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Summary
Hemivertebrae happen when the vertebrae of the canine spine are congenitally deformed so as to fuse or otherwise develop abnormally in a manner that creates a twisting wedge in what should be a very straight spine. A deformity like this in the bony spine can mean a twisting in the spinal cord as well and, potentially, a compression of this exceedingly delicate central nervous system structure.
Hemivertebrae are responsible for the characteristic "screw tail" of bulldogs, pugs and other dog breeds for which the curled tail is considered a requisite trait. In the spinal cord-less tail, however, several defective vertebrae in a row do not present a problem. In some dogs, however, one or more vertebrae other than in the tail may be affected.
Luckily, most dogs are never adversely affected by this defect. In fact, this disease is typically diagnosed as an incidental finding and clinical signs of disease seldom become evident.
Symptoms and Identification
If signs should appear as a result of hemivertebrae, they'll be related to excess pressure on the spinal cord as a result of compression in the region of the mid-thoracic spine (roughly in the middle of the back).
Weakness of the hind limbs, fecal incontinence and urinary incontinence are most typical. Most symptoms will appear in puppyhood, worsening at first until reaching a stable plateau once the vertebrae stop growing.
Diagnosis of hemivertebrae is made by taking simple X-rays. Signs of related disease, however, must be diagnosed by employing more sophisticated imaging studies to demonstrate that compression of the spinal cord is taking place as a result of the vertebrae.
In veterinary medicine, compression of the spinal cord is almost always diagnosed via myelography, an X-ray technique that uses an injectable dye to show where and how compression is occurring. CT scans and MRIs are increasingly implemented as well in this diagnosis.
Affected Breeds
Dogs bred for hemivertebrae in the form of a "screw tail" are understandably predisposed:
Other breeds affected include the German Short-haired Pointer and German Shepherd in which the disease is inherited in an autosomal recessive trait.
Treatment
Rest and anti-inflammatory drugs tend to help dogs only mildly affected by problems associated with hemivertebrae-associated spinal cord compression. Moderate to severely affected patients tend to require surgery called a hemilaminectomy to relieve the compression on the spinal cord in the location of the abnormal vertebra(e). A board-certified veterinary surgeon or veterinary neurologist is always tapped for this delicate surgical procedure.
Veterinary Cost
The cost in most cases where symptoms become evident can be very steep if advanced imaging studies are deemed necessary (almost always). $1,000 to $3,000 is a typical cost for more sophisticated imaging techniques. Surgery is definitely a high-cost proposition in these cases. Surgery itself can cost anywhere from $1,500 to $4,000 for one location, significantly more if various segments of the spinal cord are affected (this would be rare).
Prevention
In breeds for which this trait is not selected for (as when it's inherited as an autosomal recessive trait in the German shepherd and German shorthair pointer, elimination of affected dogs and their first degree from the breeding pool is the best approach to prevention.
In dogs with "screw tails" for which the trait is part of the breed standard, carefully removing those dogs with any evidence of non-tail hemivertebrae from the breeding pool (even when they are not clinically affected) is absolutely necessary.
References
Bailey CS, Morgan JP. 1992. Congenital spinal malformations. Veterinary Clinics of North America Small Animal Practice. 22(4):985-1015.
LeCouteur RA, Child G. 1995. Diseases of the spinal cord. In EJ Ettinger and EC Feldman (eds.) Textbook of Veterinary Internal Medicine, pp. 629-696. WB Saunders Co., Toronto.