What is a transitional vertebra?
The
spinal column is composed of individual vertebra through which runs
the spinal cord and spinal nerves. Each vertebra comprises of a
large cylindrical body the top of which is a tube through which the
spine runs. The vertebral bodies are attached to each other by
intervertebral discs that act as shock absorbers and by facet joints
that allow movement. There are also large bony protrusions or
processes to which the muscles are attached, the dorsal spinous
processes can be felt in the midline of the back (humans and dogs)
while the lateral processes in the chest have developed to become
the ribs.
The
spinal column is divided into segments and these are the neck or
cervical spine, the chest or thoracic spine, the lumbar spine and
the sacrum that forms part of the pelvis. Each segment has
characteristically shaped vertebrae. A transitional vertebra is one
that lies at the junction of two of the segments and tries to take
on the shape of the vertebra of both segments. A transitional
vertebra at the junction between the chest and the lumbar spine may
have a vestigial rib. However it is at the lumbosacral junction that
transitional vertebrae occur commonly, especially in German
Shepherds, and they often have clinical consequences.
The
sacrum comprises of three vertebrae that have fused together and
therefore lack intervertebral discs. The head end articulates with
the seventh lumbar vertebra and the tail end with the first caudal
(tail) vertebra. The sacrum is attached to the ilia (pelvis) by the
sacroiliac joints on its lateral aspects and it is through these
joints that forward thrust from the hind limbs is transmitted to the
body of the dog. A transitional vertebra at the level of the first
sacral vertebra takes on some of the characteristics of a lumbar
vertebra in that there is a vestigial disc space between the first
and second sacral vertebrae and there can also be an abnormal
lateral process with abnormal attachments to the ilia (pelvis). This
deformity may be either symmetrical or asymmetrical and can result
in a tilted pelvis.
Clinical consequences
There are two separate clinical conditions that can occur as a
consequence of having a lumbosacral transitional vertebra.
-
Unilateral hip dysplasia
This results from the pelvis being tilted along its long axis there
by decreasing the coverage of the femoral head by the acetabulum on
one side and increasing it on the other. The former tends to
destabilise the hip causing it to move in and out of its socket
initiating inflammation and ultimately osteoarthritis.
A
marked difference in scores between the two hips can occur naturally
without a spinal abnormality but most commonly it is due to uneven
positioning of the dog during the x-ray procedure. This should be
recognised by the radiographer and the appropriate adjustments made
to the positioning.
-
Lumbosacral stenosis (cauda equine syndrome)
This is seen in the middle aged to old dog. The cauda equina is the
name given to the spinal nerves that run from the end of the spinal
cord in the mid lumbar region, along the spinal canal into the tail.
Segmental pairs of spinal nerves leave the spinal canal between each
vertebra above the intervertebral disc.
Lumbosacral stenosis is a compression of the spinal canal from a
prolapsed intervertebral disc usually with thickening of the soft
support tissues, leading to nerve impingement with pain and
occasionally interference with hind limb movement. Clinically the
dog with low back pain is reluctant to run and especially jump or
climb stairs.
X-rays show a bony bridge between the last lumbar vertebra and the
sacrum. This is called spondylosis but it is an extremely common
finding in the older dog with no clinical signs. A more definitive
diagnosis can be made by MRI that images the soft tissues as well as
the bones. Most of these dogs will settle down with conservative
management but some require decompressive surgery.
A
lumbosacral transitional vertebra predisposes the dog to a
lumbosacral stenosis by altering the biomechanics of the spinal
column placing abnormal stresses across that area.
What is the incidence of lumbosacral transitional vertebrae
The
association between lumbosacral stenosis and lumbosacral
transitional vertebrae has been investigated in 4000 control dogs
(1). The overall incidence of lumbosacral transitional vertebrae was
3.5%, while in 92 dogs with lumbosacral stenosis the incidence was
16.3%. Therefore dogs with lumbosacral transitional vertebrae are
eight times more likely to develop lumbosacral stenosis than dogs
without the anomaly. This report also states that German Shepherd
dogs are eight times more likely to develop lumbosacral stenosis
than other breeds.
In
another publication of a study of 143 dogs with lumbosacral
transitional vertebrae, the German Shepherd dog was greatly
overrepresented (2).
What can be done to reduce the incidence
Lumbosacral transitional vertebrae are readily identified on x-rays
of the pelvis in both the standard hip extended view used for hip
dysplasia schemes and the lateral view. The condition is considered
to be genetic but a reference to the heritability has not been
found. Therefore it would be very simple to screen for the anomaly
at the same time, using the same x-ray plate, as screening for hip
dysplasia. Then perhaps the incidence of lumbosacral transitional
vertebrae may begin to decrease.
References
-
Fluckiger and others (2006). A lumbosacral transitional vertebra
in the dog predisposes to cauda equine syndrome. Vet Radio
Ultrasound. 2006 Jan-Feb; 47(1):39-44
-
Morgan JP (1999). Transitional lumbosacral vertebral anomaly in
the dog: a radiographic study. J Small Anim Pract. 1999
Apr;40(4):167-172
An
extreme example of a transitional lumbosacral vertebra.
Note the marked asymmetry.
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