BurtRepLogo02.gif (3938 bytes)

 

Anatomy of Genetic Spine Disorders

 

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.

In this illustration the yellow lines represent normal flat vertebral endplates with the fine green lines representing the passage of nutrients across these endplates by diffusion and convection.   

Absorption of nutrients is enhanced by negative pressures within the disc and decreased as pressure increases with 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 astronauts in a weightless environment rapidly gain height due to "ballooning" of their discs.

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 forms from a tubular structure called the notochord during 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 birth.

 
  Embyologic Considerations   

 

Ghosh P: The Biology of the Intervertebral Disc, Vol. I,
CRC Press, 1988

Shown above 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.
 

Ghosh P: The Biology of the Intervertebral Disc, Vol. I,
CRC Press, 1988

This drawing of the L4-5 disc interspace in a 34 week fetus shows the  notochord defects (n.c.d.) and the vascular channels (v.c.) are shown.  The vascular channels end in capillary tufts .

    Should embryologic structures not disappear prior to birth endplate deformities remain and persist into adult life.  These deformities can create significant problems for the individual.


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" have therefore come to be applied to all such defects.   In fact there are a number of different kinds of endplate deformities.  The illustration above points these out.  They are:

1. Vascular channel defects (referred to as "limbus" vertebrae)Typically related to weaknesses (left by a vascular channel)  Often misdiagnosed as a post-traumatic phenomenon.

2. Centrum abnormalities. These occur between centrum and ring apophysis.

3. Notochord endplate defects.  Residual endplate deformities which, when prominent impair impair diffusion and convection of nutrients to the disc across the endplate and thus promote disc degeneration.

The importance of these abnormally persisting endplate deformities is that they promote early and advanced disc degeneration and the sequela of this in the subsequent "degenerative cascade."  Because of the importance of early identification and subsequent preventive spine care a more detailed description of the endplates in shown below.
 


Ghosh P: The Biology of the Intervertebral Disc, Vol. I,
CRC Press, 1988

 
   
This MRI scan in a 36 year old male shows a classic picture of "Juvenile Discogenic Disease" (JDD) is seen with all of the previously described endplate deformities being represented.  In addition to abnormal segmental dynamics, often shown on MRI as "Modic II" changes (see yellow dots) which reflect the secondary sclerosis of the endplates other adverse effects of this process are degenerative changes of the facet joints as well as annular tears, disc protrusions and herniations.  In this saggital spine view a large, contained, disc herniation is marked with a red dot.  The same herniation is shown in the next image (red dot).  At the L5-S1 level there is also a circled high-intensity zone annular tear.  
This axial view of the same case (L4-5 disc) demonstrates a large, contained, disc herniation on the right side occupying about 40% of the spinal canal.  The facet joints show associated degenerative changes.  In JDD cases these changes occur 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 to the left are the plain x-ray and MRI scan of a 82 year old woman with JDD.  Remarkably, given the dramatic amount of pathology present this patient only had clinical symptoms for a matter of months.  

There is every reason to believe that if this 82 year old had been diagnosed with JDD before her 21st birthday and started in an appropriate preventive program she would have probably never have experienced the marked progression of the degenerative process.
 
Summary:

JDD is an important pathologic entity.  Those individuals afflicted with JDD experience an inordinate amount of disability and undergo an excessively higher rate of  surgery as compared to the rest of the population.  If early identification occurs and effective therapy and health maintenance are initiated the patient's long-term outlook is good.  Without this it is poor.  Because spine-related disease accounts for an inordinately large share of health care expenses in our present health care system the diagnosis of JDD must be given a high priority.

It is extremely important to point out that JDD is a congenital, and familial, abnormality of development of the lumbar spine.  When there is one congenital abnormality others are typically present.  This association is referred to by the Burton ReportŪ as the "Zebra Phenomenon."  The other "Zebras" in JDD can be:

Spondylolisthesis
Scoliosis
Spondylolysis
Congenitally Small Spinal Canal
Block Vertebrae
Tropism of Facets
Dysplastic Changes
Myelomeningocoele
Intra-Spinal Syrinx
Arterio-Venous Malformations
Benign Tumors
   and other phenomena 

Most of the associated genomic "zebras" are well known entities.  Benign intra-osseous hemangiomas developed arising from persistent embryologic vascular tufts are much less appreciated but often seen in association with other genomic disease entities.  Shown here is a prominent example.
 
  SUMMARY   

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 and other publications and presentations.  Unfortunately the saying "you only see what you know" seems to be the bane of many a patient.