Following birth there are only two tissues in the entire human body which do not have a blood supply. One of these is the cornea of the eye (nourished by tears) and the other is the intervertebral disc. The cells of the disc are nourished by diffusion and convection of nutrients across the porous surfaces of the vertebral endplates. This nutrition can be impaired by many other factors. Some of these are congenital and some are acquired. Physical insult and injury are important and one of the greatest insults known in today’s world is the smoking of cigarettes.
In this illustration the yellow lines represent an example of anatomically normal vertebral endplates. The fine green lines represent the passage of nutrients across these endplates by diffusion and convection into the normal intervertebral disc..
Absorption of nutrients is enhanced by negative pressures within the disc and decreased as pressure increases with compressive loading. This is the reason that we are all taller in the morning and shorter at night (assuming that the discs continue to have water holding capacity). It is also the reason that the original American and Russian astronauts rapidly gained body height due to a “ballooning” of their discs caused by the presence of a weightless environment .
Normal discs are composed of 85% water. Since MRIs scan hydrogen ions in the body and most of the body’s hydrogen is in water the MRI is essentially a scan of body water content and is of particular value in determining the disc’s state of health because as discs degenerate they lose their ability to hold water. The mid-saggital T2 MRI image of the lumbar spine on the left shows the “normal” discs as being light in color. The red dot is on a degenerated disc which appears dark in intensity because of decrease in its water holding capacity. Other than being dehydrated the degenerated disc shows no evidence of tearing or herniation. The spinal column is normally created from a tubular structure called the notochordduring the first few weeks of embryonic development (see below). This notochord normally disappears prior to birth. Prior to birth there are a number of other structures such as fetal vascular channels which also normally disappear prior to
Shown below is a diagram of a 7mm human embryo. The precursor notochord is the black vertical column. The light bands represent the primordium of the vertebra while the dark bands are the beginnings of the intervertebral discs.
From: Ghosh P: The Biology of the Intervertebral Disc, Vol. I, CRC Press, 1988
Should the embryologic structures described above not disappear prior to birth these endplate deformities remain and persist into adult life. These are not incidental abnormalities as they can create significant impairment in discal nutrition leading to premature disc degeneration and early initiation of the “degenerative cascade.”
German pathologist Christian George Schmorl (1861-1932) was the first to describe discal protrusions into adjacent vertebral bodies in patients with adolescent kyphosis. The term “Schmorl’s nodes” therefore came to be applied to all such defects. In fact, there are a number of different types of endplate deformities. The illustration above by Ghosh is referred to:
1. Vascular channel defects (are radiographically referred to as “limbus” vertebrae). This residual vascular channel (red dot) is often misdiagnosed as being a sequela of trauma to a vertebrae when trauma has not occurred. It is, however, a vertebral weakness and avulsion fracture can result from trauma
2. Centrum abnormalities. These occur between centrum and ring apophysis (shown by blue dot). The defect is congenital but can be expanded by trauma.
3. Notochord endplate defects. Residual notochord endplate deformities can be prominent and sequential as in the genomic spine disorder juvenile discogenic disease. These appear to be particularly significant in impairing diffusion and convection of nutrients to the disc across the endplate. Therese defects are typically not anterior (green dot in Ghosh illustration, but posterior to the centrum.
The importance of these abnormal endplate deformities in initiating premature degeneration needs to be emphasized. The contributions of William Kirkaldy-Willis in understanding this process have been unique.
This MRI scan in a 36 year old male shows a classic picture of the genomic spine disorder “Juvenile Discogenic Disease” (JDD) is seen with many of the previously described endplate deformities being represented. The associated hypermotility has produced endplate sclerosis (yellow dots) usually diagnosed as “Modic II” changes. In this saggital spine view showing multi-level degeneration a large, contained, disc herniation is marked with a red dot (the same herniation is shown in the next image also with a red dot). At the L5-S1 level there is also a circled high-intensity zone annular tear. Note that there is loss of the normal lumbar lordosis, a common clue indicating an underlying genomic spine disorder. In this axial view of the same case the red dot is on the large contained disc herniation which is occupying about 40% of the spinal canal. The facet joints show associated degenerative changes. In genomic spine disorders the pathology progresses over a period of many years and there is often a progressive acclimization of the nerve roots and other neurologic structures to this gradual insult.
This gradual adjustment may be such that the individual is asymptomatic until a slight body insult tips the delicate and precarious balance.
The images above are the plain x-ray and MRI scan of a 82 year old woman with the genomic spine disorder juvenile discogenic disease in addition to prominent scoliosis. It is remarkable that given the dramatic amount of pathology documented here that this patient only had clinical symptoms for a matter of a few months. This is but another dramatic example of how nature works to allow us to maintain function and not be disabled.
There is every reason to believe that if this 82 year old had been diagnosed with a genomic spine disorder at an early age and started in an appropriate preventive program she would have probably never have experienced the marked progression of the degenerative process.
Genomic spine disorders represent important pathologic entities. Those individuals afflicted with these typically experience a higher degree of pain and disability compared to the rest of the population.
Remarkably, even at this point in time few radiologists, physicians or even spine surgeons know much about about genomic spine disease despite the classic 1994 publication in Spine by Heithoff et al in regard to the entity “Juvenile Discogenic Disease” and other continuing publications and presentations. Unfortunately the saying “you only see (and diagnose) what you know” seems to reflect present reality.
It has now become evident that the most common etiology of multi-level degenerative disc disease is hereditary making it even more important for physicians to have a higher degree of awareness so that early identification can occur and appropriate important health factors be instituted (i.e. exercise, not smoking, avoiding spine insults, etc.). When this awareness and prevention are implemented the patient with a genomic disorder has a very good long-term outlook.The great majority of individuals with genomic spine disorders who act preventively typically self-heal and self-stabilize their spines. The other side of the coin is progressive disability particularly when a patient falls prey to a spine surgeon who recommends multi-level complex fusion based on x-ray findings alone.
Spine-related disease accounts for an inordinately large share of health care expenses in our present health care system. This can be dramatically decreased by a better understanding of genomic spine disorders and putting this knowledge to work.