Neuropsychiatric Dimensions of Movement Disorders in Sleep

Special Report: Neuropsychiatry

With the change of state from wakefulness to sleep, muscle activity and tone decrease, and they are lost completely during rapid eye movement (REM) sleep. Therefore, one would anticipate few, if any, movement problems associated with sleep. In spite of these physiologic changes, however, normal sleep is not totally free of movement. The average sleeper moves about 40 to 50 times a night and this number changes in certain situations. For example, sleep deprivation results in a sleep with fewer movements.¹

In addition to total body movements, limb jerks and twitches also occur in normal sleepers. They typically occur during sleep onset (sleep starts or hypnic jerks) or in association with REM sleep.² Such movements in sleep are increased in persons who have movement disorders while awake, such as those with Parkinson disease (PD) or Tourette syndrome (TS).

When considering disorders of movement during sleep, the physician should ascertain whether abnormal movements also occur during awake periods. Abnormal movements that are present during the day, such as the motor disturbance of PD or TS, are usually quiescent during sleep, while those occurring primarily in sleep (eg, nocturnal epilepsies, parasomnias, restless legs syndrome [RLS], periodic limb movements of sleep [PLMS]) rarely intrude into awake periods. Some disorders, such as seizures, manifest predominantly during sleep, but may occasionally occur during periods of wakefulness. A classification of sleeprelated movement disorders is presented in Table 1. I will discuss the more salient of these disorders in this article.

Previously referred to as “nocturnal myoclonus,” PLMS are regarded as a distinct nosologic entity, even though they overlap a great deal with RLS and other sleep disorders. These movements primarily occur in the lower limbs and are classically described as phasic extensions of the big toe and dorsiflexion at the ankle, occurring with a periodicity of 20 to 40 seconds. Flexion at the knee and hip may occur, and movements may involve the upper limbs. Both lower limbs are usually involved but not necessarily symmetrically or simultaneously. Sometimes only one leg is involved, or the phenomenon may alternate from one leg to the other.³ The electromyographic characteristics of the movements are varied and are usually of longer duration than those of classic myoclonus, typically 1.5 to 2.5 seconds long (range, 0.5 to 5 seconds). There may be an initial myoclonic jerk followed by a tonic contraction, or a polyclonic contraction with or without a tonic component.

PLMS are common in healthy elderly persons, with 45% of 65- to 76-year olds, women more often than men, having 5 PLMS per hour at night⁴; the condition is rare before the age of 30. PLMS occur in a number of sleep disorders, particularly RLS, but also narcolepsy, REM sleep behavior disorder (RSBD), and obstructive sleep apnea. PLMS also occur in awake subjects with RLS but only rarely in controls.⁵

The clinical significance of PLMS continues to be debated, since many studies have failed to demonstrate an association between PLMS and symptoms of sleep disturbance.^6,7 It is possible that people who complain of insomnia caused by leg movements may have a lower threshold of arousal.

The pathogenesis of PLMS is not clear. Lesion, imaging, and laboratory studies indicate neuronal hyperexcitability with involvement of brainstem and spinal cord structures, in particular, the central pattern generator for gait.⁸ There is also evidence of decreased dopaminergic transmission.⁹ PLMS have also been associated with neuroleptic-induced akathisia.¹⁰ Tricyclic and selective serotonin reuptake inhibitor antidepressants may induce or worsen PLMS,¹¹ presumably via serotonergic influences on dopaminergic transmission.