Disorders of motor control

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Motor disorders during sleep

Narcolepsy is associated with a variety of parasomnias due to fluctuations in the degree of motor inhibition in REM sleep. Sleep terrors, sleep talking and walking are common, and irregular jerking movements and periodic limb movements may be seen during REM sleep because of a failure of motor inhibition. This also underlies the appearance of REM sleep behaviour disorder. Obstructive sleep apnoeas are more common than in normal subjects, probably because of an alteration in the inhibition of the activity of the dilator muscles of the upper airway and also because of obesity.

Sleep paralysis

This is a motor disorder which has been particularly associated with narcolepsy. Over 60% of those with narcolepsy and cataplexy have sleep paralysis. It is present in only around 10% of those without cata-plexy. It is the inability to move while consciousness is retained, either at the onset of sleep or, less commonly, when waking up. The respiratory muscles, or more probably only the diaphragm, retain activity but speech is impossible. The episodes usually last for less than 2 min but can occasionally persist for up to 30 min. The intense relaxation may be pleasurable, but much more commonly it is perceived as frightening because of the inability to move, the loss of control, and the occasional association with vivid dreams.

Sleep paralysis episodes are less common in the upright position and can be terminated by an intense effort to move or by a touch from the partner, who may be alerted to the situation by a change in the pattern of breathing or faint sounds that may be uttered.

Cataplexy

This is the sudden onset of muscle weakness during wakefulness. It usually appears within 2 years of the development of excessive daytime sleepiness, although it precedes this in around 10% of narcoleptics. Excessive daytime sleepiness is usually more severe in those with cataplexy. Cataplexy is present in at least 75% of those with narcolepsy.

It may follow an abnormal sensation around the mouth or face, which is succeeded by twitching of the lower face, and occasionally the upper limbs, and rapid eye movements due to an intermittent loss of muscle tone. This usually leads to symmetrical muscle weakness which may remain localized or become generalized. Mild episodes may simply cause drooping of the face, double vision, dysarthria or nodding of the head. The weakness may spread to cause loss of strength in the arms, trunk muscles and legs, causing the knees to buckle and the subject to fall to the ground, occasionally with persisting twitching of the limbs. Consciousness and memory of the episode are retained, although paralysis may be complete apart from the respiratory and extra-ocular muscles. Incontinence and tongue biting are not features of cataplexy.

Recovery from an episode of cataplexy is usually sudden and complete. Cataplexy attacks usually last for a few seconds or up to 2 min, but can occur repetitively for up to 20-60 min (status cataplecticus) and are then usually followed by REM sleep.

Cataplexy in narcolepsy is almost invariably triggered by a sudden intense emotion. There is no disorder of emotional control, but laughter or anger, or the anticipation of these emotions, or surprise are usually the triggers. It may also occur during or after sexual intercourse, or while being tickled. It varies considerably in frequency and severity, but is more common and more severe if the subject is tired, mentally relaxed and in the company of familiar people.

Situations in which sudden or intense emotions may be felt tend to be avoided and techniques of controlling the emotions and a flattened affect are often developed to avoid cataplexy. The attacks can sometimes be prevented by tensing the muscles at the onset of an attack, for instance by clenching the fists. Injuries and accidents including drowning may occur because of cataplexy.

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