Heredity Causation to Disc Herniation and Degenerative Disc Disease
There has been much discussion in the medical-legal arena that the cause of changes in the morphology of the intervertebral disc [disc herniation, disc bulge, annular tear or schmorl’s nodes] are related to genetic make-up of the individual [heredity] rather than related to a traumatic event or it sequela. It is also suggested that these changes are predictable and therefore can be applied to the population at large.
The first such genetic based study was published by Sanbrook et al., 1999 titled, Genetic influences on cervical and lumbar disc degeneration: a magnetic resonance imaging study of twins, and set off a long series of additional research studies and speculation that genes were to blame for physical changes in the disc and not the external environment. Some in the medical-legal area called this the “twins study” and it has been a basis of denial of traumatically induced disc injury for the last 18 years. In a recent study by Rajasekaran et al., 2016, the authors stated, “Our study questions the validity of previous studies and necessitates the need for standardizing the description of disc degeneration, phenotype selection [physical appearance of disc pathology], study sample size, age, and other variables in future studies.” [pg 1649]
The authors continue, “Genetic studies on DD [degenerative discs] performed on twins observed heritability estimates > 70% for both lumbar and cervical spine. Further studies on association of different clinical and radiological features of DD with single-nucleotide polymorphisms (SNPs) in different genes implicated a wide variety of genes, albeit with poor replication, which has led to a weak credibility of reported genetic associations. Hereditary factors, ethnic variations, gene–gene interactions, and gene environment interactions have been cited as the cause for such variations.” [pg 1649] We will see in the next paragraph that this is simply untrue and based on faulty research.
In relation to this study, the authors stated, “We have documented for the first time the wide changes in genetic associations with phenotype selection [physical type of disc pathology], different age subgroups, and inclusion sequence of study subjects, in the same study population, thus questioning the reliability of genetic association studies.” [pg 1650] They continue by stated “Our study results expose many weaknesses in the genetic association studies on DD [degenerative disc disease]. [pg 1658] This also includes herniated discs.
In conclusion, the author report “This [paper] shows that genetic association studies are subject to significant variability, affected by many factors. Based on current results, it is difficult to consider one set of genes as responsible for DD considering these wide variations.” [pg 1659] In the end, it is now clear that using heredity as a basis for disc pathology observable on MRI particularly post trauma is irresponsible and does not correlate with the scientific method. Understanding this key concept is a critical component of being able to describe the causes of disc herniation and other disc pathology, particularly in acute and long standing injury.
Reference:
1. Rajasekaran, S., Kanna, R. M., Reddy, R. R., Natesan, S., Raveendran, M., Cheung, K. M., ... & Shetty, A. P. (2016). How Reliable Are the Reported Genetic Associations in Disc Degeneration?: The Influence of Phenotypes, Age, Population Size, and Inclusion Sequence in 809 Patients. Spine, 41(21), 1649-1660.
2. Sambrook PN, MacGregor AJ, Spector TD. Genetic influences on cervical and lumbar disc degeneration: a magnetic resonance imaging study in twins. Arthritis Rheum 1999;42:366–72.
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