Night-to-night variability of periodic leg movements during sleep in restless legs syndrome and periodic limb movement disorder: comparison between the periodicity index and the PLMS index.

BACKGROUND: The number of periodic leg movements during sleep (PLMS index) shows high night-to-night variability, requiring multiple nights for its reliable estimation. It is currently not known if this is also the case for the degree of periodicity of leg movements, quantified by the Periodicity index. OBJECTIVE: To compare night-to-night variability of PLMS and Periodicity indices in patients with restless legs syndrome (RLS) or periodic limb movement disorder (PLMD). METHODS: Eighteen idiopathic RLS patients and 9 PLMD patients were recruited. Subjects underwent two consecutive full night polysomnographic studies. Polysomnographic recordings were scored and leg movement activity analyzed during sleep for the computation of the PLMS and Periodicity indices. RESULTS: In both patient groups, the Periodicity index showed a significantly lower degree of variability than that of PLMS index, being >6.5 times lower in RLS patients and 2 times lower in PLMD patients. CONCLUSIONS: These data support the use of the Periodicity index in the evaluation of PLMS in RLS and PLMD and indicate that this parameter seems to be more stable than the widely used PLMS index which has higher night-to-night variability.