Professor Jim Horne

We’ve all experienced that occasional ‘waking up with a jump’, soon after going to sleep. Usually, it is a kicking of one or both the lower legs, and the real cause is unknown. For some people it becomes frequent and distressing, happening most nights every 30 seconds or so, especially around the onset of sleep and well into it. Each of these particular kicks lasts longer than the ‘normal jump’, maybe a couple of seconds at a time, as the leg muscles continue to contract after the jump. Needless to say, it is very unpleasant for the sufferer who not only can’t get to sleep because of the kicks, but when they do fall asleep, and unbeknown to the sleeper, kicking continues to disturb sleep, and doesn’t abate until sleep eventually becomes deeper. Even if they do wake up, the kicking can still continue for a while. Depending on the extent of the sleep disruption, there is sleepiness in the daytime, which is often excessive. The sleep of bed partners can also be impaired if they are the recipient of the kicks. This complaint is usually called ‘nocturnal myoclonus’, or ‘hypnic jerks’ and is one of two related disorders that come under the general heading, ‘periodic limb movements in sleep’, or PLMS in short.

This other disorder is ‘restless legs syndrome’, which can also appear in wakefulness, particularly in the evenings, when it is experienced as an unpleasant creeping-crawling sensation deep within the knees, thighs or calves, and brought on by sitting or lying. Some people liken it to a feeling of ‘insects inside the legs’. Typically, it produces an irresistible urge to stretch and move the legs about, which, when this happens in bed, further delays sleep onset. Like nocturnal myoclonus, it continues into sleep, causing momentary awakenings and sleep disruption. Sufferers can obtain some relief by getting out of bed and walking about. Even keeping the legs and feet cool, can give some relief.

Both forms of PLMS are more common during pregnancy, in the over 50s, and more so in the elderly. PLMS may well need the attention of a sleep clinic for proper diagnosis and treatment. One or both forms can appear with: iron deficiency, a build up of urea in the blood, and with heavy smoking or excessive caffeine intake. Apart from sorting out these aggravating factors, treatment for both disorders is usually by leg exercises during the day, iron supplements if necessary and, most recently, by low doses of medicines called ‘dopamine agonists’ that increase the brain’s own levels of the neurotransmitter, ‘dopamine’. These can provide immediate relief. Of course, pregnancy will limit their use, although the mother with PLMS can usually be reassured that it will probably disappear after the birth.