Unexplained chronic cough?

my 2yr old has suffered a chronic cough for over a year, she has been in hospital many times and has had every test poss done {bronchoscopy,cf test etc all negative. we constantly go from chronic coughing to a crisis point where sashe is admitted to hospital her breathing gets stabilised then we are sent home with no answers. this is heartrending to constantly watch her suffer where her body is bruised inside with the force of coughing and its hard to comfort her in my arms as her body is so sore! please any help or advice welcome