Hemivertebrae (also commonly referred to as "butterfly" vertebrae) is a condition where the vertebrae of the spine are shaped more like triangles than blocks. The breed standard "corkscrew tail" that is so desirable in the Boston Terrier is actually an example of this formation. Although this trait is desirable in the tail, it can cause serious problems when located elsewhere in the spine.
This condition results from the failure of the left and right halves of a vertebrae to fuse completely during fetal development. The resulting vertebrae resembles a butterfly when viewed from above. Each half of the vertebrae may also grow at different rates, creating wedge-shaped vertebrae (when viewed vertically).
Wedge-shaped vertebrae may cause dorsal curvature (kyphosis) or lateral curvature (scoliosis) of the spine. Spinal deformities can cause serious problems, including paralysis, if they compress the spinal cord and/or its blood supply.
Symptoms will depend on the number and the locations of the discs affected.
Milder cases require keeping the area clean and dry to reduce chances of infection and to keep your pet comfortable. More severe cases may require that the tail be amputated.