Follow-up of Occupational Asthma After Removal from or Diminution of Exposure to the Responsible Agent: Relevance of the Length of the Interval from Cessation of Exposure

Study Objective: We set the hypothesis that follow-up surveys of occupational asthma (OA) could now show better improvement in the asthmatic condition because of a more prolonged interval since removal from exposure than in previously reported studies.

Patients/methods: Ninety-nine subjects with OA were assessed and were separated into two groups according to the duration of cessation of exposure: (1) group removed for > or = 5 years: 48 subjects studied 8.9+/-2.2 years after cessation of exposure; (2) group removed for <5 years: 51 subjects with OA, comparable in terms of history and functional results at time of diagnosis, with a time lapse from last exposure of 3.1+/-1.2 years. On the follow-up visit, questionnaires including information on the current and previous use of inhaled steroids, spirometry, and methacholine tests were administered and results were compared with those obtained at the time of diagnosis.

Results: At the follow-up visit, no significant changes in spirometry were observed in the two groups. However, a significant improvement in provocative concentration of methacholine causing a 20% fall in FEV1 (PC20) from a mean value of 1.5 to 3.7 mg/mL was documented (p<0.001). The proportion of subjects having normal PC20 at the follow-up visit was significantly higher in the group removed from exposure for >5 years than in the group removed for < or = 5 years (16/33 vs 8/42; p=0.01). Stepwise logistic regression showed that follow-up PC20 could be predicted from baseline PC20 (p<0.001, odds ratio [OR]=4.1, 95% confidence interval [CI]=1.8 to 9.1), duration of exposure (p=0.04, OR=0.9, 95% CI=0.8 to 1.0), the interval since removal from exposure (p=0.002, OR=1.7, 95% CI=1.2 to 2.5), and the type of agent; subjects with OA due to high-molecular-weight agent showed a less favorable outcome (p=0.04, OR=0.2, 95% CI=0.03 to 1.0). Current and past treatments with inhaled steroids were not significant predictors.

Conclusion: Results obtained in the group of this study removed for >5 years show better prognostic figures than those reported in most previous studies. Comparison with the group removed for a shorter interval and the stepwise logistic regression analysis suggest that the longer duration of the interval from cessation of exposure appears to be a factor determining this difference.