The Twin Spine Study: Is Disc Degeneration Genetic?

There is an unfortunate tendency among doctors, employers, and patients alike to turn back pain into some sort of morality tale. Exactly what that lesson is changes depending upon who is telling it. Depending on who you ask, back pain is due to a sedentary lifestyle, poor diet, smoking, or a bad attitude.

Until the end of the twentieth century, most scientists also believed that disc degeneration was caused by lifestyle factors. The role of genetics was routinely overlooked, yet it turns out that genetics has a greater effect upon back pain (or at least, disc health) than most of our favorite bugbears.

In the late 1990s, geneticists began publishing compelling evidence that disc problems were indeed genetic. One of the most influential projects was the Twin Spine Study, which began in 1991, and involved a collaboration among Canadian, Finnish, and American scientists.

The Twin Spine Study

The researchers behind the Twin Spine Study began by selecting 300 twin pairs to focus on. Their group included 147 pairs of identical twins, and 153 pairs of fraternal twins, all of which were males drawn from the Finnish Twins Cohort.

These twins were selected because they differed in some key environmental aspect, such as a sedentary versus active job, or smoking versus not smoking.

During the course of the study, the twins were all flown to a central location in Finland, where the researchers examined, tested, and interviewed them.

The twins were weighed and measured, and interrogated about their work history and exercise habits. Scientists recorded their muscular strength and endurance, and ordered MRIs for everyone.

MRIs Provide a Better Image

By the time I developed my own back issues in 2017, an MRI for back pain had become de rigueur. But this was not always the case. The authors pointed out that when they were beginning their research in the early ‘90s, MRIs were just starting to become available.

Before that, researchers usually used x-rays to evaluate spinal pathologies. The use of new technology allowed researchers to identify and evaluate pathologies that are not always clearly visible on an x-ray.

What Is Disc Degeneration?

I have long been confused about what “disc degeneration” actually means. After all, “degenerate” is such a vague term. Debates, companies, families, buildings, and even (depending on who you ask) popular culture can all be said to degenerate.

Even scientists use the term “disc degeneration” to refer to a range of different pathologies. Discs that have dried out, shrunk, bulged, herniated, lost space to bony growth, or been sandwiched between abnormal endplates can be described as “degenerated.”

The researchers behind the Twin Spine Study were aware of this vagueness, and noted that unclear definitions often made it difficult to compare other studies, or even to know what was actually being measured.

To avoid collapsing dissimilar things into the bucket of “degenerative,” the researchers decided to evaluate each kind of pathology (including annular tears, disc herniations, end plate irregularities, and vertebral osteophytes) separately.

It should be noted, though, that many studies were done using the same cohort and data, and not all of the studies looked at all pathologies.

The Effect of Aging on Disc Degeneration

The researchers found that, while disc degeneration was assumed to be a natural aging process, the data didn’t necessarily back that assertion up. At least, aging rarely seemed to be the most important factor in disc degeneration.

Their sample set included subjects from between the ages of 35–70 years old. Within that age range, the abnormality that seemed to be most affected by age was a loss of water content, but even there, the effect was modest. Age had very little impact on disc height narrowing and disc bulging, and virtually none on disc herniations.

Do Lifestyle Factors Really Matter?

Back in the early 1990s, researchers had a short list of lifestyle factors they assumed were most important for developing disc pathologies. When the Twin Spine Study kicked off, the researchers looked for twin sets where the twins differed significantly in key areas. Then, they crunched the data, and tried to figure out whether heritable or lifestyle factors were most important.

Smoking

The first variable the researchers looked at was smoking. The considered a subset of 20 pairs of identical twins in which one twin was a heavy smoker, and the other was a nonsmoker. Although this wasn’t a large sample, the researchers could partially compensate by using a clever study design, and sophisticated statistical analysis.

They discovered that smoking did have an effect. The heavy smokers had lumbar disc degeneration scores that were 18% higher than their non-smoking siblings.

However, when the researchers considered smoking amongst all individuals, rather than between twins, the difference was minimal. Smoking explained less than 2% of differences in disc degeneration. Heritable factors, it seemed, were more influential than the decision to smoke.

A later, larger study of 115 twin pairs showed an even smaller effect. However, the twins in the second study had much smaller discrepancies in their smoking levels.

Driving

Driving was also assumed to cause back pain symptoms. Scientists speculated this was due to the whole-body vibrations endured by drivers.

The Twin Spine Study researchers decided to look into it. They studied 45 pairs of identical twins whose occupations differed significantly in the amount of driving required. To their surprise, they found that the amount of driving had no impact on disc degeneration.

Manual Labor

If you watch a gardener continue to stoop and twist, or a postal worker carrying packages, it’s hard not to assume that manual labor is hard on the back. But how much of a difference does manual labor really make?

In order to determine what sorts of physical demands each twin had experienced at work, the researchers conducted in-depth interviews about their job history, work posture, typical weight lifted, and frequency of lifting. Jobs were grouped into classes, and physical demands were rated on a scale of 1 to 4.

When they crunched the numbers, the researchers found out that physical demands at work mattered, although the effect wasn’t as strong as expected. Workers in physically demanding occupations were somewhat more likely to show signs of disc degeneration, while sedentary work seemed to offer a modest amount of protection.

However, occupational differences accounted for no more than 7% of the differences in rates of disc degeneration.

Sports and Leisure

As any middle-aged person who brought delusions of grandeur onto a basketball court knows, sports impose their own demands on the spine.

To study the effect of sports and leisure activities, the twins were interviewed about their athletic participation since the age of 12. They were also asked about other physically challenging hobbies, like gardening or construction projects.

Like physical labor, participation in sports and physically demanding hobbies had a small, harmful effect on disc degeneration.

However, the effects were so minor they were hard to quantify. They were certainly not large enough to be worth fretting about for the average Joe.

How Important Is Family Inheritance?

While researchers were teasing out which lifestyle factors affected disc degeneration, and to what degree, they were also sorting out how many of the changes ran in families, and how strong those associations were.

The identical twins used in these studies were most often brought up by the same parents, in the same household. This made it nearly impossible to sort out which similarities were due to genes, and which were due to environmental conditions the twins shared early in their lives. So, the researchers didn’t try.

Instead of claiming that similarities were due to genetics, they came up with familial aggregation percentages, which quantified how strongly a trait ran in families.

Researchers determined that job demands accounted for 7% of the individual differences, and age accounted for another 9%. In contrast, 61% of variance in disc degeneration was explained by familial aggregation.

However, the exact percentage changed depending on which spinal level they looked at. The contribution of familial aggregation was a whopping 75% at T12–L4, but only 34% at L4–S1.

Is Disc Degeneration the Same Thing as Back Pain?

It’s all well and good to understand the effects of genetics on disc degeneration, but patients might rightly want to know whether that means that back pain is genetic.

The Twin Spine Study researchers looked into this matter as well, using the full set of 300 twin pairs. For this study, they used disc height as an indicator of disc degeneration, since a previous study indicated it was most closely associated with back pain.

The researchers previously found that disc bulges and a loss of signal intensity – which indicates the disc is drying out – appeared to be influenced by the same genetic factors that influenced the loss of disc height.

They discovered that, yes, a loss of disc height does correlate with back pain. About a quarter of the genetic influence on pain could be traced back to the genetic factors that influenced disc height. However, there seemed to be other genetic factors at play, but their role remains mysterious.

The lifestyle factors that were known to influence disc height didn’t have a large effect on pain. Less than 5% of pain outcomes could be tied back to these variables.

The researchers concluded, basically, that there must be some other environmental factors that made up the difference, but they didn’t know what they were.

Conclusion

The researchers summed up their observations about lifestyle and hereditary factors by saying: “During the course of the exposure-discordant twin studies, the striking observation of anyone who had the opportunity to view twin sibling images side-by-side was the strong resemblance in disc degeneration, not just in the degree of degeneration, but also in the types of findings and spinal levels involved.”

I enjoy blaming my parents for things, and the Twin Spine Study gives me a reason to hold them 34% responsible for my bad back. I’m glad to have them as a partial scapegoat, because that’s more fun than evaluating my posture, diet, or stupid exercise habits.

References

1. Battie, Michele C, Tapio Videman, Esko Levalahti, Kevin Gill, and Jaakko Kaprio. 2008. “Genetic and Environmental Effects on Disc Degeneration by Phenotype and Spinal Level.” Spine (Lippincott Williams & Wilkins) 33 (25): 2801–2808. doi:10.1097/BRS.0b013e31818043b7.
2. Battie, Michele C, Tapio Videman, Jaakko Kaprio, Laura E Gibbons, Kevin Gill, Hannu Manninen, Janna Saarela, and Leena Peltonen. 2009. “The Twin Spine Study: Contributions to a changing view of disc degeneration.” The Spine Journal (Elsevier) 9 (1): 47–59. doi:10.1016/j.spinee.2008.11.011.
3. Battie, Michele C, Tapio Videman, Laura E Gibbons, Lloyd D Fisher, Hannu Manninen, and Kevin Gill. 1995. “Determinants of Lumbar Disc Degeneration: A Study Relating Lifetime Exposures and Magnetic Resonance Imaging Findings in Identical Twins.” Spine (Lippincott-Raven Publishers) 20 (24): 2601–2612. doi:10.1097/00007632-199512150-00001.
4. Videman, Tapio, Michele C Battie, Eric Parent, Laura E Gibbons, Pauli Vainio, and Jaakko Kaprio. 2008. “Progression and Determinants of Quantitative Magnetic Resonance Imaging Measures of Lumbar Disc Degeneration.” Spine (Lippincott Williams & Wilkins) 33 (13): 1484–1490. doi:10.1097/BRS.0b013e31818043b7.