October 1997
Back To The Beginnings
I guess we had better have our "learning" page and for that we are going back to the start of life.
In the Beginning, OOPS, sorry, we don't have to go that far back.
We are going to discuss problems with the vertebrae. The majority of congenital anomalies of the vertebral column don't cause any clinical problems. However, if they cause a deformity or instability of the vertebral canal, spinal cord compression may occur with the resultant clinical signs.
Now, on to the "fun" stuff. Disturbances in embryonic development can result in vertebral anomalies. Hemivertebrae, block vertebrae, butterfly vertebrae and transitional vertebrae are some of the anomalies that may develop.
The vertebrae develop from the embryonic mesoderm. The mesoderm differentiates into a structure called the central notochord and bilateral serially arranged somites. the somites are divided by a transverse cleft. The blood vessels will develop in this cleft. Each somite then differentiates into 3 layers: laterally there is a dermatome, in the middle there is a myotome, and medially there is a sclerotome. We all remember our Latin, don't we? Derma refers to skin, myo to muscle and sclero means hard. The sclerotomes give rise to the vertebrae. Each of the sclerotomes divides into a cranial half and a caudal half (cranial means in the direction of the head and caudal is the opposite or the direction of the tail). These divided sclerotomes then recombine with the cranial half of one sclerotome combining with the caudal half of the adjacent one. This results in the blood vessels penetrating each vertebrae to nourish it (remember the blood vessels develop in the cleft between the somites). The recombined sclerotomes migrate to surround the developing spinal cord(called the neural tube at this stage) and form the primordial vertebrae. The embryonic notochord disappears except at each intervertebral disk where it forms the center of the disk(the nucleus pulposus). Cartilage forms from 4 different spots in each vertebrae that merge to form a single cartilaginous vertebrae. Ossification(bone formation) also occurs from 4 distinct centers( 2 in the vertebral body that combine into one, and one on each side in the vertebral arch.
The vertebrae in the neck are called cervical, the chest vertebrae are thoracic, the ones over the abdomen are lumbar and the ones that the pelvis connect to are the sacral. Tail vertebrae are the coccigeal vertebrae.
Now on to a couple of the problems.
Hemivertebrae
The term means half or partial vertebrae. They result from hemimetameric displacement of somites during the recombination of sclerotomes to form primordial vertebrae(Huh? I had to quote that sentence directly from my source since it was put so eloquently). Hemivertebrae are wedge shaped and the apex may point dorsally, ventrally or medially. A defect in the development of the vascular supply can result in a failure of ossification (turning into bone) of 1/2 of the vertebral body and may cause unilateral hemivertebrae or dorsal or ventral hemivertebrae. Moderate to severe angulation of the spine may result and the hemivertebrae may be displaced dorsally during growth by adjacent vertebrae. Kyphosis, lordosis, or scoliosis may be seen depending on the shape of the hemivertebrae.
The top radiograph shows the severe kyphosis in the thoracic spine of a 3 month old Malamute that had never been able to use rear legs since birth. The bottom photo is the necropsy of the same dog. The spinal column has been cut in the saggital plane. The arrow points to the area where the spinal cord has been completely pinched off. This picture is from "Textbook of Veterinary Internal Medicine" by Ettinger and Feldman.
Please excuse the rather odd looking animals in the drawings above (a former tech of mine says they should be on a cave wall in France) but I think you get the idea of what these 3 terms refer to.
Hemivertebrae occur most commonly in the thoracic spine of the "screw-tailed" brachycephalic breeds (Boston terriers, Pugs, and French and English bulldogs). However, they can occur at any location in any breed. A kinked tail may result from a caudal hemivertebrae. Brachycephalic means short skull, mesaticephalic is a normal skull(German shepherd, Samoyed), and dolichocephalic refers to a long skull(Collie, Borzoi). A familial tendency has been reported in English bulldogs and Yorkies. An autosomal recessive can cause thoracic hemivertebrae in German shorthair pointers.
Butterfly Vertebrae
Remember that the embryonic notochord normally is obliterated with the development of the vertebral bodies and remains only as the center of the intervertebral disk. If the notochord persists, a sagittal(front to back) cleft may result. This results in a vertebrae that resembles a butterfly on a D/V(dorsal/ventral) radiograph. Again this abnormality is most common in the brachycephalic "screw-tailed" breeds.
Block Vertebrae
Block vertebrae can occur at any level in the vertebral column and results from a disturbance in the segmentation of the somites into primordial vertebrae. Any or all parts of the vertebrae may be involved(vertebral body, vertebral arches, and/or spinous processes). The sacrum is a normal block vertebrae. The block vertebrae may be the same length as the number of involved vertebrae or may be shorter and may result in an abnormal angulation of the spine.
Transitional Vertebrae
There can be a variation in the number of cervical, thoracic, lumbar and sacral vertebrae. Each of the 4 groups of vertebrae share common characteristics with respect to dorsal spinous processes and transverse processes. Vertebrae that share characteristics of two of the different groups are referred to as transitional vertebrae and occur at the cervical/thoracic junction, the thoracic/lumbar junction, or lumbar/sacral junction. These are an oddity seen on radiographs and seldom result in clinical signs.
Clinical Signs
Butterfly and block vertebrae are usually stable and rarely result in spinal cord dysfunction. Severe alterations in transitional vertebrae may result in instability that results in arthritis, nerve root pressure or intervertebral disk changes. By far, hemivertebrae cause most of the neurologic dysfunction associated with vertebral anomalies. They result in instability and narrowing of the spinal canal due to the severe angulation of the spine. Compression and/or intermittent trauma to the spinal cord results. Clinical signs depends on the location of the defect. Onset may be acute following trauma to the unstable spine, or my develop gradually as the dog grows. Signs usually are seen in dogs less than 1 year old.
Treatment
Surgical decompression and stabilization may be attempted. The prognosis is guarded in these cases because there may be more than one vertebral abnormality in the spine and congenital spinal cord anomalies may also be present.