Brittle asthma.

Asthma is recognised as a heterogeneous disease and, within this heterogeneity, brittle asthma is identified as a distinct phenotype. Two types of brittle asthma have been recognised in adults: type 1, showing more than 40% diurnal variability in PEF rate on most days over long periods and type 2, with acute severe attacks on a background of apparently good asthma control. The exact incidence of these phenotypes in adults is unknown but both seem to carry an increased risk of death and considerable morbidity.