TRANSITIONAL VERTEBRAL SEGMENT, TVS
There has been a flurry of questions in recent years about conditions known
as TVS and APA (asymmetrical pelvic attachment), which makes me wonder
it is the incidence or the diagnosis that is on the rise. A malformation
of vertebrae, usually the last lumbar or first sacral vertebra (sometimes
more than one per case) is simply and aptly called “Transitional Vertebrae”.
This congenital and inherited defect begins in that stage
of embryo development when differentiation is nearly complete, but a vertebral
body “can not make up its mind whether it wants to be part of the lower
lumbar vertebral column or part of the sacrum”, as colourfully described
by OFA’s Dr. Greg Keller, and it winds up taking on bony characteristics
of both lumbar vertebrae and sacrum. Less common are the transitional vertebrae
seen at the thoracic-lumbar junctions.
Veterinary-science observer Barbara Nibling describes mild cases this way:
“The processes are just a bit odd, one set of processes looking as if the
T13 belongs to the lumbar group, while the other side looks like a normal
T13.” Many go unnoticed unless there is a rib on one side but not on the
other, in which case the difference leaps out at you. The affected part
is called TVS, Transitional Vertebral Segment. It apparently can happen
anywhere along the spinal column, but in the lumbo-sacral area most often.
If it results in less symmetry there, the condition is usually referred
to as APA.
The transitional vertebral body takes on some of the characteristics of
both, and is an abnormality that may have a causative role in the severity
or nature of HD, although by itself it seems not to have any bad effects.
It is developmental, and its inherited nature is extremely probable, having
been identified as familial and being similar in many ways to other known
genetic defects. Frequency of transitional vertebrae runs from quite low
(zero in Saints, 3% in Labs) to somewhat higher (8-11% in German Shepherd
Dogs), and most cases (69%) are associated with HD. That is, most but not
all cases are found in dysplastic dogs. This is one reason why the frequency
of asymmetrical HD is as high as it is (30% in the GSD, for example — more
than in other breeds).
The trait is probably polygenic; at least it cannot be said to be simple.
It is definitely familial; for example, Borzoi breeder Bonnie Dalzell reports
around 40% of the dogs that were radiographed in a program she participated
in had lumbo-sacral transitional vertebrae! She described them as “much
slower gallopers at field trials than the dogs with normal anatomy” or
having reduced interest in running at a fast gallop. Top racers, she said,
can go 33 mph, while dogs with “weird backs” generally top out at 23 to
24 mph. She also surmised that many cases might be missed, if the transitional
vertebrae were further toward the front of the dog, because of the great
size of her breed — the crest of the ilium of the pelvis and the last lumbar
vertebrae might not be included in the standard 17-inch film, and thus
any TVS not observed. Field trial performance, for which she is well known
and highly respected, is affected, so she naturally pays more attention
to searching for the anomaly than most people might. She has had eager
coursing dogs with TVS, and found them not as fast as the ones without
it. Her meticulous observation has uncovered other little-known “symmetry
anomalies” such as one in which there is an extra lumbar vertebra. This
would be highly unusual in dogs, although there is a breed of pigs with
an extra pair of ribs, and differences in number of cervical vertebrae
between animals such as giraffes and squirrels (the numbers may surprise
you!). Dogs with the extra vertebrae in the loin and other lumbo-sacral
anomalies seem to be prone to having symptoms of
something that vaguely resembles “Wobblers”, also displayed neurological
deficiencies in the hind legs, and were relatively poor gallopers, according
to Dalzell. Another fairly location of transitional vertebrae is in the
thoraco-lumbar region, in which, she says, “the dog may have one less rib
on one side than the other. This does not seem to affect performance at
a gallop.”
The abnormal attachment of these segments causes the pelvis to be rotated
a little in a longitudinal fashion, which means the pelvis is tilted so
one femoral head gets more coverage by the acetabulum than the other
side does, and as we see in Chapter 8 of the second edition of my HD &
Orthopedic Disorders book, this tends to draw laxity or joint ligament
weakness into subluxation. Yet it is not so much the fact that transitional
vertebrae are there, as what effect they have on pelvic attachment that
is important. TVS does not participate in causing HD unless
it does so through intermediately causing APA. Asymmetry in pelvic attachment
is highest in Labs and GSD’s (around 7-8%) and 79% of these cases are associated
with HD. While transitional vertebrae and asymmetrical pelvic attachment
may not cause all cases of unilateral HD, they do appear to uncover it
or make bilateral laxity look like unilateral HD, at least on the
hip-extended view. An important conclusion of this ten-year study is that
the quality of hip joints is not independent of the nature of pelvic vertebral
attachment.
More acronyms are not what I enjoy, but these abbreviations are helpful.
In orthopaedic lingo, the condition of asymmetrical (without symmetry)
pelvic attachment of the last vertebrae before the sacrum, is called APA.
The condition of transitional vertebral segments is referred to as TVS.
The fact that TVS and APA are associated with unilateral HD or differences
in the severity of one side vs. the other, is why the OFA looks for these
signs and requires a full picture of the pelvis be included on each hip
radiograph submitted for evaluation/certification. The incidences of both
APA and TVS in “normal” dogs are quite low, but these conditions do exist
independently of HD in a small number of dogs with otherwise normal hips.
For example, APA was 2.8% and TVS 1.4% in Labrador Retrievers with normal
hip joints, but 13.8% and 5.2%, respectively, in dysplastic Labs. There
are similar or perhaps identical conditions in humans.
The L7 to S1 region of the spinal column is where some see what they call
“block vertebra”, referring to a somewhat unusual shape. Not only are affected
coursing sight hounds slower, according to some in the sport, but most
dogs upon very careful motion analysis will evidence an asymmetric gait.
The last lumbar vertebra can be fused on one side to either the first sacral
vertebra, or its process (“wing”) can bridge over to near the top of the
ilium. In any case, the dog has more articulation, and range of movement
and flexion/extension on one side than the other. Many have noticeably
shorter stride, but this can also be the result of other problems, such
as arthroses in any of the four limbs’ joints. In people, this is an indicator
of future problems in the lower back due to points of stress in humans.
Is TVS a serious problem? Many think not, but I believe their lack of concern
is based on old information that indicated TVS was not a significant problem.
In a conversation between the OFA and GSD fancier Rita Ledda (via e-mail
chat list) about transitional vertebrae, she was told that it is not a
big problem; yet OFA recommends an affected dog not be bred to a mate that
also has transitional vertebrae. They say that normally, TVS or APA by
itself (without concurrent HD or another disorder) does not affect the
dog. There might be some concern about dogs that engage in some type of
pulling competition, a big sport in Malamutes, American Bulldogs, and American
Pit Bull Terriers. The stress may cause irritation at the sacrum joint.
It must be kept in mind that there could be a relationship between TVS
and other disorders, too. Morgan et al found that in German Shepherd Dogs,
at least, the presence of lumbosacral transitional
vertebrae is a predisposing cause of cauda equina syndrome; this disorder
is treated in another article.
Another GSD fancier, trainer, and writer named Ricardo Carbajal reported
(on an e-mail list) a December 1996 conversation with Dr. Keller of OFA
in which Keller said that TVS is a developmental problem. When the puppy
is developing in utero, some of the lumbar vertebrae attach firmly or tenuously
to the sacrum. You usually see bony changes that are more similar to the
sacral area; one of those changes is that the transverse process (the side
wing) deforms or “bends” and fully or partially fuses to the ilium. Some
times you also see that the inter-vertebral space is diminished or absent.
Usually this problem does not affect the health of the individual. But
it does tend to run in families, so it should be considered a genetic defect
and given whatever amount of serious attention you decide it deserves.
Morgan has done much work in this area, and says that transitional vertebrae
“can occur at the site of junction of the major portions of the vertebral
column, i.e., occipito-atlanto-axial, cervico-thoracic, thoraco-lumbar,
and lumbosacral. The spinal segments at these sites are altered so they
have features in common with those either cranial or caudal” [to them].
When we speak of transitional vertebrae, we should also describe
the specific nature of the change, such as where the attachment is directed,
and whether it is unilateral or bilateral. Some TV are symmetrical, equal
on both sides. Others are asymmetrical (APA); “for example,
a lumbar transverse process on one side and a heavy wing on the other that
attaches to the sacrum.” Transitional lumbo-sacral vertebra (TLSV) was
found in 38% of German Shepherd Dogs diagnosed with cauda equina syndrome.
Is there a biochemical/genetic connection, or are such dogs selected for
by sloppy breeding practices in general?
For a long time, it was generally thought that these abnormal vertebral
segments and attachments did not cause pain and therefore had little clinical
or breeding significance throughout a dog’s lifetime. Currently, the feeling
is that such lesions at the lumbosacral junction predispose the dog to
further weakness, instability, excessive motion around that disc, creation
of cauda equina syndrome, and possibly a worsening of hip joint laxity
or instability. TVS can weaken an already unstable sacro-iliac joint and
put additional stress on the joint between the sacrum and the last lumbar
vertebra.
