Cognitive Function in Sleep Disorders
 by Meir H. Kryger


INTRODUCTION
    Sleep disorders, which can be broadly classified into disorders causing daytime sleepiness and those leading to problems in initiating or maintaining sleep, have been associated with cognitive function abnormalities. Cognitive function includes a variety of skills such as memory, decision making, psychomotor attention, sustained attention (vigilance), all of which are measured with performance tests.

    This report will review the putative mechanisms causing such dysfunction and then present an overview of published peer-reviewed data on this topic. The primary disorders to be examined include sleep apnea syndrome, and narcolepsy. Because sleep disorders are so common, and because cognitive dysfunction is so commonly affected, it is clear that cerebral dysfunction caused by these disorders is common. It is also highly probable that such disorders represent the largest number of cases of reversible cerebral dysfunction in the population.



BACKGROUND
    The central nervous system abnormalities in patients with sleep disorders to be discussed below can be caused by several mechanisms. First, patients may have less sleep than needed (i.e., disorders causing insomnia). Second, patients may have sleep fragmentation (i.e., obstructive sleep apnea). Third, patients may have abnormal regulation of sleep/wake (i.e., narcolepsy). These disorders can all lead to excessive daytime sleepiness. The excessive daytime sleepiness can cause, or be a correlate (by unclear mechanisms) of abnormal cognitive function. The sleep deprivation literature confirms that sleepy brain may not function normally.

    The repetitive episodes of cessation of breathing in sleep apnea, besides being associated with sleep fragmentation, may also lead to hypoxemia (1), and changes in cerebral blood flow, both which may also cause abnormal cerebral cortical function.

    In this report the focus will be on the two general areas of abnormalities that have been the most studied: cognitive and psychomotor performance. The former is usually assessed by a battery of neuropsychological tests. The terms "performance", "alertness", and "vigilance" are often used synonymously to describe the ability to perform specific tasks.



COGNITIVE FUNCTION IS ABNORMAL IN OSA
    Many studies from several countries have documented abnormalities in sleep apnea patients (1-7). The studies have documented various degrees of problems with memory (3,8), learning, intellect (9), and a decreased ability to initiate new mental processes (i.e., planning) (2).

    The many studies have used many different methodologies to reach the above conclusion. It is beyond the scope of this report to review the methodologies in detail, but a few points are worth emphasizing.

    First, no clear pattern of deficit has been consistently observed in the various studies. This is not surprising since the reports deal with different patient groups with differing severity of apnea and differing degrees of hypoxemia. Second, some studies suggest that hypoxemia may be associated with frontal lobe dysfunction which may explain propensity for distraction and problems in planning and initiating new tasks (2).

    Whether patients with mild to moderate OSA have cognitive dysfunction is not clear (10,11); some studies have shown that even mild OSA is associated with abnormal memory, while executive functions may be preserved. This is consistent with hypoxemia contributing to abnormal executive function (11). That even patients with mild OSA have measurable deficits can be inferred from improvement in some measures of cognitive function with nasal CPAP treatment. (12).

    There is little published about the pediatric population, especially effect on school achievement. One paper suggested documented impaired school performance (13), while another documented impaired cognitive function (14) . This is an important area in that the long term consequences or deficits may be different in pediatric populations.



PSYCHOMOTOR PERFORMANCE IS IMPAIRED IN OSA
    Performance is impaired in OSA and has been confirmed using various techniques (15,16). It has also been shown that abnormalities exist when tasks such as operating a motor vehicle are assessed (17-19).

    There is a poor quality of life with both family and workplace sequelae which is likely also related to the impaired ability to perform psycho-motor tasks (20).

    There is little published about the pediatric population but school performance is likely impaired (13).



CONSEQUENCES OF OSA
    The most widely studied consequences of abnormal performance in OSA is an increase in automobile accident rate (18,21-23). In many communities doctors are required to report patients who may pose a risk if they operate a motor vehicle. Some patients with sleep disorders are impaired while others are not at risk. At present there does not exist a validated instrument or test that will help in this assessment before or after treatment. This is problematic, since in many communities physicians are required to assess "suitability to drive"

    Doing poorly at school is a likely outcome of OSA is children (13).



EFFECT OF TREATMENT OF OSA
    Treatment has been shown to improve some measures of cognitive function (12,16) as well as performance (9,17), and has been shown in two studies to reduce automobile accident rate (9,24). Some of the cognitive changes in OSA may be irreversible (25). Also, it is possible that abnormal cognitive function may not be totally reversible in some children.

    There is little published about the pediatric population. One paper suggested that treatment of OSA improved impaired school performance (13), while another reported improved cognitive function with treatment (14).



THERE IS LITTLE PUBLISHED ABOUT COGNITIVE PROBLEMS IN OTHER SLEEP DISORDERS
    In contrast to the large number of studies published about OSA there are few studies in other specific sleep disorders such as narcolepsy or periodic limb movements in sleep. The few studies about narcolepsy suggest that there is impairment of cognitive function and performance (18,26-29) and probable improvement with treatment (30,31). In young insomniacs there is minimal or no objective impairment of daytime performance, while such deficiencies have been documented in elderly insomniacs (32).


CURRENT PROBLEMS
    There are several issues that confront the physician dealing with patients with sleep disorders with respect to cognitive function and performance.
    • Should physicians assess cognitive function and performance? If so, what efficient and cost effective evaluation should be performed and by whom? What is the role of the physician in evaluating the ability of the patient to assess the patients' ability to operate motor vehicles?
    • What is the effect of performance and cognitive dysfunction in the more common sleep disorders on actual work performance, quality of life, effect on automobile accident rate, and success in school in children?


FUTURE NEEDS
  • Evaluation of cognitive function in children with sleep disorders; this will require research studies.
  • Development of standardized tools to efficiently assess cognitive function and performance during the clinical assessment that are specific to the needs of sleep disorders patients. This will require research studies to develop the instruments and multi-centre trials to evaluate them.
  • Development of standardized methodology to evaluate driving performance and response to treatment. This will require research studies to develop systems that are correlates of driving.


FUNDING RECOMMENDATIONS
  • Fund research in pediatric age group to examine the effect of sleep disorders on development, cognitive function and school performance.
  • Fund research in the development of sleep-disorders-specific clinical evaluation instruments of cognitive function and performance.
  • Fund research on objective methods to evaluate the ability to operate a motor vehicle.




References

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Meir H. Kryger, Sleep Disorders Center, University of Manitoba Winnepeg, Manitoba Canada


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