PLMs are limb movements of 0.5-5 sec duration that typically occur in batches of 30 minutes or more with a periodicity of 5-90 minutes during the night. These are recorded during nocturnal polysomnography using anterior tibialis electromyogram (EMG) recordings (limb electrodes).
It is important to distinguish periodic legs movements from leg movements that do occur if the patients is waking up for other reasons, for example because of sleep apnea. A series of four or more PLM's with an interval between 5 and 90 sec between the onset of each limb movement is considered periodic. It is also helpful to note whether the periodic movement are leading to an arousal, and disturbing sleep.
As mentioned above, a PLM index (PLMI) above 5/hour may be supportive of RLS. Although a PLMI above 5/hour is not considered a formal disease in isolation of RLS symptoms, some authors have suggested it is part of the same disease continuum (PLM disease, or "PLMD" if associated with significant sleep disruption). Indeed, several of the genes associated with RLS, notably MEIS1, BTBD9 are also associated with PLMS independently of RLS. PLMD typically does not require treatment.