Prevalence of nocturnal asthma in a general population sample: determinants and effect of aging

Nocturnal asthma (NA) is important because of clinical and prognostic implications. Previous data on prevalence may be overestimated, because they are derived from selected series. Observations on monitoring of peak expiratory flow in elderly asthmatics suggested that prevalence of NA may increase with age. This study was designed to estimate the prevalence of NA-related symptoms in a sample drawn from a general population and evaluate the role of aging. Subjects (1,100, mean age 41.9, SD 22.8 years) were randomly selected from the lists of seven general practitioners. A questionnaire on nighttime and morning NA-associated symptoms was used and frequency of occurrence was rated as never, sometimes (less than once a week), and often (once a week or more). In the overall sample, symptoms were experienced "sometimes" by 2.3%-4.9% of subjects, whereas the response "often" was given by 0.9%-1.6% of subjects. Among subjects with a diagnosis of asthma, symptoms occurred sometimes in 16.7%-23.7% and often in 5%-15%. Symptoms reported the morning after were significantly more frequent among patients aged 65 years and older (p < 0.005), whereas the difference for nighttime symptoms was not statistically significant in different age groups, confirming an age-related blunted sensitivity. Logistic regression analysis shows that a diagnosis of asthma is the most important correlate of symptoms, with odds ratio (OR) up to 14.78 for cough; advanced age also proved to be an independent risk factor (OR 3.35-4.97). In conclusion, although the prevalence of NA was previously overestimated, our results indicate its importance, particularly among elderly patients who are exposed to a prominent risk of underdiagnosis and undertreatment.