Periodicity Index limits variability when measuring restless legs

By Andrew McCulloch, medwireNews Reporter

The Periodicity Index is a more stable measure of leg movements during sleep in restless legs syndrome (RLS) than the widely used periodic leg movements (PLMS) index, investigators have found.

Their study showed that the Periodicity Index had a significantly lower night-to-night variability than the PLMS index.

"The significant night-to-night variability limits the potential application of the PLMS index," say the researchers. "Especially because a single recording night is often the rule for polysomnographic studies."

In their study of 18 patients with idiopathic RLS and nine PLMD patients, Raffaele Ferri (Oasi Institute for Research on Mental Retardation and Brain Aging, Troina, Italy) and colleagues found that the degree of variability with the Periodicity Index was 6.5-times lower compared with the PLMS index in RLS patients and two times lower in PLMD patients.

The patients underwent two consecutive full-night polysomnographic studies, during which polysomnographic recordings were scored and leg movement activity analyzed for the computation of the PLMS and Periodicity indices.

"These results… suggest that the Periodicity Index characterizes a different and more stable feature of LMs during sleep in RLS and PLMD," Ferri and team report.

They also note that "in recent years, additional features of PLMS, such as time of the night distribution and the Periodicity Index have been demonstrated to distinguish 'true' PLMS from more irregular leg motor activity during sleep."

The researchers therefore suggest in Sleep Medicine that a combination of three main indices, ie, PLMS index, Periodicity Index, and night-time distribution of the leg motor activity, may be a better way of characterizing leg movement activity during sleep than the PLMS index alone.

However, they acknowledge that as the Periodicity Index targets the time structure of leg movements specifically, accurate computation requires a minimum amount of leg movements and this still needs to be better established.

Currently, preliminary mathematical considerations indicate that the minimum number should be approximately 80 total leg movements, which would be about 10 per hour during 8 hours of sleep.

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