Nocturnal asthma: not a separate disease entity

Circadian rhythms and sleep-related breathing disorders

Recent studies have provided evidence that the human circadian timing system has an influence on respiration and respiratory control. Both sleep and circadian mechanisms combine to mediate the rise in lower airway resistance in nocturnal asthma. In rats, circadian rhythms in minute ventilation are present in both wakefulness and sleep, implying that circadian and sleep mechanisms also combine to influence the control of breathing. The circadian timing system causes a nocturnal increase in the chemoreflex threshold and it is suggested that this may increase the propensity for nocturnal sleep apnea. This hypothesis is supported by a model analysis of human chemoreflex control, and relevant published data are reviewed. The clinical implications of this putative circadian contribution to sleep apnea are potentially very significant, but relevant data are scarce and directions for future research are discussed.

Nocturnal asthma

Lung function in a healthy individual varies in a circadian rhythm, with peak lung function occurring near 4:00 PM (1600 hours) and minimal lung function occurring near 4:00 AM (0400 hours). An episode of nocturnal asthma is characterized by an exaggeration in this normal variation in lung function from daytime to nighttime, with diurnal changes in pulmonary function generally of > 15%. The occurrence of nocturnal asthma is associated with increased morbidity and inadequate asthma control, and has an important negative impact on quality of life (QOL). Newer data have shed light on physiologic and immunologic mechanisms that underlie the nocturnal development of airway obstruction. It remains controversial whether nocturnal asthma is a distinct entity or is a manifestation of more severe asthma. The current data do not resolve these two alternatives, as well-controlled studies have reached opposite conclusions. However, the clinical associations of gastroesophageal reflux disease and obesity appear to be strong. The treatment of asthma with effective controller agents can reduce nighttime symptoms, improve psychometric outcomes, and improve QOL.

Exhaled nitric oxide is higher both at day and night in subjects with nocturnal asthma

Nitric oxide in exhaled air is thought to reflect airway inflammation. No data have been reported so far on circadian changes in NO in subjects with nocturnal asthma. To determine whether exhaled NO shows a circadian rhythm inverse to the circadian rhythm in airway obstruction in subjects with nocturnal asthma, we conducted a study involving six healthy controls, eight individuals without nocturnal asthma (4-h to 16-h variation in peak expiratory flow [PEF] <= 15%), and six individuals with nocturnal asthma (4-h to 16-h PEF variation > 15%). Smoking, use of corticosteroids, and recent respiratory infections were excluded. NO concentrations were measured at 12, 16, 20, and 24 h, and at 4, 8, and 12 h of the next day, using the single-breath method. At the same times, FEV1 and PEF were also measured. Mean NO concentrations were significantly higher in subjects with nocturnal asthma than in subjects without nocturnal asthma, and higher in both groups than in healthy controls at all time points. Mean exhaled NO levels over 24 h correlated with the 4-h to 16-h variation in PEF (r = 0.61, p < 0.01). Exhaled NO did not show a significant circadian variation in any of the three groups as assessed with cosinor analysis, in contrast to the FEV1 in both asthma groups (p < 0.05). At 4 h, mean +/- SD NO levels were higher than at 16 h in subjects with nocturnal asthma; at 50 +/- 20 ppb versus 42 +/- 15 ppb (p < 0.05); other measurements at all time points were similar. Differences in NO and FEV1 from 4 h to 16 h did not correlate with one another. We conclude that subjects with nocturnal asthma exhale NO at higher levels both at night and during the day, which may reflect more severe diurnal airway-wall inflammation. A circadian rhythm in exhaled NO was not observed. NO levels did not correspond to the circadian rhythm in airway obstruction. The small increase in NO at 4 h may indicate an aspect of inflammation, but it is not associated with increased nocturnal airway obstruction.

Sleep disturbances in clinically stable young asthmatic adults

BACKGROUND

Although nocturnal asthma is a well documented clinical problem, the effect of asthma on sleep itself and morbidity caused thereby have not received adequate attention. It is now recognized that partial sleep deprivation has a considerable effect on human functions.

OBJECTIVES

With this in view, we conducted a study to determine the occurrence and nature of sleep disturbances in a homogeneous group of patients with clinically stable bronchial asthma in their home environment.

METHODS

Thirty young adult unmarried university students with clinically stable bronchial asthma attending the Institute's outpatient department were asked to respond to an asthma/sleep questionnaire and were requested to maintain a sleep diary for 1 week. A similar group of 30 unmarried healthy young university students also underwent an identical workup and functioned as controls. Ten subjects from each group maintained a peak expiratory flow rate (PEFR) diary.

RESULTS

The questionnaire revealed that 28 (93%) of the 30 patients experienced sleep disturbances (average 2.7/subject) as compared to 10 (33%) subjects in the control group (average 0.4/subject). The sleep diary confirmed that 27 (90%) of the 30 patients had sleep disturbances (2.4/subject/wk) as compared with 8 (27%) subjects (0.3/subject/wk) in the control group. These differences were statistically significant. Daytime sleepiness and tiredness (63%) and difficulty in maintaining sleep (60%) along with early morning awakening (46%) were more frequently seen. The asthmatic patients also had a shorter duration of sleep (427 min) as compared with the controls (474 min). These findings were significantly different from the control group.

CONCLUSIONS

Increased daytime sleepiness and tiredness was perhaps a reflection of the poor quality of sleep experienced by patients with clinically stable asthma. This consequently may lead to impaired daytime performance which can have a potentially serious effect for the patient and society.

The association of nocturnal asthma with asthma severity

The objective of this study is to assess nocturnal asthma as a marker for poor control of asthma. Cross-sectional study of asthmatic patients was conducted in six general practices in East Anglia, England. Subjects were 240 asthma patients, between the ages of 20 and 54 years, from the above general practices. A continuous asthma severity score (range 6-26), consisting of the summation of ordinal responses to questions regarding asthma symptoms during the last 6 months, was used for analysis. The crude association between nocturnal asthma and severity score was statistically highly significant. Regression analysis suggests that, after controlling for possible confounding variables, there remains a strong association between nocturnal asthma (NA) and severity, which is modified by self-report of current consultation for "nervous/emotional trouble" (current NT). In the absence of current NT, NA raises the severity score by 5.3 (95% CI 4.5, 6.2), whereas in the presence of current NT, NA raises the severity score by 8.2 (95% CI 4.8, 11.6). NA appears promising as a marker for poorly controlled asthma. Confirmation of the association found in this study by objective measures of severity would strengthen the utility of NA as a marker.