Prevalence of Cognitive Impairment in Obstructive Sleep Apnea and Its Association with the Severity of Obstructive Sleep Apnea: A Cross-sectional Study

INTRODUCTION

Obstructive sleep apnea (OSA) is a common disorder characterized by repetitive episodes of apnea during sleep and oxygen desaturation. OSA is a major public health problem, but the majority of physicians and the general public are unaware of this problem. Most of the time, OSA remains undiagnosed, which has serious health implications.

The development of OSA is associated with several factors like age, gender, anatomical variations, obesity, alcoholism, and drug intake.¹ There are many health implications of OSA. OSA has been associated with serious cardiovascular and metabolic comorbidities. Some of the conditions associated with OSA are hypertension, diabetes, cardiovascular disease, stroke, depression, endocrine abnormalities, neurocognitive impairment, and increased risk of automobile accidents.¹ It has also been found that undiagnosed or untreated OSA among workers in industry has adverse effects on their work performance as well as their safety in work place¹ by affecting their attention and vigilance.²

One of the major health impacts of OSA is in cognitive function.² Many studies have reported a range of cognitive impairment in OSA including selective and sustained attention, short-term memory, and executive functioning.^3-7 Cognitive impairment can be due to the excessive daytime sleepiness in OSA patients and also due to other mechanisms affecting the brain due to poor sleep quality, sleep fragmentation, and hypoxia. Neuroinflammation, oxidative stress, and sympathetic overactivity have also been implicated in the mechanism of cognitive impairment in OSA. Studies suggest that severe OSA can increase the risk of dementia in the elderly.⁸ It is considered to be one of the potentially modifiable risk factors for dementia.

OSA is diagnosed by sleep study or polysomnography. Studies conducted previously measure cognitive impairment objectively with tests such as Mini Mental State Examination, Montreal cognitive assessment, Wechsler test, etc. Excessive daytime sleepiness is measured by the Epworth Sleepiness Scale in studies of cognitive impairment. Recently neuroimaging is used to find the areas of the brain affected in patients with cognitive impairment.²

Cognitive impairment and sleepiness have been found to be improved with continuous positive airway pressure (CPAP) therapy for OSA. Therefore, early recognition and treatment of OSA are important.² Even though studies have demonstrated consistent improvement in cognition and performance, the exact magnitude of improvement has been found to be variable.

OSA typically shows a “tip of the iceberg” phenomenon, that is, very few cases are diagnosed even though the prevalence in populations, both Indian and Western, have been found to be high.⁹ It has been found that cognitive impairment due to OSA can lead to increased risk of automobile accidents and workplace-related accidents. Thus it becomes very important to assess the effect of OSA on cognitive functions in the population.

Studies conducted previously for assessing cognitive function in OSA have shown variable results.⁹ Some studies assess only particular neurocognitive domains and their impairment. Hence, more research is required in this field to assess the levels of neurocognitive impairment due to OSA and the benefit of treatment.

In this background, this study was done to determine the prevalence of cognitive impairment in patients diagnosed to have OSA and its association with the severity of OSA.

MATERIAL AND METHODS

RESULTS

DISCUSSION

In this study, 58.3% of patients (out of the total of 96 OSA patients) were found to have cognitive impairment as indicated by a Montreal cognitive assessment (MoCA) score < 26. The prevalence of cognitive impairment in OSA in this study is 58.3%, which is the primary outcome of the study. It was also found that as the severity of OSA increases, the MoCA score decreases, indicating an association between cognitive impairment and severity of OSA. The study could not find a significant association between ESS and MoCA score. This is the secondary outcome of the study.

This study had more patients with severe OSA, similar to the study by Chen et al.¹² This study was comparable to previous studies in terms of age and sex distribution and showed an increased prevalence of OSA with increasing age. There were more males than females in the study group. Obesity, systemic hypertension, and Type 2 diabetes mellitus were common in the study group and the number of patients with these comorbidities was more in the severe OSA group. The increased number of patients with obesity, systemic hypertension, and diabetes mellitus in this study could be due to the increased prevalence of these conditions among the general population in our part of the state.

In this study, the number of patients with excessive daytime sleepiness, as indicated by an Epworth Sleepiness Score of > 10 was 56. The number of patients with excessive daytime sleepiness was more in the severe OSA group. This was similar in the study by Chen et al.¹²

Studies conducted previously to assess cognitive impairment in OSA have shown variable results for the prevalence of cognitive impairment. In the case-controlled neuro electrophysiological study by Wen et al.,¹³ the prevalence rates of cognitive impairment using mismatch negativity and Montreal Cognitive Assessment scores were 73.99 and 76.54%, respectively. A total of 127 out of 348 obstructive sleep apnea-hypopnea syndrome (OSAHS) (36.5%) patients were found to have cognitive impairment in the study by Chen et al.¹² In a prospective sleep and cognition study conducted on 298 elder women without dementia, with a mean age of 82.3 years, it was found that 44.8% of women with sleep-disordered breathing developed mild cognitive impairment or dementia after a five year follow up.¹⁴ Kales et al.¹⁵ have found a prevalence of 76% for cognitive impairment in a study on 50 patients with OSA. In a cross-sectional study by Yusop et al.,¹⁶ in moderate and severe OSA patients, using Mini Mental State Examination to assess cognitive impairment, only 5 patients with severe OSA among the total 38 patients in the study had cognitive impairment (prevalence: 13.2%). The prevalence rate of cognitive impairment obtained from our study (58.3%) was intermediate between that obtained from other studies that were conducted previously. Both the studies by Chen et al.¹² and Wen et al.¹³ assessed and analyzed the various MoCA subdomain scores. But this was not done in our study. Similar to other studies, the mean MoCA score of the severe OSA group in our study was indicative of cognitive impairment.

The number of patients with cognitive impairment was found to be the highest in the severe OSA group. This may be due to the fact that mean AHI scores and mean ESS scores were higher in the severe OSA group, indicating the severity of OSA and also increased daytime sleepiness both of which can contribute to the cognitive impairment. It is also important to note that out of 4 patients who reported a history of automobile accidents, 3 were in the severe OSA group. Though studies by Chen et al.¹² and Yusop et al.¹⁶ showed that patients with increased severity of OSA had cognitive impairment, a statistical correlation was not obtained in these studies between severity of OSA and cognitive impairment.

CONCLUSION

OSA is an important health condition but early recognition and treatment of this problem seldom happen. One of the important health implications of OSA is the impairment of cognitive function. It becomes very important for the early detection and treatment of OSA and assessment of cognitive impairment in patients suspected of having OSA. The prevalence of cognitive impairment in our study is 58.3%, and it was found to be associated with the severity of OSA. It is recommended that cognitive impairment should be assessed in patients with moderate to severe OSA and in those with significant excessive daytime sleepiness.

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