Background The natural history and time course of the onset of exacerbation events of chronic obstructive pulmonary disease (COPD) is incompletely understood.

Methods A prospective cohort of 212 patients with COPD was monitored using daily symptom diaries for a median of 2.8 years to characterise the time course of COPD exacerbation onset. Decision rules based on weighted self-reported symptoms were used to define opening and closing of exacerbation events. Event time intervals were analysed and logistic regression was used to determine the effects of patient covariates on exacerbation events.

Results Patients recorded 4439 episodes of worsening respiratory symptoms from baseline; 2444 (55%) events resolved spontaneously and 1995 (45%) resulted in a COPD exacerbation. In 1115 of the 1995 COPD exacerbations (56%) the onset was sudden and the exacerbation threshold was crossed on the same day symptoms began. In contrast, 44% of exacerbations were characterised by gradual onset of symptoms (median duration from symptom onset to exacerbation 4 days). Patients who experienced sudden onset exacerbations had greater mean daily symptom scores (7.86 vs 6.55 points, p<0.001), greater peak symptom scores (10.7 vs 10.2 points, p=0.003), earlier peak symptoms (4.5 vs 8.0 days, p<0.001) and shorter median recovery times back to baseline health status (11 vs 13 days, p<0.001). Multivariable analysis showed that gradual onset exacerbations were statistically associated with a longer duration of exacerbation recovery (OR 1.28, 95% CI 1.06 to 1.54, p=0.010).

Conclusions COPD exacerbations exhibit two distinct patterns—sudden and gradual onset. Sudden onset exacerbations are associated with increased respiratory symptoms but shorter exacerbation recovery times.