PMS is thought to affect between 5 and 10% of women to the extent that it prevents them getting on with their daily lives, according to the Royal College of Obstetricians and Gynaecologists (RCOG) . However, it's thought that just a handful of these women experience PMDD.While the precise causes of PMS – and PMDD – remain unknown, the impact these conditions can have on women’s lives is devastating, with repercussions on their careers, their everyday routine and the relationships with their family, friends and partners.De Vere admits missing out on many professional opportunities because of her illness. “I’m self-employed on purpose, so that I can work around my PMDD. I kind of know my cycle now, and on my bad days I don’t plan to do anything, I don’t see anyone socially.”Looking back, she realises that PMDD put a strain on all her relationships. It caused her to lose many friends, who would either blame her for being flaky and inconsistent, or put her under a lot of pressure. “The last thing you want is to feel guilty about feeling the way that you’re feeling,” she explains.And her disorder almost put family bonds at risk, too. “My sister and I have fallen out for months on end, because she might have said something to me on a bad day, and I have just gone crazy at her because I couldn’t tell and I couldn’t control it,” she recalls. “It’s just something you can’t control; I’m not making any excuses, you just can’t. To me, it’s almost like an out-of-body experience.”De Vere is now on a hormone treatment that induces a temporary "medical menopause" to manage her PMDD symptoms. One of her options is to get a hysterectomy and remove all or part of her womb, but she doesn’t want to rule out the idea of settling down with the right person and having biological children in the future.For most PMDD patients, the biggest hurdle seems to be getting a correct diagnosis, as De Vere also points out: “There are women in the world who are being diagnosed with different mental illnesses and their doctors are not even considering it could be something to do with their cycle.”We asked Dr. Panay, spokesperson for the RCOG and chair of NAPS, to explain some of the major symptoms of PMDD, and how it differs to PMS. Click through the slides to see what he said.