Caudal Cervical Vertebral Malformation/Malarticulation

Anatomy and Pathophysiology

Caudal cervical malarticulation/malformation (CCVMM), also known in lay terms as "Wobbler's syndrome", is referred to by many names in the literature including: cervical vertebral instability, cervical vertebral spondylopathy, caudal cervical spondylomyelopathy and cervical spondylolithesis [1-19]. The plethora of terminology indicates the confusion and lack of understanding about the underlying pathogenesis of this disease. This is a fairly common spinal column disease of mature Doberman pinschers that appear to have instability-related pathology and young Great Danes and Mastiffs that appear to have congenital stenosis of the spinal canal [2,5,6,9,11-14,16,17,19]. The Boerboel, an African relative of the Mastiff, which is becoming a more popular breed in the United States, appears to have similar pathology to the Mastiff breed. These syndromes have been observed much less frequently in other large breed dogs such as the Weimaraner, Dalmatian, German shepherd, Borzoi, and Samoyed. The Borzoi may have a recessive mode of inheritance for this disease.20 A similar syndrome of vertebral malformation/malarticulation has been recorded in many breeds, but the changes are often recognized in the cranial part of the cervical spine [1-3,5-7,8-12,14,16-22]. The author has recognized similar severe changes in the caudal cervical vertebrae of middle-aged-to-older Yorkshire terriers as well. Whether this finding is related to a similar underlying pathogenesis is not known.

The cervical spinal column possesses the greatest range of motion of all the vertebrae. In addition to the paracervical musculature, several soft tissue structures play a role in the anatomy and stability of the cervical spine [13,21,23 ]. A more complete description of this anatomy is found in the first section of Intervertebral Disk Disease. The dorsal longitudinal ligament, the articular facet joint capsules, the intervertebral disk, and ligamentum flavum bind the vertebral elements together and help prevent excessive motion that could damage the spinal cord and nerve roots. Motion induced by physiologic loads should not cause spinal cord damage in an anatomically stable spine. Malformation and malalignment of the cervical spine may result in spinal cord injury when the intervertebral movement is within normal or less than normal ranges.

The true underlying cause of CCVMM is not known; the pathogenesis is highly complex and may include genetic, nutritional and growth rate factors. The pathologic changes that account for clinical signs are well described [2,3,5,8-12,14,16,17,19,20]. For purposes of understanding the pathophysiology, the disease can be subdivided into instability and congenital/developmental stenosis. In the Doberman pinscher, the most likely basis for the degenerative changes is a malformation and malarticulation of the caudal vertebrae, which result in abnormal stressors on the soft tissues (ligaments, intervertebral disks, joint capsule of the articular facet, and paraspinal musculature) [2,3,5,9,13,14,16,17,19]. With this micro-instability, the ligamentous elements that confer stability, hypertrophy and osteophytes (spondylosis, articular facet osteophytes) form at joint spaces. These degenerative changes result in static and dynamic compression of the spinal cord (Fig. 48-1 and Fig. 48-2). The early events that lead to these changes are not completely known but likely include genetic and nutritional factors as is suspected for most diseases of abnormal bone development; evidence of these changes has been shown in very young Doberman pinscher puppies [2]. The three chronic degenerative changes of the spinal column that are associated with this type of CCVMM are chronic degenerative disk disease (Hansen's type II), osteoarthritic changes, and hypertrophy of the ligamentum flavum and dorsal longitudinal ligament. A dog with this disease may have one, multiple, or all of the changes described [2,3,5,9,11,12,14,16,17,19].

Figure 48.1. Lateral myelogram of caudal cervical malformation/malarticulation with instability at C6-7 in a Doberman pinscher. A. The neck has been placed in a neutral position. A considerable amount of extradural compression by the intervertebral disk and dorsal longitudinal ligament is observed (arrow). Ventral spondylosis is indicated by the arrowhead. The C7 vertebra is abnormally shaped. B. The neck has been positioned in extension. The compression on the spinal cord becomes more severe (arrow) and the interarcuate ligament may contribute dorsally to the compression. C. The neck is now positioned in flexion and the compression is significantly relieved (arrow). D The neck has been pulled into traction. Compression on the spinal cord still exists but it has been relieved to some extent (arrow). (From Seim HB, Withrow SJ [14])

Figure 48.2. A T2-weighted lateral MRI of caudal cervical malformation/malarticulation. The C6 and C7 vertebrae are malformed. The C5-6, C6-7, and C7-T1 intervertebral disks are mildly degenerative as indicated by loss of signal intensity. Mild spinal cord compressions are at C5-6 and C6-7. Mild hydromyelia is observed in the spinal cord over the C6-7 disk space.

Chronic Degenerative Disk Disease (Hansen's type II)

This change is observed mostly as a proliferative (hypertrophied, hyperplastic, or both) dorsal aspect of the annulus fibrosus and hypertrophied dorsal longitudinal ligament causing compression of the spinal cord from the ventral aspect. Although fibrous metaplasia and degeneration of the IVD is seen, extrusion of the nucleus pulposus is uncommon. It is not known if the type II disk degeneration is a cause or effect of the pathology. One source claims it is the underlying disease; however, from the chronicity of the anatomic changes, it would appear that the disk degeneration is an effect of chronic abnormal anatomy and instability.

Osteoarthritic Changes

Articular facet joint capsule hypertrophy, spondylosis, and rounding of the cranial ventral aspect of involved vertebrae may be observed. Articular facet osteophytes and capsule hypertrophy may contribute to spinal cord compression from a dorsal or dorsolateral direction. Osteochondrotic changes (cores of retained cartilage, subchondral cysts, and osteoporosis) have been observed in the articulations of dogs with CCVMM.

Hypertrophy of the Ligamentum Flavum and Dorsal Longitudinal Ligament

Hypertrophy of the ligamentum flavum is probably an attempt at stabilization and can be a source of significant dynamic compression on the spinal cord from the dorsal direction. The dorsal longitudinal ligament also hypertrophies (although some sources disagree). The dorsal longitudinal ligament and type II disk herniation cause compression of the spinal cord from the ventral direction.

Congenital or developmental osseous anomalies are the more common changes seen in young Great Dane, Mastiff, Boerboel, and other large breed dogs [1,3,5,9,11,14,16,19]. A component of cervical stenosis often exists. The most common change seen is enlarged articular facets resulting in a dorsolateral stenosis at the level of the joint (Fig. 48-3). Multilevel stenosis is a common finding. In other cases, the neural canal may be flattened in the dorsoventral direction, but lateral stenosis from the laminae/pedicle is also observed. Instability may also be involved in the development in some of these dogs, because spondylosis and articular facet osteoarthrosis are not unusual findings.

Figure 48.3. A. T2-weighted parasagittal MRI image from an 11-month-old Boerboel with severe ataxia and mild tetraparesis. The arrows indicate the location of the multiple enlarged facet joints that are compressing the cord laterally. B. T2-weighted axial MRI image at the level of C2-3. The stenotic change at the facets has resulted in severe spinal cord compression and changes in spinal cord signal intensity (increased signal) that may indicate edema or malacia. C. T2-weighted axial MRI image at the level of C5-6. Less severe stenosis has occurred with severe spinal cord compression. D. Postmortem evaluation of the spinal canal reveals severe stenosis of the canal at C2-3 and a sharp indentation (hemostats) of the canal at C5-6.

In both types of this disease, the primary pathophysiology is a reduction in the diameter of the spinal canal, which results in spinal cord compression and traction injury, causing neurologic deficits [1-3,5,7-10,14-16,19,20,22,24]. An extensive description of the pathology of spinal cord compression is found in Spinal Cord Compression.

Clinical Presentation

The disease is usually chronic and progressive over many months to years. Clinical signs may be present from two days to two years. The average age of Doberman pinschers with clinical signs of disease is six to eight years (range 1-11 years), whereas the average age for Mastiffs and Great Danes is 11 to 24 months [2-5,7-11,14-20,22-24]. Animals may present with acute exacerbation with neurologic deterioration. The underlying malformation and instability occur during development. The chronic changes probably occur over many years [2-5,7-10,14-20,22-24]. By the time dogs show clinical signs with CCVMM, permanent damage to the spinal cord may be significant. Although some animals may present with an apparently acute onset of severe clinical signs, the pathology suggests chronicity. The degree of neurologic dysfunction is highly variable. Initial signs are wide-based stance and abnormal gait in the pelvic limbs. Clients may not recognize these signs early in the course of the disease. Dogs usually begin with pelvic limb ataxia and conscious proprioceptive deficits followed by or concurrent with a short, stilted thoracic limb gait if the lesion occurs at C6-7. Signs usually progress over time to mild-to-moderate tetraparesis and dysmetria. Some dogs become non ambulatory. Signs progress at variable rates; some dogs deteriorate acutely from mild trauma or massive intervertebral disk extrusion.

The most common lesions are at C6-7, with the next most common changes at C5-6 [2-5,7,8-10,13,14,16-19,21-24]. This is particularly true for the instability seen in the Doberman pinscher. The gray matter in this area contributes to brachial plexus nerves responsible for supplying innervation to the shoulder and flexors of the elbow (suprascapularis and musculocutaneus nerves). When closely examined, these dogs often have some degree of atrophy in the scapular and biceps brachii muscles. When severely affected, these dogs may not be able to flex their limbs to withdrawal of their limbs or to lie in sternal recumbency. Thus, the lesion may result in lower motor neuron signs to the flexors and upper motor neuron signs to the extensor muscles. The extensor muscles are the most dominant muscle group in the thoracic limb of the dog. When opposing tone and function to flexor groups are lost, the extensor muscles display increased tone.

CCVMM is not typically considered a painful disease. Dogs may carry their head in ventroflexion. Although dogs may sometimes display discomfort when the head and neck are extended, they do not often display the degree of pain that is seen in type I disk extrusions, vertebral tumors, and meningitis. Mild discomfort may be representative of meningeal compression, arthritis, or nerve root pain. Dogs that display significant pain may have a massive disk extrusion or radiculopathy from nerve root impingement. Discussions of the pathogenesis of pain associated with intervertebral disk disease and nerve root compression are found in Chapter 46: Intervertebral Disk Disease, and Chapter 50: Lumbosacral Disease, respectively.