Back pain: a study of twins

Twin studies for the prognosis, prevention and treatment of musculoskeletal conditions

BACKGROUND

Musculoskeletal conditions are highly prevalent in our ageing society and are therefore incurring substantial increases in population levels of years lived with disability (YLD). An evidence-based approach to the prognosis, prevention, and treatment of those disorders can allow an overall improvement in the quality of life of patients, while also softening the burden on national health care systems.

METHODS

In this Masterclass article, we provide an overview of the most relevant twin study designs, their advantages, limitations and major contributions to the investigation of traits related to the domain of musculoskeletal physical therapy.

CONCLUSIONS

Twin studies can be an important scientific tool to address issues related to musculoskeletal conditions. They allow researchers to understand how genes and environment combine to influence human health and disease. Twin registries and international collaboration through existing networks can provide resources for achieving large sample sizes and access to expertise in study design and analysis of twin data.

Genome-wide association scan of neuropathic pain symptoms post total joint replacement highlights a variant in the protein-kinase C gene

Neuropathic pain-like joint symptoms (NP) are seen in a proportion of individuals diagnosed with osteoarthritis (OA) and post total joint replacement (TJR). In this study, we performed a genome-wide association study (GWAS) using NP as defined by the painDETECT questionnaire (score >12 indicating possible NP) in 613 post-TJR participants recruited from Nottinghamshire (UK). The prevalence of possible NP was 17.8%. The top four hits from the GWAS and two other biologically relevant single-nucleotide polymorphisms (SNPs) were replicated in individuals with OA and post TJR from an independent study in the same area (N=908) and in individuals from the Rotterdam Study (N=212). Three of these SNPs showed effect sizes in the same direction as in the GWAS results in both replication cohorts. The strongest association upon meta-analysis of a recessive model was for the variant allele in rs887797 mapping to the protein kinase C alpha (PRKCA) gene odds ratio (OR)_possNP=2.41 (95% CI 1.74-3.34, P=1.29 × 10^-7). This SNP has been found to be associated with multiple sclerosis and encodes a functional variant affecting splicing and expression of the PRKCA gene. The PRKCA gene has been associated with long-term potentiation, synaptic plasticity, chronic pain and memory in the literature, making this a biologically relevant finding.

A118G polymorphism in the μ-opioid receptor gene and levels of β-endorphin are associated with provoked vestibulodynia and pressure pain sensitivity

Background and aims

Provoked vestibulodynia (PVD) is the most common cause of dyspareunia among young women. The aetiology is largely unknown and treatment is often extensive and longstanding with varying outcomes. Patients display general pain hypersensitivity and there are correlations with other chronic pain syndromes such as fibromyalgia later in life. The A118G polymorphism in the μ-opioid receptor (OPRM1) gene influences endogenous pain regulation and pain sensitivity, but has not been studied in this patient group before. We aimed to investigate a possible association between A118G polymorphism and PVD, with correlation to plasma levels of β-endorphin, and to explore relationships between this polymorphism and pain sensitivity among women with PVD and healthy controls.

Methods

This case-control study included 98 women with PVD and 103 controls. Participants filled out study-specific questionnaires and underwent quantitative sensory testing of pressure pain thresholds on the arm, leg and in the vestibular area. Levels of β-endorphin were analyzed by radioimmunoassay using the EURIA-beta-endorphin kit, and the A118G single-nucleotide polymorphism (SNP; rs1799971) in the OPRM1 gene was analyzed using the TaqMan SNP genotyping assay.

Results

The 118G allele was more common in controls (44%) than in patients (30%) (p = 0.042). The odds ratio of having PVD was 1.8 in participants carrying the 118A allele compared to participants hetero- or homozygous for the 118G allele (OR = 1.846, CI: 1.03-3.31, p = 0.039). Pressure pain thresholds on the leg were higher for participants carrying the 118G allele (mean 480 kPa, SD 167.5) than for those carrying the 118A allele (mean 419, SD 150.4, p = 0.008). Levels of β-endorphin were higher in patients (mean 17.9 fmol/ml, SD 4.71) than in controls (mean 15.8 fmol/ml, SD 4.03) (p < 0.001).

Conclusion

We found an association between the A118G polymorphism in the OPRM1 gene and an increased risk of PVD and increased pain sensitivity among participants carrying the 118A allele. PVD patients were more sensitive to pressure pain and had higher levels of plasma β-endorphin than controls. The results indicate that differences in endogenous pain modulation involving the opioid system could contribute to the pathophysiology of PVD and the general pain hypersensitivity seen in these women.

Implications

The data support the conceptualization of PVD as part of a general pain disorder with a possible genetic predisposition. The age of onset of PVD is usually between 18 and 25 years and already at this age general pain hypersensitivity is present but rarely causing disability. We believe that early recognition and treatment, with the risk of further development of chronic pain taken into consideration, might prevent future aggravated pain problems in this patient group.

Nature or nurture in low back pain? Results of a systematic review of studies based on twin samples

Twin studies are becoming popular to investigate risk factors for low back pain (LBP) because they consider the genetic factor and allow for more precise estimates of risks. We aimed to identify and summarize the results of studies based on twin samples investigating risk factors for LBP. The MEDLINE, CINAHL, LILACS, Web of Science and EMBASE databases were searched. Prospective and cross-sectional observational studies of LBP involving twins were included. The exposure factors could be genetics (heritability) or environmental such as smoking, alcohol consumption, body mass index and medical history. Pooling was attempted using an inverse variance weighting and fixed effects model. Twenty-seven studies were included. Estimates of heritability effects ranged from 21% to 67%. The genetic component was higher for more chronic and disabling LBP than acute and less disabling LBP. Smoking was significantly associated with LBP [pooled odds ratio (OR) = 3.0; 95% confidence interval (CI) 2.8-3.3] with a longitudinal and a cross-sectional study also identifying a dose-response relationship in people with chronic LBP. Obesity was associated with LBP (pooled OR = 1.9; 95% CI 1.6-2.2) with a cross-sectional study identifying a dose-response relationship. No association between alcohol consumption and LBP was identified. Co-morbidities such as asthma, diabetes and osteoarthritis were associated with LBP (pooled OR ranging from 1.6 to 4.2). The contribution of genetics to LBP appears to be dependent on the severity of the condition. Twin studies could be better used to explore possible causation paths between lifestyle factors, co-morbidities and LBP.

The human μ-opioid receptor gene polymorphism (A118G) is associated with head pain severity in a clinical cohort of female migraine with aura patients

Migraine is a painful and debilitating, neurovascular disease. Current migraine head pain treatments work with differing efficacies in migraineurs. The opioid system plays an important role in diverse biological functions including analgesia, drug response and pain reduction. The A118G single nucleotide polymorphism (SNP) in exon 1 of the μ-opioid receptor gene (OPRM1) has been associated with elevated pain responses and decreased pain threshold in a variety of populations. The aim of the current preliminary study was to test whether genotypes of the OPRM1 A118G SNP are associated with head pain severity in a clinical cohort of female migraineurs. This was a preliminary study to determine whether genotypes of the OPRM1 A118G SNP are associated with head pain severity in a clinical cohort of female migraineurs. A total of 153 chronic migraine with aura sufferers were assessed for migraine head pain using the Migraine Disability Assessment Score instrument and classified into high and low pain severity groups. DNA was extracted and genotypes obtained for the A118G SNP. Logistic regression analysis adjusting for age effects showed the A118G SNP of the OPRM1 gene to be significantly associated with migraine pain severity in the test population (P = 0.0037). In particular, G118 allele carriers were more likely to be high pain sufferers compared to homozygous carriers of the A118 allele (OR = 3.125, 95 % CI = 1.41, 6.93, P = 0.0037). These findings suggest that A118G genotypes of the OPRM1 gene may influence migraine-associated head pain in females. Further investigations are required to fully understand the effect of this gene variant on migraine head pain including studies in males and in different migraine subtypes, as well as in response to head pain medication.

High heritability for concurrent low back and neck-shoulder pain: a study of twins

STUDY DESIGN

A cross-sectional study on 21,225 twins based on a Web-based questionnaire was performed in 2005-2006 and administered by the Swedish Twin Registry.

OBJECTIVE

To investigate the importance of genetic factors for the occurrence of "Concurrent low back (LBP) and neck-shoulder pain (NSP)" as well as of "Solely LBP," and "Solely NSP" in an adult population of women and men.

SUMMARY OF BACKGROUND DATA

Many individuals suffering from LBP also have concurrent NSP, and little is known about the factors influencing the occurrence of this spinal comorbidity.

METHODS

Heritability of Concurrent LBP and NSP, solely LBP, and solely NSP was analyzed in 2934 monozygotic twin pairs, 2009 same-sex dizygotic (DZ) twin pairs, and 1960 opposite-sex DZ twin pairs without any known rheumatic disorders using structural equation modeling (SEM).

RESULTS

The SEM showed that 60% of the total variance for concurrent LBP and NSP can be explained by additive genetic effects, which was twice as large as for solely LBP (30%) and more than twice as large as for solely NSP (24%).

CONCLUSION

Genetic factors had a considerably greater importance for the occurrence of concurrent LBP and NSP compared with solely LBP or solely NSP. The influence of genetic factors was similar for solely LBP and solely NSP.

Pharmacogenetics in pain management: the clinical need

Genetic research heralds a new therapeutic approach to pain management. Increasing literature demonstrates individual genetic vulnerabilities to specific pain types and mechanisms, partially explaining differing responses to similar pain stimuli. Furthermore, analgesics demonstrate great variability among polymorphic genotypes. Family history and genotyping promise to play an important role in future pain therapies. As advances continue in the genetics of pain and analgesia, pharmacotherapy will depend more on an individualized, targeted approach and less on empiricism.

Bone health and back pain: what do we know and where should we go?

Bone health is generally not considered in patients who present with chronic back pain. Nonetheless, bone health and back pain share common genetic and environmental correlates suggesting a co-dependence. Evidence exists for a relationship between back pain and impaired bone health. Here we present the evidence, theoretic framework and clinical relevance. Bone health and back pain are important determinants of musculoskeletal health. Back pain experienced in youth is a risk factor for future back pain, while suboptimal bone health during development increases the risk of skeletal fragility in later life. Generally, bone health is not considered in patients with chronic back pain who do not demonstrate other well-recognised bone health risk factors or associated conditions. Nonetheless, evidence suggests that back pain and impaired bone health share common environmental and genetic correlates, indicating that bone health ought to be considered in the context of back pain in otherwise healthy individuals. This review describes the likely mechanisms explaining the relationship between back pain and impaired bone health, evidence concerning the relationship and suggestions for future research. A narrative literature search was conducted using CINAHL, Medline, PubMed and Web of Science electronic databases. A history of back pain is associated with decreased bone mineral density in adults, yet this tends to be site-specific. No studies were identified examining this association in youth, yet the negative effects of childhood skeletal trauma and obesity on bone and spinal health provide indirect evidence for an association. Further research is required to clarify the impact of back pain on bone health at different lifespan stages using prospective cohort designs.

Burned-out discs stop hurting: fact or fiction?

STUDY DESIGN

Cross-sectional study of a general-population sample.

OBJECTIVE

The aim was to test the hypothesis that moderate disc degeneration (DD) is stronger associated with low back pain (LBP) than severe degeneration, and also to identify a possible dose-response relationship between numbers of degenerated discs and LBP.

SUMMARY OF BACKGROUND DATA

Among many clinicians there has been a belief that DD initially causes pain because of the penetration of fluid nuclear material through annular fissures, and that pain eventually resolves as the nucleus becomes fibrotic and can no longer penetrate the fissures.

METHODS

From a population-based cohort of 412 individuals, all aged 40, three groups were created for a primary analysis: those with exclusively normal discs (white nuclei on a T2-weighted MRI), those with moderate DD (grey) + normal, and those with severely degenerated (black) + normal discs. The number of individuals reporting LBP during the past year (the most relevant for chronic/intermittent pain) and the past month were separately related to DD. Second, a possible dose-response in terms of numbers of abnormal discs was analyzed.

RESULTS

The percentage of people with LBP was virtually not influenced by grey discs, where the fraction with pain during the past year was close to 69% irrespective of the presence or number of grey discs. Black discs, however, increased the corresponding fraction from 60%, if no black discs, to 86% for those with 2 (odds ratio of 2 per number of black disc). The risk for LBP during the past year attributed to black discs was 11%.

CONCLUSION

The data could not support the hypothesis that severely degenerated discs are "burned out" and become less painful. People with black discs had a higher prevalence of LBP compared to those with grey or normal discs, the latter 2 being equally often painful.

The molecular epidemiology of pain: a new discipline for drug discovery

Recent candidate gene studies have identified and replicated the first associations between several common polymorphisms and pain severity in humans. Moreover, human studies in twins suggest high heritability for responses to experimental pain stimuli. Human genome-wide association studies of pain phenotypes might identify novel analgesic targets, help to prioritize research among current targets, and increase the likelihood of success for analgesic candidates emerging from animal studies. However, clinical research in pain has largely focused on small neurophysiology-based studies, so expansion of epidemiological understanding will be essential to the success of genetic or proteomic dissection of complex pain disorders. This Perspective outlines how methods of molecular epidemiology, proved effective in the study of other diseases, can enhance the returns from human genomic studies and expedite the development of new drugs to prevent or treat pain.

Identical lumbar spinal disk pathology in twins (clinical presentation and review of literature)

BACKGROUND

The occurrence of similar pathology in identical twins is well recognized. Spinal pathologies have been suggested to have a genetic predisposition. Degenerative lumbar spinal disease is one of them, with low back pain as one of its common presentations.

CASE DESCRIPTION

We describe a case of monozygotic twins presenting with lumbar disk herniation at same level, that is, L4/L5. Both the twins underwent surgery.

CONCLUSIONS

We believe such a scenario is a rare occurrence and does highlight a probable role of genetic factors, over and above the environmental factors, in lumbar disk pathology.

Genetic and environmental influences on non-specific low back pain in children: a twin study

Aggregation of low back symptoms in families of children with low back pain (LBP) has been described. However, this may be due to genetic factors or common exposure to environmental factors. The aim of this study was to evaluate the relative contribution of genetic and environmental factors to childhood LBP by comparing the pairwise similarity of LBP in pairs of monozygotic (MZ) and dizygotic (DZ) twin children. Data was collected from 1995 to 1998 from a national sample of Finnish 11-year-old twins born between 1984 and 1987. This study sample constituted of 1,790 twin pairs: 610 MZ pairs, 598 same-sex DZ pairs, 582 opposite-sex DZ pairs. LBP pain was determined by using a validated pain questionnaire designed to assess musculoskeletal pains during the preceding 3 months. The outcome measure, LBP, was considered in three categories: none, once a month and at least once a week. Twin similarity in the report of LBP was quantified by correlations. Variance components for genetic and environmental factors were estimated by using biometric structural equation modelling techniques. The prevalence of LBP at least once a month was 15.7%, and at least once a week was 6.6%. The prevalence of frequent LBP in boys was significantly higher than that in girls (P = 0.04). In both genders, there were no differences in LBP reporting by zygosity (P > 0.2). There were no statistically significant differences between polychoric correlations in male MZ and DZ pairs and between polychoric correlations in female MZ and DZ pairs, suggesting little genetic influence. Results obtained from the best-fitting genetic model suggests that, of the total variance in LBP, 41% (95% CI 34-48) could be attributed to shared environmental factors within families; and 59% (52-66) to unique (unshared) environmental factors. Our results suggest that genetic factors play, at most, a minor role in LBP in children; instead, symptoms seem to be related to a mixture of shared and unshared environmental factors. This study underscore the need for further high-quality research, preferably prospective studies, to identify important modifiable risk factors in order to guide interventions that may prevent LBP in childhood.

Sex-specific variability and a ‘cage effect’ independently mask a neuropathic pain quantitative trait locus detected in a whole genome scan

Sex and environment may dramatically affect genetic studies, and thus should be carefully considered. Beginning with two inbred mouse strains with contrasting phenotype in the neuroma model of neuropathic pain (autotomy), we established a backcross population on which we conducted a genome-wide scan. The backcross population was partially maintained in small social groups and partially in isolation. The genome scan detected one previously reported quantitative trait locus (QTL) on chromosome 15 (pain1), but no additional QTLs were found. Interestingly, group caging introduced phenotypic noise large enough to completely mask the genetic effect of the chromosome 15 QTL. The reason appears to be that group-caging animals from the low-autotomy strain together with animals from the high-autotomy strain dramatically increases autotomy in the otherwise low-autotomy mice (males or females). The converse, suppression of pain behaviour in the high-autotomy strain when caged with the low-autotomy strain was also observed, but only in females. Even in isolated mice, the genetic effect of the chromosome 15 QTL was significant only in females. To determine why, we evaluated autotomy levels of females in 12 different inbred stains of mice and compared them to previously reported levels for males. Strikingly larger environmental variation was observed in males than in females for this pain phenotype. The high baseline variance in males can explain the difficulty in detecting the genetic effect, which was readily seen in females. Our study emphasizes the importance of sex and environment in the genetic analysis of pain.

Heritability of low back pain and the role of disc degeneration

Twin studies suggest that both disc degeneration and back pain have a genetic component. We were interested in estimating the heritability of low back pain in men and examining whether genetic influences on back pain are mediated through genetic influences on disc degeneration. Thus, we conducted a classic twin study with multivariate quantitative genetic models to estimate the degree to which genetic (or environmental) effects on back pain were correlated with genetic (or environmental) effects on disc degeneration. Subjects included 147 monozygotic and 153 dizygotic male twin pairs (N=600 subjects) from the population-based Finnish Twin Cohort. All subjects underwent lumbar magnetic resonance imaging and completed an extensive interview, including back pain history and exposure to suspected risk factors. Disc height narrowing was the degenerative finding most associated with pain history, and was used to index disc degeneration in the models. Statistically significant genetic correlations were found for disc height narrowing and different definitions of back pain, such as duration of the worst back pain episode (r(g)=0.46) and hospitalization for back problems (r(g)=0.49), as well as disability in the previous year from back pain (r(g)=0.33). The heritability estimates for these back pain variables ranged from 30% to 46%. There also were statistically significant, but weaker, environmental correlations for disc height narrowing with back symptoms over the prior year. A substantial minority of the genetic influences on pain was due to the same genetic influences affecting disc degeneration. This suggests that disc degeneration is one pathway through which genes influence back pain.

Adolescent flexibility, endurance strength, and physical activity as predictors of adult tension neck, low back pain, and knee injury: a 25 year follow up study

OBJECTIVE

To examine whether adolescent flexibility, endurance strength, and physical activity can predict the later occurrence of recurrent low back pain, tension neck, or knee injury.

METHODS

In 1976, 520 men and 605 women participated in a sit and reach test (flexibility) and a 30 second sit up test (endurance strength). In 1976 and 2001 (aged 37 and 42 years) they completed a questionnaire. Lifetime occurrence and risk of self reported low back pain and self reported, physician diagnosed tension neck and knee injury were calculated for subjects divided into tertiles by baseline results of strength and flexibility tests.

RESULTS

Men from the highest baseline flexibility tertile were at lower risk of tension neck than those from the lowest tertile (odds ratio (OR) 0.51, 95% confidence interval (CI) 0.28 to 0.93). Women from the highest baseline endurance strength tertile were at lower risk of tension neck than those from the lowest tertile (OR 0.60, 95% CI 0.40 to 0.91). Men from the highest baseline endurance strength tertile were at higher risk of knee injury than those from the lowest tertile (OR 1.96, 95% CI 1.05 to 3.64). Men who at school age participated in physical activity were at lower risk of recurrent low back pain (OR 0.61; 95% CI 0.42 to 0.88) than those who did not.

CONCLUSIONS

Overall good flexibility in boys and good endurance strength in girls may contribute to a decreased risk of tension neck. High endurance strength in boys may indicate an increased risk of knee injury.

pain1: A neuropathic pain QTL on mouse chromosome 15 in a C3H×C58 backcross

We have produced a backcross (BC) population of 267 mice from the parental strains C3H/HeN and C58/J. The mice were phenotyped for neuropathic pain using the neuroma model. Subsequently all BC mice were genotyped in a region of chromosome 15 that has been previously suggested to contain a quantitative trait locus (QTL) for this trait. We have confirmed the linkage of the QTL, named pain1, to the central region of chromosome 15. Our finding provides the necessary robustness to justify efforts towards identification of the underlying gene.

Low back pain in military recruits in relation to social background and previous low back pain. A cross-sectional and prospective observational survey

Background

Traditionally, studies on the etiology of low back pain have been carried out in adult populations. However, since low back pain often appears early in life, more research on young populations is needed. This study focuses on the importance of social background factors and previous low back pain in the development of low back pain in military recruits.

Methods

During a three-month period, Danish military recruits with different social backgrounds live and work under the same conditions. Thus, there is an opportunity to investigate the influence of social background on the development of low back pain, when persons are removed from their usual environment and submitted to a number of new stressors. In addition, the importance of the recruits' previous low back pain history in relation to low back pain during military service was studied. This was done by means of questionnaires to 1,711 recruits before and after this three-month period.

Results

Sedentary occupation was negatively associated with long-lasting low back pain (>30 days during the past year) at baseline with an odds ratios of 0.55 (95% CI: 0.33–0.90). This effect vanished during service. Having parents with higher education increased the risk of low back pain during service (OR: 1.9;1.2–3.0, for the highest educated group), but not of the consequences (leg pain and exemption from duty), whereas high IQ decreased the risk of these consequences (odds ratios as low as 0.2;0.1–0.8 for exemption from duty in the group with highest IQ). Long-lasting low back pain prior to service increased the risk of long-lasting low back pain (OR: 4.8;2.1–10.8), leg pain (OR: 3.3;1.3–8.3) and exemption from duty during service (OR: 5.9;2.4–14.8).

Conclusion

Sedentary occupation is negatively associated with low back pain at baseline. This protective effect disappears, when the person becomes physically active. For predicting trouble related to the low back during service, the duration of low back pain prior to service and IQ-level are the most important factors.

Structural, psychological, and genetic influences on low back and neck pain: a study of adult female twins

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.

METHODS

The subjects comprised 1,064 unselected women (181 monozygotic [MZ] and 351 dizygotic [DZ] 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 MZ when compared with DZ 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 [OR] 3.6, 95% confidence interval [95% CI] 1.8-7.3]) and between neck pain and response on the GHQ (OR 3.3, 95% CI 2.1-5.0). These associations were mediated genetically.

CONCLUSIONS

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.

Genetic and environmental contributions to back pain in old age: a study of 2,108 danish twins aged 70 and older

STUDY DESIGN

Self-reported 1-month prevalence of back pain in older twins assessed at intake in a population-based longitudinal survey.

OBJECTIVES

To determine the relative contribution of genetic and environmental factors to back pain in old age.

SUMMARY OF BACKGROUND DATA

To date, genetic contributions to back pain in old age have not been assessed, to the authors' best knowledge.

METHODS

Interview data given at entry into a nationwide cohort-sequential population-based survey of Danish twins aged 70 years and older in 1995, 1997, 1999, and 2001 form the basis of this analysis. Analysis of twin similarity was estimated using probandwise concordance rates, odds ratios, and tetrachoric correlations for back pain. Heritability (proportion of the population variance attributable to genetic variation) was estimated by bivariate probit estimation and adjusted for known significant environmental factors. Odds ratios for known environmental effects were estimated after controlling for age, sex, and genetic effects.

RESULTS

Modest and nonsignificant differences between monozygotic and dizygotic twin pairs were found for probandwise concordance rates, odds ratios, and tet-rachoric correlations for both men and women. In the bivariate probit estimation, a current or previous diagnosis of osteoporosis, degenerative joint disease, or lumbar disc prolapse was found to significantly affect the risk of back pain. Additive genetic effects explained approximately one fourth of the liability to report back pain in men and none of the occurrence in women. Individual environmental effects were found to explain roughly 75% of the occurrence of back pain in men and 100% in women.

CONCLUSIONS

Additive genetic effects are modest contributors to back pain in older men but not in women. A current or previous medical diagnosis of osteoporosis, degenerative joint disease, or lumbar disc prolapse is-strongly associated with back pain, also when genetic factors are controlled for. Because of inherent methodologic issues, this estimate of the genetic influence on back pain in old age is probably conservative.

Associations between back pain history and lumbar MRI findings

STUDY DESIGN

Retrospective monozygotic twin cohort study.

OBJECTIVES

Our goal was to investigate the associations between different spinal MRI findings and current, past year, and lifetime low back pain after adjusting for occupational physical loading, smoking, genetics, and early family influences.

SUMMARY OF BACKGROUND DATA

The role of spinal pathology in back symptoms continues to be controversial.

METHODS

The study participants consisted of 115 monozygotic male twin pairs 35 to 69 years of age. The qualitatively assessed MRI parameters were as follows: disc height, bulging, herniations, anular tears, osteophytes, spinal stenosis, and endplate changes. Signal intensity was measured quantitatively.

RESULTS

After controlling for age, disc height was associated with all back pain variables studied and anular tears with LBP frequency and intensity during the 12 months before imaging. Both were associated with lifetime frequency of low back pain interfering with daily activities, disability, and intensity of the worst lifetime pain episode. Other MRI findings did not explain the various symptom histories. Adjusting for physical loading in the past 12 months increased the associations of anular tears and "low back pain today" and 12-month low back pain parameters. After controlling for genotype and other familial influences, the within-pair differences in disc height and anular tears accounted for 6% to 12% of the total variance in the within-pair differences of low back pain variables.

CONCLUSION

These findings raise new questions about the underlying mechanisms of LBP. The sensitivities of the only significant MRI parameters, disc height narrowing and anular tears, are poor, and these findings alone are of limited clinical importance.

Ambiguous relation between physical workload and low back pain: a twin control study

AIMS

To examine the association between self reported physical workload and low back pain (LBP) in younger twins. To investigate whether genetic factors interact with physical workload in relation to LBP.

METHODS

A twin control study was performed within a population based twin register using 1910 complete monozygotic (MZ) and same sexed dizygotic (DZ) twin pairs aged 25-42 and discordant for LBP. LBP in the affected twins was divided into two groups: "LBP for </=30 days during the past year", and "LBP for >30 days during the past year". Physical workload was divided into four categories: "sitting", "sitting/walking", "light physical", and "heavy physical". Data were analysed in a matched design using conditional logistic regression. MZ and DZ twins were analysed separately and together in order to determine possible genetic influences in relation to physical workload and LBP.

RESULTS

Statistically significant graded relations were found for increasing workload and LBP of longer duration but not for LBP of shorter duration (</=30 days during the past year). In both LBP groups the "sitting" and "sitting/walking" groups were not statistically different. MZ and DZ twins did not differ significantly with respect to LBP in the various workload groups.

CONCLUSIONS

There is evidence for a dose-response relation between physical workload and LBP of longer duration. Attention to clinically relevant subgroups based on duration, for example, is necessary in epidemiological studies dealing with LBP. Physical workload might be more important than genetic factors in LBP.

The genetic mediation of individual differences in sensitivity to pain and its inhibition

The underlying bases of the considerable interindividual variability in pain-related traits are starting to be revealed. Although the relative importance of genes versus experience in human pain perception remains unclear, rodent populations display large and heritable differences in both nociceptive and analgesic sensitivity. The identification and characterization of particularly divergent populations provides a powerful initial step in the genetic analysis of pain, because these models can be exploited to identify genes contributing to the behavior-level variability. Ultimately, DNA sequence differences representing the differential alleles at pain-relevant genes can be identified. Thus, by using a combination of "top-down" and "bottom-up" strategies, we are now able to genetically dissect even complex biological traits like pain. The present review summarizes the current progress toward these ends in both humans and rodents.

Intragenic polymorphisms of the vitamin D receptor gene associated with intervertebral disc degeneration

STUDY DESIGN

A study in genetic epidemiology of disc degeneration, based on lifetime exposure data, findings on magnetic resonance imaging, and genotyping of intragenic markers.

OBJECTIVES

To pursue the potential correlation between common allelic variations in the vitamin D receptor locus and degeneration of the intervertebral disc.

SUMMARY OF BACKGROUND DATA

Familial aggregation has been observed in intervertebral disc degeneration, but the relative significance of the genetic component and shared environmental influences is unknown. The identification of relevant candidate genes associated with disc degeneration would specify a genetic component and increase our understanding of the etiopathogenesis of disc degeneration.

METHODS

From the population-based Finnish Twin cohort, 85 pairs of male monozygotic twins were selected based on exposure to suspected risk factors for disc degeneration. Interview data were gathered on relevant lifetime exposures, and thoracic and lumbar disc degeneration was determined through quantitative and qualitative assessments of signal intensity on magnetic resonance imaging, and qualitative assessments of disc bulging and disc height narrowing. Possible associations were examined between disc degeneration measures and two polymorphisms of the coding region of the vitamin D receptor locus.

RESULTS

Two intragenic polymorphisms of the vitamin D receptor gene revealed an association with disc degeneration. Quantitatively assessed signal intensities of thoracic and lumbar (T6-S1) discs were 12.9% worse in men with the Taql tt genotype and 4.5% worse in men with the Tt genotype, compared with signal intensity in men with the TT genotype (age adjusted P = 0.003). A similar pattern was found between disc signal intensity and Fokl genotypes; men with the ff and Ff genotypes had mean signal intensities that were 9.3% and 4.3% lower, respectively, than those in men with FF genotypes (age-adjusted P = 0.006). The summary scores of qualitatively assessed signal intensity, bulging, and disc height were 4.0% and 6.9% worse in men with Ff and ff genotypes, respectively, when compared with those in men with the FF genotype (age-adjusted P = 0.029).

CONCLUSION

Specific vitamin D receptor alleles were associated with intervertebral disc degeneration as measured by T2-weighted signal intensity, demonstrating for the first time, the existence of genetic susceptibility to this progressive, age-related degenerative process.

Similarities in degenerative findings on magnetic resonance images of the lumbar spines of identical twins

Although the etiology of most degenerative changes in the lumbar spine is unclear, genetic factors may play an important role. To investigate this link, we reviewed magnetic resonance images of the lumbar spines of identical twins to assess the degree of similarities in degenerative findings in the discs. Observers who were blinded to twinship evaluated sagittal T1-weighted and T2-weighted magnetic resonance images of the lumbar spines of forty male identical twins (twenty pairs) with respect to changes in the end plates, desiccation of the discs, bulging or herniated discs, and decrease in the height of the disc space. Similarities between co-twins were significantly greater than would be expected by chance. Whereas smoking status and age explained 0 to 15 per cent of the variability in the various degenerative findings in the discs, 26 to 72 per cent of the variability was explained with the addition of a variable representing co-twin status. These findings are compatible with a marked genetic influence and warrant further investigation.

Twin studies in rheumatic diseases

Twin studies attract both clinicians and geneticists because of the value of the twin method in helping unravel the genetic predisposition to diseases and the role of environment in their causation. In the field of rheumatology, there are many case reports on twins concordant or discordant for diseases. Interesting as such reports may be, very few generalizations can be made from them. The concordance rate among monozygotic (MZ) twins indicates the maximum level of genetic contribution. Based on studies of systematically compiled twin series, the concordance rate is about 15% for rheumatoid arthritis; the rate is probably of the same order of magnitude for systemic lupus erythematosus (SLE). The fine specificity of antinuclear antibodies of MZ twins at least one of whom is affected by SLE is very similar, despite somatic generation of immune diversity. Up to now, twin studies have provided little information on the role of environmental factors in rheumatic diseases. A notable exception is a case-control study of MZ twins discordant for smoking that reinforced the role of smoking as a cause of lumbar intervertebral disc degeneration.