A closely related disturbance is periodic leg movements of sleep. Like the restless legs syndrome, it may result in sleep deprivation and daytime somnolence or, more often, in disturbance of a bed partner. However, the diagnosis of periodic leg movements depends on finding them during polysomnographic recordings, whereas restless leg syndrome is identified on clinical grounds. There is less certainty as to the importance of periodic compared to restless leg movements in disrupting sleep. Originally described as “nocturnal myoclonus,” periodic leg movements are slower than myoclonic jerks. They consist of a series of repetitive movements of the feet and legs occurring every 20 to 90 s for several minutes to an hour; mainly the anterior tibialis is involved, with dorsiflexion of the feet and big toes, sometimes followed by flexion of the hip and knee. The movements are similar to the triple-flexion (Babinski) response, which can be elicited in normal sleeping persons. These movements may produce frequent microarousals or, if severe and periodic, full arousals. The patient, usually unaware of these sleep-related movements at the time they occur, is told of them by a bed mate or suspects their occurrence from the disarray of the bedclothes.
Periodic leg movement is closely associated with the restless legs syndrome and many sleep specialists consider it an integral part of the syndrome, but it also occurs independently with narcolepsy, sleep apnea, with the use of tricyclic and serotonin reuptake inhibiting antidepressants, and withdrawal from anticonvulsants and sedative-hypnotic drugs. Approximately 80 percent of individuals with restless leg syndrome will display periodic leg movements, but the opposite is not the case, as only 20 to 30 percent of patients with periodic leg movements have restless leg syndrome.
A genetic finding by Stefansson and colleagues derived from several populations, including the homogenous Icelandic, is that a nucleotide variant in a short segment of chromosome 6p is associated with periodic leg movements of sleep. This was found to hold in those with and without restless legs syndrome. If nothing else, as pointed out by the authors, this establishes that periodic limb movements are a distinct entity as defined in the era of genomics. The biologic significance and frequency in other populations of this variant is not yet known. Nonetheless, we continue to be impressed at the frequent cooccurrence of the two conditions and several shared underlying conditions such as iron deficiency, and treatments that are effective in both.
Treatment of RLS and PLS
A search for iron deficiency, and its correction if present, is indicated in almost all cases. It is appropriate in many patients to explore the reason for iron deficiency. A large number of symptomatic medications have proved helpful in the treatment of both the restless legs syndrome and periodic leg movements. Until recently, many practitioners have favored treatment with either dopamine agonists such as pramipexole (0.25 to 0.75 mg) or ropinirole (0.5 to 1.5 mg) taken 1.5 to 2 h before bedtime. Long-acting combinations of l-dopa/carbidopa (12.5/50 or 25/100 mg dose) taken at bedtime have also been successful, but l-dopa, and sometimes the dopamine agonists, causes some patients to develop the movements earlier, that is, in the daytime, which become more intense and spread to other body parts. A longer acting dopamine agonist, rotigotine patch is available to treat patients who have this augmentation phenomenon. An association between augmentation, even mild forms and with low doses of medication, and impulsive behavior has been noted by Heim and associates in a group of patients but the mechanism for their finding is not known.