The Neuropsychology of Sleep Disorders
Raymond Cluydts and Edwin Verstraeten
Within the vast area of the life sciences, sleep and sleep disorders have received increasing attention and interest during recent decades from governmental and health authorities, the news media, and the public in general. A big step forward in the recognition of sleep as an important aspect of our health was based on findings revealing the numerous effects of poor or inadequate sleep on daytime functioning and quality of life in general.
Epidemiological studies show (Partinen and Hublin, 2000) that a vast majority of children, adolescents, adults, and the elderly can expect to suffer for either a short or longer period in their lives, from some kind of sleep problem. In addition to primary insomnia, sleep disturbances associated with breathing problems, muscular contractions, or irritability during the night, as well as narcolepsy and circadian rhythm disorders, have been identified. These conditions have selective effects on either sleep stages, such as REM or non-REM sleep, or sleep onset, the continuity of sleep, or sleep architecture. This disturbance or lack of sleep has been associated with serious safety consequences not only for the patient, but also for the public. Daytime sleepiness has been recognized as a major intervening factor. A sleepy brain cannot function normally and attend to ongoing activities. Many disasters witnessed in recent years, such as train, coach, and car crashes, may be related to ignorance of the limits of human performance after sleep loss.
On the individual level, extensive clinical and neuropsychological investigation of the sleep-disordered patient seems necessary to understand fully the impact of sleep disorders on the patient's daily activities. But, next to the process of diagnosis, the results obtained from this neuropsychological examination can provide the clinician with clues for a comprehensive treatment plan and outcome measures of the therapy.
The fact that sleep loss and poor sleep result in daytime impairment of cognitive functioning is inferred not only from real disasters but also mainly from two types of research and clinical findings. First there are research data on the effects of total and partial sleep deprivation and sleep fragmentation; secondly, we can rely on clinical data gathered from patients suffering from sleep disorders. Decades of research on sleep deprivation show the consistently detrimental effects of sleep shortage on next-day performance. Good reviews on this topic are numerous (Walsh and Lindblom, 1997; Bonnet, 2000; Pilcher and Hufcutt, 1996; Dinges and Kribbs, 1991). Most of these studies were performed to reveal the function(s) of sleep, providing us with key information about areas in the neurocognitive field that are susceptible to these experimental manipulations. These areas include attentional performance (impact of daytime sleepiness), mood changes, and memory function. But can these experimentally induced sleep disturbance be compared to the sleep disturbance experienced by insomniacs? Probably not. At best, this experimentally induced sleep fragmentation and partial sleep deprivation can be compared to the effects of an acute shift-work or jet-lag situation. We do not consider induced sleep deprivation a condition of 'insomnia' but one of 'sleeplessness'. Furthermore, these experiments are conducted in laboratory situations and are very much affected by selection bias (students as volunteers), experimental set-effects (motivation), and testing and instrumentation effects (learning), so that a generalization of these findings to a sleep disorder population is absolutely unwarranted. Our patients are mostly confronted with non-stimulating situations and lack of motivation in everyday life. Nevertheless, the findings in the three areas described above can give us some clues on promising areas to explore in insomniacs. However, we should certainly not restrict our studies to these areas only. Moreover, the finding that, in many studies on partial sleep deprivation in volunteers, few neurocognitive effects were evidenced cannot be simply generalized to the patient population.
From clinical practice, we know that patients suffering from sleep disorders complain not only of poor sleep, but also of significant impairment of their daytime activities. This includes cognitive impairment such as memory deficits (both encoding and retrieval) and limited attention span; some patients even report specific disorders such as visuoperceptual and spatial orientation difficulties. These clinical observations became the focus of some population-based studies in recent years. These studies confirm that patients with insomnia experience more problems with memory, concentration, thinking, and ability to accomplish tasks (Roth and Ancoli-Israel, 1999). These problems not only affect quality of life scales (Zammit et al., 1999) but are also associated with poor occupational performance and increased absenteeism. Leger (1999) estimated the annual economic impact of insomnia in France to be around US$ 2 billion.
A crucial question is whether these neurocognitive phenomena are just one part of a larger 'neurotic' complex that is 'complaint'driven, or whether a causal relationship between the sleep complaint and the reported daytime neurocognitive impairment can be demonstrated. In the next sections, we will discuss the data available on neurocognitive functioning in patients suffering from following sleep disorders: primary insomnia, sleep apnoea syndrome, and narcolepsy.
Insomnia is usually defined according to the Diagnostic and Statistical Manual of Mental Disorders (4th edition) (DSM-IV) (American Psychiatric Association, 1994) or the International Classification of Sleep Disorders (ICSD) (Revised) (American