Structural, Psychological, and Genetic Influences on Low Back and Neck Pain: A Study of Adult Female Twins
From: Arthritis & Rheumatism (Arthritis Care & Research) Vol. 51, No. 2, April 15, 2004, pp 160–167
Genetic factors have an important influence on back and neck pain reporting in women. These factors include the genetic determinants of structural disc degeneration and an individual’s inherited tendency toward psychological distress. MRI changes are the strongest predictor of low back pain.
Objective. To assess genetic and environmental influences on low back and neck pain in a classic twin design and to examine the extent to which these are explained by structural changes seen on magnetic resonance imaging (MRI) and psychological and lifestyle variables.
The emphasis of studies of the epidemiology of back pain to date has been on occupational and environmental factors. In general, these have confirmed modest effects of weight, height, smoking, and occupation, and psychological factors similar to those seen in this study. Psychological factors and previous pain experience also emerge as the most important in longitudinal studies that have examined risk factors for new episodes of pain. By contrast, data on genetic risk factors for pain are limited.
Methods. The subjects comprised 1,064 unselected women (181 monozygotic and 351 dizygotic twin pairs) recruited from a national registry of twin volunteers. Outcome measures included lifetime history of low back and neck pain (using a range of increasingly stringent definitions), MRI scores of disc degeneration in the lumbar and cervical spine, psychological distress as assessed by the General Health Questionnaire (GHQ), and lifestyle variables assessed by questionnaire.
Results. For all definitions of pain, there was a consistent excess concordance in monozygotic when compared with dizygotic twins, equating to a heritability for low back pain in the range of 52–68% and for neck pain in the range of 35–58%. The strongest associations were between low back pain and MRI change (odds ratio 3.6, 95% confidence interval 1.8 –7.3]) and between neck pain and response on the GHQ (odds ratio 3.3, 95% confidence interval 2.1–5.0). These associations were mediated genetically.
Discussion. The strongest predictor of severe low back pain in this study was MRI score. The use of radiographic and MRI measures of degenerative spinal disease have hitherto been regarded of little importance in predicting back pain in population samples. Indeed, several studies have noted an apparent poor correlation between MRI findings, such as disc prolapse or bulging, and clinical symptoms. For example, Jensen et al performed MRI scans in 98 asymptomatic subjects and found 52% had a bulge at 1 level, 27% had a protrusion, and 1% had an extrusion. Similarly, Boden et al examined 67 subjects who had never had low back pain or sciatica and found 20% had MRI evidence of a herniated disc. To date, however, no study has addressed the association in a population sample including subjects both with and without pain. Our data are the first to show a clear relationship between disc degeneration in the lumbar spine and the propensity to report pain in the lumbar spine, although interestingly no clear relationship exists for neck pain. MRI of the lumbar spine thus provides an objective measure against which to assess the influence of risk factors for the development of back pain.
The results are consistent with there being a genetically mediated association between clinical low back pain and degree of MRI change. In addition to showing a structural basis for low back pain reporting, our data also confirm its multifactorial nature. In the low back, associations were found with physical characteristics such as weight, height, and smoking exposure and with measures of psychological distress as assessed by the GHQ. The strongest association with pain reporting in the neck was with GHQ score. This risk factor profile is consistent with a number of other crosssectional population-based studies that also demonstrate a contrast between risk factors that determine pain in the low back and neck.
Additionally, the data shows for the first time that the psychological component of both low back and neck pain reporting has a genetic basis. While this observation itself does not resolve the question of cause and effect, it provides a different perspective on the origin of an individual’s propensity for pain. A genetic basis for psychological distress has been shown previously in twin studies (7). Our data suggest that the progress in identifying genes associated with anxiety and depression may be of direct relevance in understanding the development of clinically important back pain.
In conclusion, this study shows a substantial genetic contribution to the occurrence of severe back pain in the community. Our results also suggest that low back pain has a complex genetic etiology that is only partly accounted for by the genetic determinants of disc degeneration. The search for specific genes for back pain should therefore extend to include a broad range of pathologic, physiologic, and behavioral mechanisms. The strong predictive value of MRI in low back pain and psychological assessments in neck pain should help clinicians and researchers to progress the understanding and treatment of these common and complex complaints.
