I can not understand if you son has had asthma since he was 2 why he nenver had oral steroid? How many episodes he has a year? It is almost a generic practice that if an asthma patient has attack, oral steroid is being used to dampen the inflammation. I saw 7 pedtriacians and 3 specialists. It is a very common way for all of them. The inhaling one is just for maintenance and not strong enough for asthma attack.
What is prednisolone ? you mean prednisone? if you mean prednisone, I guess it is oral.
If the doctor didnot even give him oral steroid, why need CT?
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The most important thing to get lung function back. It is dangerous to have low lung function without any symptoms.