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The parasomnias are conditions in which sleep is disrupted by inappropriate activation, sometimes in the brain centers that control body movements and sometimes in the autonomic nervous system, which governs various physiological and emotional functions. Some parasomnias are characteristic of non-REM (NREM or dreamless) sleep and occur mainly in the first third of the night. Others are typical of REM (rapid eye movement or dreaming) sleep and occur mainly in the last two-thirds of the night. Still others may occur at the transition between sleeping and waking or at any time during the night.
The most familiar parasomnias are sleepwalking (somnambulism) and tooth grinding (bruxism), which are caused by motor activation, and enuresis (bed-wetting), which is caused by autonomic activation of the bladder. Other motor parasomnias are sleep paralysis, rhythmic movement disorder, REM sleep behavior disorder, hypnagogic starts, and nocturnal leg cramps. Autonomic parasomnias include sleep terrors, nightmares, and painful erections.
Motor parasomnias result from the effects of sleep on the brain's motor control system. To fall asleep we must usually change our posture and muscle tone by lying down and relaxing our muscles. As a result, the centers in the brainstem that preserve the waking state are deactivated, and the slow waves of NREM sleep appear on the electroencephalograph (EEG). As NREM sleep deepens through four stages, organized movement gradually becomes more difficult. Several times a night, beginning about 90 minutes after sleep begins, REM periods interrupt slow-wave sleep. During these periods, motor centers are active (dreams are full of motion), but their commands are not obeyed because neurons in the spinal cord are inhibited and almost all muscle activity is suppressed.
The reticular formation of the brain stem, which generates the impulses responsible for major body movements, contains cells that produce several kinds of neurotransmitters, including norepinephfine, serotonin, acetylcholine, and dopamine. Norepinephrine and serotonin neurons become less active in NREM sleep and completely inactive in REM sleep. Neurons that produce acetylcholine are quiescent in NREM sleep but reactivated during REM periods. Nerve circuits using dopamine, which are necessary for all motor control, never become completely quiescent. This may explain why complex movement is never entirely suppressed and can occur even in very deep sleep. The position of the body changes throughout the night, and occasional muscle twitches as well as eye movements occur even during the REM stage.
NREM sleep disorders
Rhythmic movements, especially head rolling, rocking, and cries, are common during NREM sleep in childhood, especially in early infancy. The movements, probably a soothing mechanism, run strongly in families and occur far more often in boys than in girls. The sleeper is usually half awake and, if old enough, responds to a command to go back to bed. The movements almost always subside with age and rarely require treatment unless there is a risk of head injury.
Nocturnal leg cramps, which occur mainly in the elderly, may be intensely painful and cause partial or complete awakening. Women are more vulnerable. Pregnancy, the use of oral contraceptives, diabetes, and prolonged vigorous exercise raise the risk, possibly by depleting calcium and potassium. Hormone and mineral replacement therapy may be helpful.
The brain strongly resists arousal from NREM sleep. This sleep inertia may lead to confusion, anxiety, and involuntary movements when a person is awakened early in the night. Sleep deprivation raises the risk. For example, when mathematics students are kept awake for a long time, allowed to fall asleep, and then aroused after a short time, they cannot even subtract two-digit numbers.
Young children, who have the deepest NREM sleep and the greatest sleep inertia, are especially susceptible to confusional arousal and night terrors, as well as to sleepwalking. The risk of all NREM sleep disorders declines with maturity and disappears almost entirely by age 40, along with stages III and IV, the deepest phases of NREM sleep. Since the hormones that promote growth and sexual maturation are released during this stage of the sleep cycle, its disorders may be a price we pay for crucial processes of development.
REM sleep disorders
Children, again, are especially susceptible, because they have so much REM sleep. Rare in adult life, nightmares occur in 50% of children at the ages of 3 to 6, and they are also apparently common in younger children who cannot give a good account of them. They are not related to personality and not correlated with waking anxiety, but they are more common among depressed adults, who also have more REM sleep than normal. Drugs that may cause nightmares include Ldopa (prescribed for parkinsonism), propranolol (prescribed for migraine), and the antidepressants fluoxetine (Prozac) and clomipramine. Cognitive behavioral therapy may be helpful for adult nightmares. Another treatment is cyproheptadine, which inhibits the activity of serotonin.
REM sleep also produces several types of motor parasomnia. In sleep paralysis, the suppression of muscle movement persists for a few minutes after awakening from a dream. In this olden frightening state, the higher brain is awake, but the spinal cord is still asleep. More sleep usually solves the problem. If that is not sufficient, treatment with drugs that suppress REM sleep may help.
Another motor parasomnia, REM sleep behavior disorder, arises when sleepers act out scenarios from their dreams because the region in the brain stem that generates muscle tone becomes disconnected from the neurons that normally inhibit it. Although this disorder has a comical side, it can be dangerous. Sleepers may dive off their beds into imaginary swimming pools, tackle chests of drawers taken for opposing football players, or strike bed partners as they make life-saving wheel corrections in out-of-control dream automobiles. The typical patient is a man in his 50s or 60s with an incipient degenerative brain disease such as multiple sclerosis, Alzheimer's disease, amyotrophic lateral sclerosis, or (most commonly) Parkinson's disease.
Mood-altering drugs, including fluoxetine and other popular selective serotonin reuptake inhibitors (SSRIs), may induce REM sleep behavior disorder by changing the balance of serotonin and dopamine in the brain stem and the basal ganglia, which control involuntary body movements. Like tardive dyskinesia, the body movement disorder that develops in some schizophrenic patients after long exposure to antipsychotic drugs, SSRI-induced REM sleep behavior disorder may continue after the drug is discontinued. Other drugs that may cause the disorder are MAO inhibitors and caffeine, as well as withdrawal from alcohol, barbiturates, and benzodiazepines. The treatment of choice is clonazepam at bedtime. Alternatives are imipramine, clonidine, and L-dopa.
A motor disorder that occurs during the transition between sleep and waking is sleep starts or hypnagogic starts. These brisk, jerking movements of the head, neck, and shoulders may occur either spontaneously or as a reflexive response to sounds or touch, and they are usually intensified by emotional stress. Sleep starts are rarely serious enough to require treatment. They should not be mistaken for other motor disorders such as benign neonatal myoclonus and the restless legs syndrome. Benign neonatal myoclonus is the name for muscle twitches that occur in some infants during sleep. It is often misdiagnosed as epilepsy. Restless legs syndrome is a sensory-motor disorder that occurs mainly in the elderly. It often causes insomnia and can be difficult to treat.
Some parasomnias, especially enuresis and bruxism, can occur at any stage of sleep. Young children often sleep so deeply that they do not awaken in response to signals from the bladder and repeatedly wet their beds. Children almost always grow out of enuresis. Bruxism is caused by the automatic activation of the brain centers that govern chewing motions. As teeth are clenched rhythmically by the powerful muscles of the jaw, a dramatic crunching sound may awaken everyone in the house except the bruxist. The unfortunate social and dental side effects usually subside on their own but if necessary can be controlled by pharmacotherapy or by a boxer's mouthpiece.
Parasomnias are common because the control of motor activity during sleep is complex and difficult. In any system as intricate as the brain, one part may become so overexcited or inhibited that its activity is dissociated from the rest, especially during transitional states such as the passage between sleep and waking or between different stages of sleep. Parasomnias are the natural consequences of errors in timing and balance that are inevitable in a system with such broad functions and so many centers of control. Almost everyone experiences at least one of these symptoms at least once as normal patterns of sleep are differently affected by changes in the brain's activity throughout life.
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