(Also called Butterfly Vertebrae)
Photo of hemivertebra by MarVistaVet
Most of us know that our backbones are more technically called vertebrae, that they make up our vertebral column, and consist of small block-like bones joined by spongy discs, supporting our skeletons and protecting our spinal cords. A hemivertebra is a congenital (present at birth) condition where vertebrae are deformed. It occurs when two or more vertebral bodies (the block-like portion of the vertebrae) fuse and develop asymmetrically; creating a wedging effect in what should be a fairly rectangular block.
The normal vertebrae have cylindrical vertebral bodies that connect like beads on a necklace. The vertebral body of the hemivertebra has not developed symmetrically leading to a wedge shape. Fitting the hemivertebrae into the spine causes the spine to bend or twist. Graphics by MarVistaVet.
The English Bulldog, French Bulldog, Pug and Boston Terriers are famous for having what is called a "screw tail". The screw tail results from hemivertebrae in the vertebrae of the tail and is characteristic, even desirable, in these breeds.
Hemivertebrae of the tail are not a problem as there is no spinal cord tissue that far down and the result is an interesting-looking tail with a possible risk of a skin fold infection.
Mostly, this is of cosmetic interest; the screw tail is part of what makes a bulldog. The problem is that by selecting for hemivertebrae in the tail through breeding, we have also selected for hemivertebrae in more important areas of the spine. When the hemivertebrae occur higher up in the middle of the back, problems occur with the spinal cord. The wedging effect of the hemivertebrae’s body does not connect normally to the vertebral bodies next to them, and the spine becomes twisted. The spinal cord can actually become compressed as well.
A bulldog's screw tail
Photo by Christina Thomas-Virnig. PhD
The most common area for problematic hemivertebrae is around the 8th (out of 13) thoracic vertebrae (the upper back). Most of the time, hemivertebrae are incidental findings on a X-ray and are not associated with clinical signs, but should there be abnormal pressure on the spinal cord, the following signs can be seen:
- Weakness of the rear limbs
- Fecal incontinence
- Urinary incontinence
In most cases, signs plateau at age nine months when the vertebrae stop growing. It is important to understand that the presence of hemivertebrae is usually of no significance and are just an interesting finding on a radiograph. That said, if they create enough of an abnormal angle in the spine, there will most likely be symptoms. It is also not unusual for a dog with hemivertebrae to have other spinal malformations as well. Testing may be needed to fully assess the condition of the spinal cord. This commonly involves special imaging (myelography, MRI, or CT imaging) to see if the spinal cord is actually compressed by the hemivertebrae.
Rest and possibly corticosteroid injections during flare-ups may be adequate to treat a mild case; more severe cases require surgery. Each case is individual and a spinal surgeon can perform stabilization.