Periodic limb movement (PLM) disorder is unique in that the movements occur during sleep. Most other movement disorders manifest during wakefulness. The condition is remarkably periodic, and the movements may cause poor sleep and subsequent daytime somnolence. PLM disorder may occur with other sleep disorders and is related to, but not synonymous with, restless leg syndrome (RLS), a less specific condition with sensory features that manifest during wakefulness. The majority of patients with RLS have PLM disorder, but the reverse is not true. Treatment involves either dopaminergic medication in an attempt to modify activity of the subcortical motor system or, more commonly, sedative medications to allow uninterrupted sleep. Many new agents are proving efficacious for treatment as well.

Symonds first described PLM disorder in 1953. The original name, “nocturnal myoclonus,” does not describe the condition accurately, since the movements are slower than are those of myoclonus. The original name seldom is used today.

The etiology of the primary form of PLM disorder is uncertain. Suprasegmental disinhibition of the descending inhibitory pathways may be a factor. Because the etiology is not clear, treatment is primarily symptomatic and does not modify the disease. Studies differ regarding the frequency of polyneuropathy in cases of PLM disorder. Martinez-Mena and Pastor found that only 1 of 9 patients had signs of neuropathy.

The secondary forms of PLM disorder may be due to diabetes mellitus, spinal cord tumor, sleep apnea syndrome, narcolepsy, uremia, or anemia. Many authors report an association between attention deficit hyperactivity disorder (ADHD) and PLM disorder. Antidopaminergic, dopaminergic, or tricyclic drug therapy or cessation of treatment with barbiturates or benzodiazepines may initiate the syndrome as well. Voderholzer and colleagues noted an increased incidence of periodic limb movements during sleep in patients with Gilles de la Tourette syndrome.However, the authors emphasized that the different responses to pharmacological treatments are evidence against a pathophysiological relationship between PLM disorder and Gilles de la Tourette syndrome.

The presenting symptom may be stereotyped periodic limb movements that cause awakening during the night, but often the presenting complaint is poor sleep and daytime somnolence. Haba-Rubio et al report that sleep changes induced by periodic limb movements during sleep (PLMS) are associated with decreased physical and psychological fitness on awakening.
• Occasionally, a bed partner may provide the history of limb movements.
• Nozawa and colleagues studied arousal index and movement index in PLM disorder and noted that the sleep-wake disorders associated with PLM relate to threshold of awakening.

Leg movements are stereotyped and involve one or both limbs.
• The movement simulates triple flexion with leg flexion, ankle dorsiflexion, and great toe extension; it lasts approximately 2 seconds and thus is not consistent with the rapid jerk that defines true myoclonus.
• The periodicity ranges from 20-40 seconds with a variable duration. The movements are said to occur mainly in non-rapid eye movement (REM) sleep.