Brittle asthma.

We believe that the asthma phenotypes we have defined as types 1 and 2 brittle asthma appear to be defined subgroups of asthma. For example, we have characterised patients with type 1 brittle asthma, as defined in this review, on the basis of peak flow variability and treatment and these patients remain a separate group when assessed by other means such as psychosocial factors, immunoglobulin levels, and atopy. The question remains as to whether they are truly separate groups with entirely different pathogenetic influences or whether they simply represent the severe end of the spectrum. Our suggested classification into types 1 and 2 forms a useful start for studies of this condition, although prospective evaluation of patients with severe asthma is the only way of substantiating the validity of these definitions which will then enable investigation of possible mechanisms. However, these patients are rare and in order to study them as a group a national register would need to be set up along the lines of the West Midlands Brittle Asthma Register, perhaps recruiting all "at risk" patients and then using this resource as a means of exploring the different asthma phenotypes within this broad grouping, including brittle asthma.