INTRODUCTION

Cervical spondylomyelopathy is a common neurological syndrome of large and giant breed dogs, characterized by cervical spinal cord and nerve root compression. Doberman pinchers and Great Danes are the breeds most commonly affected (80% of cases). This syndrome has also been called caudal cervical spondylomyelopathy (CCSM), Wobbler syndrome, cervical malformation-malarticulation, cervical vertebral instability and cervical spondylolisthesis.

The exact etiology is unknown. Multiply factors including "over nutrition" and hypercalcitoninism, mechanical factors, genetic factors and degenerative disk disease are probably involved. Myelopathy may result from stenosis of the vertebral canal, vertebral instability, disc herniation (Hansen type II), ligamentous hypertrophy, joint capsule proliferation or osteophyte production.

The lesion is located more frequently to C₄-C₆ vertebrae in the Great Danes and C₅-C₇ in Doberman pinchers.

CLINICAL SIGNS

The age of onset of clinical signs is variable, ranging from 3 months to 9 years. In general, Great Danes are affected at less than 2 years of age, where Doberman pinschers and other breeds more frequently manifest clinical signs at 2 years of age or older. There is no sex predisposition in affected dogs.

The clinical signs reflect either damage to the spinal cord (myelopathy) or the spinal nerve roots (radiculopathy), or both. Radiculopathy is probably an important contributory factor to the pain, lameness and shoulder muscle atrophy.

Clinical signs are related to the severity of spinal cord compression and therefore are variable in nature and degree. The most common presentation is a gait disturbance that is most severe in the pelvic limbs. This ranges in severity from mild ataxia to marked pelvic limb hypermetria. The same time the thoracic limbs are moving in a short, choppy manner and often held in extension when the dog is in lateral recumbency. Eventually, the dog may become sufficiently tetraparetic, to have difficulty rising from lateral recumbency or from sitting position or unable to walk. Cervical hyperesthesia, guarding of the neck or a low carriage of the head may be seen because this position results in the least amount of spinal cord compression. Extension of the neck often accentuates spinal cord compression with consequent worsening of motor deficits. Lameness and atrophy of the infraspinatus and supraspinatus muscles in one thoracic limb or pain when traction is applied to the limb (root signature), suggest that nerve root compression is present.

Although dogs affected by cervical spondylomyelopathy will usually develop symptoms over a long period, occasional dogs will present with acute tetraparesis. In these cases it is suggested that an episode of hyperextension has occurred which has caused spinal cord concussion. Myelography of these cases will reveal a swollen spinal cord.

Results of neurological examination are variable but conscious proprioceptive responses are usually depressed in the pelvic limbs. Local reflexes in all four legs are intact, with frequent hyperreflexia and crossed extensor responses in the pelvic limbs.

DIAGNOSIS

Diagnostic evaluation of dogs suspected of suffering from cervical spondylomyelopathy includes careful evaluation of the history, physical and neurological examinations, laboratory evaluations including cerebrospinal fluid analysis and radiological assessment including plain radiography and myelography. Computed tomographic myelography and magnetic resonance imaging may potentially provide prognostic information by detecting spinal cord atrophy.

Plain radiographs are valuable in ruling out other conditions (i.e., trauma, neoplasia of vertebral body, and discospondylitis), but they are not accurate in locating the site of spinal cord compression.

Myelography is very important in diagnosis of cervical spondylomyelopathy. Myelographic study should include both lateral and ventrodorsal projections as well as linear traction views. On lateral radiographs one may see ventral midline spinal cord compression due most commonly to hypertrophied dorsal longitudinal ligament and dorsal annulus fibrosus, dorsal compression from hypertrophied ligamentum flavum or circumferential (hourglass) compression due to the previous two problems plus lateral impingement upon the cord by facet osteophytes and joint capsule enlargement Strong linear traction of the head usually results in a marked reduction of spinal cord compression caused by redundant annulus fibrosus or ligamentous tissue. A lesion that improves in this manner is termed "dynamic". In contrast, herniation of nucleus pulposus is not significantly improved by linear traction and is termed "static". Differentiation between static and dynamic compression is very important for the choice of surgical procedure.

DIFFERENTIAL DIAGNOSIS

The differential diagnosis in young dogs includes atlantoaxial subluxation, arachnoid cyst, syringohydromyelia, inflammatory central nervous system disease, discospondylitis and trauma. In adult dogs, degenerative intervertebral disk extrusion (Hansen type I), degenerative myelopathy, neoplasia and fibrocartilaginous embolism are more likely.

TREATMENT

The decision on the best way to treat each patient is based on the presenting history, neurological deficits and myelographic findings.

Dogs with mild neurological deficits may respond favorably to conservative management which consists of strict confinement, a neck brace to help immobilize the caudal cervical spine and anti-inflammatory medications (prednisolone, carprofen). This treatment regimen should be continued for 3 to 4 weeks. If the patient responds favorably during the treatment period, gradual return to normal activity should be encouraged. If there is minimal, or no response to conservative medical management, or if the patient's neurological status deteriorates, surgery should be considered.

Several techniques for surgical treatment of this syndrome have been described. The three basic types of surgery are: ventral decompression, vertebral distraction/fusion and dorsal decompression. The choice of type of surgical procedure must be based on the myelographic study, particularly the linear traction view.

Ventral decompression ("ventral slot") is indicated primarily for the relief of static ventral compression of the spinal cord, such as type II degenerative disk disease. Ventral decompression can only be considered to have been completed satisfactory when the dura is clearly visible in the depth of the slot after the removal of the compressive mass. To promote vertebral fusion at the surgical site, autogenous cancellous bone graft may be packed around the slot. Cancellous bone graft enhances fusion, which usually occurs within 8 weeks. Intervertebral disk fenestration is contraindicated because it does not remove the protruding dorsal annulus fibrosus, and it may decrease vertebral stability and collapse the disk space.

Vertebral distraction and fusion are indicated in the presence of a dynamic compression to the spinal cord. This technique was devised to achieve cervical spinal interbody fusion, with the affected interspace in traction and the spinal cord decompressed. Two surgical techniques will be described that allow ventral decompression by linear traction and subsequent stabilization. Metal implant and bone cement method and screw and washer method. In the metal implant and bone cement method the metal implant can either be Steinmann pins or bone screws but neither are suitable for distraction at more than one interspace. Despite potential disadvantages, (risk of implant infection, difficulty in revision of a surgical failure) this technique has good long term follow up results. The main disadvantage of screw and washer technique is the resorption of vertebral end plate with subsequent collapse of the distracted intervertebral space. However, the temporary stability provided by the implants seems to allow fusion to occur. A major advantage is that unlike metal implant and cement technique that technique can be applied to more than one side.

The dorsal decompression technique involves a dorsal laminectomy for decompression, with or without articular facet screws or pins to help stabilize the joint. The dorsal approach may be preferable when multiple disk spaces are affected, or the compression is primarily dorsal or lateral. Long-term results of dorsal laminectomy appear to be favorable, but several authors have reported significant postoperative mortality and deterioration in neurological status.

PROGNOSIS

The prognosis for dogs with cervical spondylomyelopathy depends on the severity of neurological deficits at the time of presentation, the number of the involved disk spaces and the duration of the clinical signs. Dogs with more than one lesion generally have a worse prognosis than dogs with a single affected intervertebral space. Dogs with chronic tetraparesis have a very guarded prognosis. In contrast, dogs with acute tetraparesis respond well to surgery and have favorable prognosis for functional recovery within 2 months.

Following any type of surgery in which fusion of adjacent vertebrae occurs, there is a potential risk of development of secondary lesions at an adjacent site. This effect has been termed the "domino" effect and has also been observed in human patients following surgery of similar cervical conditions.