Neuropathic pain has much in common with chronic pain that may occur in several bodily areas. This may also include the responsible genes.
There are pains that are self-sufficient. They have lost their function and raise alarm although the danger has been banned, the injury has healed for weeks, the wound has closed, the muscle regenerated. Chronic widespread pain (CWP) not only fires at one site but up and down, left and right in the body. And then there is neural (neuropathic) pain, when specific nerves have been damaged during injury, surgery, or a neural disease. Sometimes the entire nervous system is deranged, with stinging and burning at the least touch or temperature sensation.
Both are independent diseases, but there are indications that they could have something in common. British researchers investigated the links between both diseases in thousands of twins (1). Scientists, led by William Frances from London’s King’s College identified among twins with CWP strikingly many with neuropathic pain as well. In addition, they found evidence that not only the same genes may at least in part be responsible for both, but also that they may share certain risk factors such as smoking, advanced age, and obesity.
For their study, the researchers collaborated with TwinUK, the largest register for monozygotic and dizygotic twins in the UK and North Ireland. More than 12,000 twins are registered there, including health status data and information on their lifestyle: an invaluable data pool. With their help, Frances and his colleagues have investigated whether CWP and neuropathic pain coincide noticeably. For this they distributed thousands of questionnaires to the twins, asking for typical symptoms. They identified 636 twins with CWP. Of those, 16% also suffered from neuropathic pains with high likelihood. These are twice as many as in the general population, where neuropathic pain occurs in 6-8%, significantly less frequent.
But how does this association manifest? Maybe both diseases are favored by the same risk factors, was one of the speculations of the scientists. When they evaluated data from the register they indeed found many commonalities: smoking, obesity, higher age, and specifically female sex determined both CWP and neuropathic pain. Social status as well seems to play a role: the higher it is, the lower the risk for both pain types. However, these factors appear not to be specific, as smoking and obesity are also evident risk factors for the development of other types of chronic pain.
Next, the researchers compared the genetically identical monozygotic twins to the dizygotic ones, which share only 50% of their genetic heritage. This was that wanted to find out which role genes (in general) may play in the development of neuropathic pain. Are among them mostly monozygotic twins, then this argues in favor of predominantly genetic responsibility. Although the groups with CWP and neuropathic pain was relatively small, a specific software allowed them to conclude that genes account only for about one-third of the effect, while two-thirds are attributable to environmental factors. Some of the genetic factors that are responsible for the neuropathic pain are the same than for CWP, according to the scientists. This, however, has to be confirmed by further studies.
Other researchers have already identified specific gene variants that may play a role in neuropathic pain, and others, that may contribute to CWP. One of the next steps, therefore, needs to be to see whether and where there are overlaps between these genes, the scientists say.
This is part 4 of a series covering twin health provided by Paul Enck from the Tübingen University Hospital and science writer Nicole Simon.
- Momi SK, Fabiane SM, Lachance G, Livshits G, Williams FM. Neuropathic pain as part of chronic widespread pain: environmental and genetic influences. Pain. 2015;156:2100-6.