THE WOBBLER SYNDROME

CERVICAL VERTEBRAL INSTABILITY SYNDROME

The wobbler syndrome, or cervical (neck) vertebral instability syndrome, is a malformation of a vertebra (or vertebrae) that leads to an intervertebral disc rupture and compression of the spinal cord. This compression injures the portion of the spinal cord necessary for the animal’s normal strength and coordination to stand and move. Therefore, the spinal cord disease is secondary to the vertebral cord abnormality. The primary disease condition resides in the skeletal structures that surrounds the spinal cord.

In Doberman Pinscher and Great Dane dogs the skeletal abnormality occurs predominantly in the last three cervical vertebrae: the fifth, sixth and seventh. The cause of the skeletal malformation or malarticulation (joint abnormality) is unknown. Clinical studies suggest both genetics and nutrition may play a role in the development of the defect. A genetic predisposition is suggested by the distinct breed predilection and by one study where mating of affected individuals produced a considerable number of affected offspring. No definitive pedigree studies have proven a genetic basis or the genetic factors involved. Nutritional factors have been implicated by the appearance of this disease in young Great Dane dogs. Research has shown that, in some young dogs, excessive intake of a diet high in protein, energy, calcium and phosphorus accelerates growth. This may induce skeletal changes such as those seen in some of these "wobbler" dogs.

The most common vertebral abnormality is a narrow opening into the bony passageway through which the spinal cord must pass. The opening is reduced in height and mildly compresses the spinal cord, especially during extension of the neck. Other abnormalities in these cervical vertebrae include an overgrowth of the body of the vertebrae into the passageway for the spinal cord, an abnormal shape of the vertebral body and the joints between the vertebrae, too narrow a passageway in width through the vertebrae; and excessive mobility of the cervical vertebrae, causing displacement or subluxation (partial dislocation). All of these abnormalities, which provide an early normal life, eventually place so much stress on the Intervertebral Disc that it degenerates and ruptures, placing additional pressure on the already compressed spinal cord. This results in an acute problem which is actually secondary to a chronic abnormality of the last three vertebrae.

There is no evidence published to date that the disease can be predicted by radiographic (x-ray) study of clinically normal dogs. Undoubtedly, a severe malformation in the vertebrae could indicate that an individual dog would have a good chance of experiencing spinal cord compression. However, dogs with minimal radiographic changes may remain free of clinical signs of spinal cord compression.

In general, the onset is more common in younger Great Danes and older Doberman Pinschers, but exceptions to this generality are common. Radiographs should be taken in the normal position and with the neck extended and flexed. A myelogram, (a contrast dye study of the spine) is necessary to define the lesion because the extent of the compression cannot be outlined on regular radiographs.

SYMPTOMS

In most dogs, the onset of clinical signs is slow and insidious and not associated with any known external injury. The initial signs of weakness and incoordination occur rapidly, and are most apparent in the hindlimbs. The signs worsen slowly over succeeding weeks. The hindlimbs often are spread wider apart than normal, causing the hindquarters to sway from side to side. The hindlimbs may not fully extend, causing a crouched posture, with the toes scuffing on the ground with each step. The degree of forelimb involvement varies from no observable abnormality to an obvious stiffness and awkward use of the forelimbs. In mild cases, or early in the disease, these signs may be most obvious as the dog turns corners, and are less apparent when it walks or runs along a straight path. An abrupt change in speed or direction may exacerbate the neurological signs.

Because the nervous system involvement is limited to an injury of a small section of the cervical spinal cord, these dogs remain alert and responsive, and eat and excrete normally. Despite this injury, these dogs usually do not exhibit pain on manipulation of the neck.

The reason that some dogs do not show clinical signs until they are a number of years old probably is because their initial vertebral abnormalities are mild. With time, movement at the abnormal articulations causes secondary changes in the intervertebral disc which result in degeneration and disc rupture. When watching affected dogs move it must be determined if the gait abnormality is because the dog cannot function normally, as with the Wobbler Syndrome, or does not want to function normally. The latter occurs from pain associated with skeletal disease in the limbs, including hip dysplasia, osteochondrosis dissecans of the cervical vertebrae in young, giant breed dogs, and hypertrophic osteodystrophy. In dogs with these skeletal diseases, the stride is usually shorter than normal, often creating a choppy gait. However, these patients always know the position of their limbs, which are kept directly under the body. Joint pain may be determined by palpation.

DIAGNOSIS

The nature of the "wobbler" syndrome requires a more extensive evaluation than regular radiographs (x-rays). The myelogram is used to confirm and document not only the location of a compression, but also the amount of spinal cord swelling. With new gas anesthetics, advanced monitoring equipment , and modern "contrast" agents for the dye study, the myelogram is now a common and safe diagnostic procedure when performed with care and under the proper conditions. Laboratory tests on blood and cerebrospinal fluid usually are normal.

TREATMENT

With the first clinical episode, treatment is directed primarily at the spinal cord injury and consists of corticosteroids to attempt to reduce edema (swelling) that may be present in the compressed segment of the spinal cord. However, medical therapy usually provides only temporary improvement at best. Surgery provides for spinal cord decompression and an opportunity to directly repair some of the vertebral abnormalities. Although many techniques are described, the most common surgical procedure is to remove the ruptured disc material and distract the collapsed vertebrae while allowing time to heal in a stable position.

PROGNOSIS

The prognosis with Wobbler Syndrome depends on the severity of the signs and radiographic evidence of the degree of skeletal disease present. If the dog is paralyzed and unable to stand, the prognosis for recovery after surgery is guarded. The dog that ambulated freely but with incoordination has a better prognosis.

While we anticipate that surgery will stop the progression of the signs and result in improved function, it is the degree of permanent spinal cord damage already sustained that ultimately determines the final outcome.

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