapy.

But it can't yet offer help to sufferers like Sophie, whose independence is dwindling. Could this affect her marriage? "I've thought about it. Do I fear it? Not at the moment, and he's been fantastically supportive. Contrary to what's said about him, Dave is actually quite a selfless person, although he's not a people-pleaser. But he hasn't really been put to the test yet. I mean, the level of care my father now gives my mother is enormous. It's something I can't really afford to worry about."

The couple argue about leaving the lights on. "David keeps turning them off. He can't understand that if I turn off the lights, when I want to go back into that room I can't see where the light switch is." Maybe it's David's thrifty Ulster Presbyterian background coming to the fore? "Yes, absolutely."

Sophie feels her role as a parent is compromised: "What really gets to me is when the older girls are out at night and I'm worrying and want to pick them up. I can't drive and do those maternal things."

Her painting is already affected. Ironically, Sophie's work — bold abstracts and river scenes — has improved in the medium term. "Because of the ticking clock, I have an even greater motivation to paint," she says.

Still, Sophie hasn't had her children tested. She adds: "I sincerely hope by the time any of the kids develop it, there will be a treatment for them, even if it simply inhibits the progress".

To help the cause, Sophie and her father act as trustees of the British Retinitis Pigmentosa Society (www.brps.org.uk), which funds research and helps sufferers