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Restless legs syndrome - a pain in the leg
Dr Alison Bentley
The human body is exposed to many sensations at any one time. These include
vision, touch, hearing, vibration, ‘pins and needles’, itch, tickle and
pain. Most of these sensations we just notice but only for pain and tickle do we
feel the need to avoid and get away from the sensation. A subset, up to 15% at
last count, of the general population have another type of sensation that they
try to avoid. This is the sensation associated with Restless Legs Syndrome.
This sensation has an unusual feature in that it seems to occur mainly in the
evening. The need to escape the strange feelings in the legs forces the
sufferers to move their legs almost continuously. This movement does bring some
relief but only until the person stops moving – then the sensations come back.
Having to move the legs to keep the sensations at bay can significantly impair
the ability to fall asleep.
While the disorder itself was described over 60 years ago the origin of the
sensations of restless legs syndrome has remained a mystery. Evidence has been
accumulating that the pain pathways may be involved. Many patients remark that
painkillers, particularly those containing codeine, are often more useful in
getting a good night’s sleep than sleeping tablets. Codeine is a derivative of
morphine and opium, long used for treatment of severe pain. Some sufferers of
severe restless legs describe the sensations as painful. The results of a
standard battery of sensory tests indicated a deficiency in pain processing.
Thus research was initiated trying to define the sensations of restless legs
and compare them to previously described forms of pain. There are two types of
pain recognized by the International Association for the Study of Pain. The
first type – nociceptive – is the type usually associated with damage to
tissue such as when skin is burnt or when under-exercised muscles are suddenly
over-exercised. The other type of pain – neuropathic - occurs when there is
damage, of even a minor type, to the nerves of the pain pathways themselves. In
the absence of obvious tissue damage, this second type then becomes the most
likely source of the sensation associated with restless legs syndrome.
Other causes of neuropathic pain include those caused by damage to nerves in
the periphery of the nervous system, such as occurs in long-standing diabetes,
alcoholism or HIV infection, damage to the spinal cord such as occurs in back
injuries or damage to the brain itself such as may occur after a stroke.
Researchers have previously tried to define the differences between noci-ceptive
and neuropathic pain by using the descriptions given by patients with these
types of pain. In the 1970’s a questionnaire, the McGill pain questionnaire,
was developed in order to use 78 of these different words to describe pain in
order to identify both the type and intensity of any pain. The McGill
questionnaire has been used for many different pain studies and found to be very
useful.
Researchers at the Wits Dial.a.Bed Sleep laboratory used the McGill
questionnaire to investigate the sensations of restless legs. They found that
all patients were able to use the words given as descriptors on the McGill
questionnaire as descriptors for the sensations of restless legs and that the
severity of restless legs could be measured on the McGill questionnaire.
However, the sensations that were used by the patients with restless legs
syndrome did not match those sensations typically described by patients from
other studies with either noci-ceptive or neuropathic pain. Thus while restless
legs can be measured by pain measures there is no clarity as to which type of
pain the sensations of restless legs most resembles.
Hopefully, this information will help in defining the site of the lesion in
restless legs syndrome as well as leading to more treatment options. At present
dopamine, a chemical used as an information transmitter in the brain, is being
used as treatment. Given this new data it is possibly not surprisingly then
that, in other situations, the administration of dopamine can cause resolution
of some forms of neuropathic pain.
Research will be continuing in this field
particularly with comparisons of the pain produced by various causes of
neuropathic and noci-ceptive pain to those sensation occurring in restless legs
syndrome. Other research being done in the sleep laboratory includes the
measurement of reflexes in patients with restless legs in order to see whether
the spinal cord is, as suspected, the site of the lesion. Other active research
fields in the sleep lab include the metabolic effects to the mother of having
babies who don’t sleep properly, novel drugs for the treatment of insomnia,
the relationship between sleep loss and exercise and various other projects to
do with the relationship between pain and sleep. - Wits news
More information:
 University
of the Witwatersrand: www.wits.ac.za
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