There are many studies that are even quite old you should be able to find easily in a library search. I recall that Diamond reported the comlete cure of 40 cases with the approach used in: Diamond, H.H. HYPNOSIS IN CHILDER: ASTHMA. Am. J. Clin. Hypn., 1:124, 1959. Most researchers stress that the symptoms should not be supressed and that appeals to the "Ericksonian in me" too. There are many others: Margonet, 1960 in Int. J. Clin. Exp. Hypn.., 8:121; etc. Those are the older ones I recall without going to the library. But you won't have trouble finding these and more.