While the study of post-traumatic stress disorder dates back more than 100 years, to the victims of steam-train accidents in Victorian Britain, the condition is now commonly linked to exposure to war or extreme disasters. The experiences of Vietnam veterans from the US highlighted the way the syndrome affects those involved in armed conflicts. 'I had one client who was a former Vietnamese soldier,' said New York psychologist Toan Phan. 'He'd been engaged in a battle with four other enemies and he shot them all. At the end, he was covered in the blood of his final victim. Ever since, his dreams always come back to that event. 'Four years afterwards, he developed a numbness in his hand - to this day he is unable to hold dishes or glasses. He says he feels he did something truly evil - so maybe the numbness of his hand is a result.' Some patients recover from the disorder within months; for others, the suffering can last for years. Treatment combines psychological therapies and medication such as drugs for depression and anxiety. Dr Phan tells the story of another client - a former US spy dropped into North Vietnam in 1965 and captured. Detained as a prisoner for 18 years, he came to the US in 1983, where he still suffers from his ordeal. 'He was tortured, physically and psychologically in the dark room for a long time. One of the results is that he is now very forgetful whenever he is sitting with me. If I help him focus on recollecting things that happened to him, his memory is not good - probably down to the fact that so many terrible things have happened to him, leaving him with a numbness in his mind. 'He had five years in the darkness, with a shackle on his ankle. He now says he can still feel the shackle on his ankle, even though it is years since he got out of prison. That feeling cannot go away, and every time he feels it, it causes him great depression.' Dr Phan uses cognitive-behavioural practices to help people understand their condition and their thought processes, and learn how to change their behaviour or reaction to their symptoms.