Transitional vertebral segments (TVS) disorder belongs to congenital anomalies of the spine which include hemivertebrae, spina bifida, block vertebrae and subarachnoid cysts. It has characteristics of one portion of the vertebral column, yet when the vertebrae are counted, they belong in another segment. In such a case the anomalous vertebra is known as "transitional" as it lies half way between a lumbar vertebra and a sacral segment. This congenital and inherited defect begins in that stage of embryo development when differentiation is nearly complete, but a vertebral body develops in such a way that it is a part of the lower lumbar vertebral column and part of the sacrum at the same time, and has bony characteristics of both lumbar vertebrae and sacrum. Many go unnoticed unless there is a rib on one side but not on the other. The affected part is called transitional vertebral segment. It apparently can happen anywhere along the spinal column, but is most often seen in the lumbo-sacral area. The condition affects more frequently female than male dogs.
Transitional vertebral segments have been associated with several vertebral abnormalities. Most cases of TVS are associated with hip dysplasia, meaning that most but not all cases are found in dysplastic dogs. TVS has also been suggested as a possible cause of a disease or injury involving multiple spinal nerve roots, especially in the German Shepherd Dog. The effect of the weakening of the sacroiliac attachment is thought to result in premature disc degeneration, which, together with spinal canal stenosis, results in potential compression of the overlying spinal nerves and creation of a cauda equina syndrome. The condition is thought to have clinical significance and should be selected against in breeding, especially in the German Shepherd dogs. Vertebral body anomalies are readily recognized by veterinarians, as it is possible to diagnose transitional vertebral segments using radiographic test.
- Morgan JP. Transitional lumbosacral vertebral anomaly in the dog: a radiographic study. (J Small Anim Pract. 1999 Apr;40(4):167-72)
- Fred Lanting. TVS, Caudal Equina Syndrome, and Spondylosis
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- A Guide to Canine and Feline Orthopaedic Surgery. Hamish Denny, Steve Butterworth