Approach

A diagnosis of PLMD is based on a supportive history, including, wherever possible, a collateral history from a partner, together with exclusion of alternative causes of sleep disturbances. Definitive diagnosis requires polysomnographic confirmation and is defined as a frequency of periodic limb movements of sleep (PLMS) >15/hour in adults, accompanied by sleep disturbance or daytime impairment.[1]​​[2][3]​​​[5]

Clinical features

PLMD is frequently associated with nocturnal arousals of which patients are aware. Partners sleeping in the same bed may be woken by limb movements (mostly of the legs). These limb movements may be reported as the presenting symptom by the patient, but more often by the bed partner. Patients frequently report feeling unrefreshed following sleep.[1]​ PLMD is associated with subjective diminished physical and psychological fitness on awakening, and with symptoms of daytime sleepiness, fatigue, and poor performance at work.

The prevalence of PLMD is as high as 80% in individuals with restless legs syndrome (RLS). Patients with RLS may report an irresistible urge to move their legs, and unpleasant sensory symptoms in the lower limbs, with symptoms worse in the evening and at night. RLS is troublesome during wakefulness, whereas PLMD usually occurs during sleep.

Physical examination is typically normal in primary (idiopathic) PLMD, but may reveal evidence of an underlying cause for PLMD, such as pallor in anaemia.

PLMD has been reported to occur in individuals with narcolepsy, which is characterised by episodes of sudden onset of sleep during the day, and may or may not be associated with loss of muscle tone. It may also occur in RLS, obstructive sleep apnoea syndrome, rapid eye movement sleep behaviour disorder (RBD), narcolepsy, congestive heart failure, essential hypertension, end-stage renal disease, spinal cord injury, syringomyelia, alcohol dependence, Parkinson's disease, and Tourette's syndrome.

A history of snoring, early-morning dry mouth, headache, and elevated body mass index may all be supportive of PLMD associated with obstructive sleep apnoea. In PLMD associated with RBD, body movements may occur as dreams are re-enacted during rapid eye movement (REM) sleep.

A thorough medication and recreational drug history is important, because PLMD has also been reported to occur in individuals withdrawing from benzodiazepine and barbiturate therapy, and in those taking neuroleptic and dopaminergic medications.

Electromyography (EMG) evaluation of limb movements during polysomnography

EMG activity is recorded over the tibialis anterior muscles. World Association of Sleep Medicine (WASM) 2016 standards for recording and scoring leg movements in polysomnograms define candidate leg movements (CLM) as any monolateral leg movements lasting 0.5 to 10 seconds or bilateral leg movements lasting 0.5 to 15 seconds. Periodic leg movements (PLM) are defined by runs of at least four consecutive CLM with an intermovement interval ≥10 and ≤90 seconds without any CLM preceded by an interval <10 seconds interrupting the PLM series.[3]

EMG activity during movements may assume varying forms. Movements include tonic activity lasting several hundreds of milliseconds, with or without associated myoclonic activity; an initial myoclonic jerk, followed by tonic activity; or several myoclonic jerks in clusters (polymyoclonus), sometimes followed by tonic activity.

A PLMS index (PLMS-I; defined as number of PLMS per hour of total sleep time) of >15 for the night, accompanied by clinically significant sleep disturbance or daytime impairment, is diagnostic for PLMD.[1]​​[3][27]

PLMS associated with arousals are termed PLMS-Ar, and these have been postulated to be clinically more relevant in contributing to disrupted sleep. The PLMS-Ar index is determined as the number of PLMS-Ar per hour of total sleep time.[1]​​[3][27]

Electroencephalogram (EEG) evaluation during polysomnography

EEG recording in polysomnography is performed with 2 'reference' electrodes and 6 'exploring' electrodes attached with paste to the scalp over the frontal, central, and occipital areas.[27]​ Recordings from these electrodes can be grouped into different stages of sleep (non-REM, stages 1 to 4, and REM sleep) and wakefulness.

PLMS predominantly arise during non-REM stages 1 and 2 of sleep, and progressively diminish during the deep sleep stages 3 to 4. They almost never occur during REM sleep, although they may persist in extreme cases.[1]​​​

Assessment of sleep-related breathing disorders by polysomnography

PLMS may be associated with sleep-related breathing disorders, such as obstructive sleep apnoea.​[1]​​​ Movements may occur in close temporal association with apnoeas, or independently from apnoeic episodes. Apnoeas and hypopnoeas should be scored.[27]

Evaluation of respiratory disorders is important, as these may independently be the cause of sleep disturbances, and may be amenable to treatment with non-invasive ventilation techniques. In these cases, any excessive daytime sleepiness associated with PLMS may not be due to the leg movements, but merely an associated condition; therefore, treatment suppressing the leg movements may not resolve associated daytime somnolence.

Laboratory investigations

Laboratory investigations may include the following.

  • FBC: anaemia, particularly if related to iron deficiency, may be associated with PLMD. This finding; however, is very non-specific, and iron replacement may not result in resolution of symptoms.

  • Serum ferritin: in PLMD associated with RLS, iron replacement is widely recommended if serum ferritin levels are <101 pmol/L (<45 micrograms/L or 45 nanograms/mL) (normal range for adults is 45-674 pmol/L [20-300 micrograms/L or 20-300 nanograms/mL] for men and 45-337 pmol/L [20-150 micrograms/L or 20-150 nanograms/mL] for women), as patients may experience symptoms when ferritin is in the low to normal range.[28]

  • Urea: uraemia may be associated with PLMD. Uraemia in end-stage renal failure may also result in PLMD.

Actigraphy

This non-invasive technique has been proposed as an alternative method for home monitoring of periodic limb movements, and as a more cost-effective and widely available alternative to polysomnography in the diagnosis of limb movement disorders.[29]​​ Actigraphy is not recommended as an alternative to EEG for the diagnosis of PLMD, but it may be useful to assess night-to-night variability.[1][30]​​

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