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Brittle asthma is a type of asthma distinguishable from other forms by recurrent, severe attacks. There are two subtypes divided by symptoms: Type 1 and Type 2, depending on the stability of the patient's maximum speed of expiration, or peak expiratory flow rate. Type 1 is characterized by a maintained wide PEF variability despite considerable medical therapy including a dose of inhaled steroids, and Type 2 is characterized by sudden acute attacks occurring in less than three hours without an obvious trigger on a background of well controlled asthma.
Brittle asthma is one of the "unstable" subtypes of "difficult asthma", a term used to characterize the less than 5% of asthma cases that do not respond to maximal inhaled treatment, including high doses of corticosteroids combined with additional therapies such as long-acting beta-2 agonists.