Aetiology of periodic limb movements

Periodic limb movements are common in normal subjects, especially the elderly. A PLMI of less than 5/h is usually considered to be normal, but this cut-off point may be too low in older subjects. When PLMS occur with other features of the restless legs syndrome, they are conventionally considered to be part of this, rather than a separate disorder. They are equally frequent in primary and secondary RLS.

Periodic limb movements in sleep also occur in the following situations.

1 Synucleinopathies, e.g. Parkinson's disease, multiple system atrophy, Lewy body disease.

2 REM sleep disorders, e.g. narcolepsy, REM sleep behaviour disorder.

3 Huntington's disease.

4 Tourette's syndrome.

5 Obstructive sleep apnoeas. The relationship of PLMS to obstructive sleep apnoeas is complex. The diagnosis of PLMS may be difficult in the presence of obstructive sleep apnoeas because arousal from the apnoeas often causes a limb movement indistinguishable from a PLM. The sleep fragmentation due to obstructive sleep apnoeas may also induce PLMS. Treatment of sleep apnoeas with, for instance, nasal continuous positive airway pressure may clarify the situation by abolishing the limb movements, but equally it may lead to more consolidated sleep and increase the number of PLMS.

6 Drugs. PLMS like RLS can be precipitated by a wide range of drugs (see page 215).

The periodic limb movement disorder (PLMD) is defined as the presence of an abnormal number of PLMS for the subject's age in the absence of RLS or any other disorder or drug that causes PLMS and with symptoms, particularly insomnia or excessive daytime sleepiness, due to the limb movements (Table 9.10). PLMD can be graded as mild (PLMI 5-24), moderate (PLMI 25-49) or severe (PLMI 50 or more or a PLMAI of 25 or more per hour).

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