Firstly the term 'mild' should not really be used in conjunction with classic CAH. Perhaps mild can be used with non-classic but not with classic CAH, even if it is simple virilising (SVCAH). SVCAH usually only has about 2 - 7% enzyme function which is not much - depending on which mutations are present. Moderately severe is a better term.
Secondly if your daughter has SVCAH she will be on corticosteroids (and possibly a mineralocorticoid - Florinef). The very issue of being on long term corticosteroids above a certain dose usually causes the adrenal gland to cease its production of cortisol. (This is for anyone on longterm doses and high doses of corticosteroids, asthmatics, transplant patients etc). Therefore in the event of illness (fever, vomiting, diarrhoea & dehydration) or severe injury (eg. broken bone) in a CAH person, the adrenal gland is unable to mount a sufficient adrenal response. On these occasions an adrenal crisis may occur.
As to how often this may happen is anybody's guess! The best way to avoid it happening is to carry a Solu-cortef (or equivalent) emergency medication injection kit and to be proactive if such an event occurs. Also wearing or carrying some medic alert information to inform ambulance personnel and others of these potential scenarios.
Realistically adrenal crises tend to reduce, once children reach about five years old. However this is no reason for complacency as they can occur anytime. I know for myself, even as an adult, I can go downhill very quickly given adverse circumstances.
