Ronaldo Casimiro Da Costa,     Columbus, Ohio

Causes and Pathophysiology

The cause of CSM remains unresolved. Proposed causes include genetic, congenital, body conformation, and nutritional factors, with the latter two playing a less significant role. Based on current evidence CSM appears to be congenital. The genetic contribution to the development of the disease remains unclear, although recent evidence in Dobermans suggests that CSM is inherited as an autosomal dominant trait with variable penetrance in this breed.

The pathophysiology of CSM involves both static and dynamic factors. The key static factor is vertebral canal stenosis. It may be an absolute vertebral canal stenosis, which causes direct spinal cord compression and neurologic signs, or a relative vertebral stenosis, which by itself does not lead to myelopathic signs but predisposes the patient to development of myelopathy. Despite some degree of overlap, the spinal cord compressions can be divided into osseous compression and disk-associated compression based on pathophysiology.

Disk-associated compression typically is seen in middle-aged large-breed dogs (mostly Dobermans). It is caused by intervertebral protrusion with or without hypertrophy of the dorsal longitudinal ligament or ligamentum flavum (Figure 236-1). Three factors act in combination to explain the pathophysiology of disk-associated CSM: (1) relative vertebral canal stenosis, (2) more pronounced torsion in the caudal cervical spine leading to intervertebral disk degeneration, and (3) protrusion of larger-volume disks into the caudal cervical spine. Affected dogs apparently are born with a congenital vertebral canal stenosis that predisposes them to the development of clinical signs.

The vast majority of disk-associated spinal cord compressions are located in the caudal cervical spine, affecting the disks at C5-6 and C6-7. The biomechanical features of the caudal cervical spine explain the high incidence of lesions there. This region experiences three times more axial rotation or torsion than the cranial cervical spine, and torsion (more than axial compression) is the main biomechanical force leading to intervertebral disk degeneration in nonchondrodystrophic dogs. Additionally, a study found that Dobermans with CSM have larger intervertebral disks than clinically normal Dobermans (da Costa et al, 2006b). This would cause a larger volume of disk protrusion into the vertebral canal. Since dogs with CSM have a narrow vertebral canal, the combination of a stenotic vertebral canal and protrusion of disks with larger volumes ultimately leads to clinical disease.

The pathophysiology of osseous or bony CSM is different. Osseous CSM is seen predominantly in young adult giant-breed dogs, especially in Great Danes. Although a hereditary basis still is unproven, a familial predisposition has been identified. Giant breeds usually have severe absolute vertebral canal stenosis secondary to proliferation of the vertebral arch (dorsally), articular facets (dorsolaterally), or articular facets and pedicles (laterally) (Figure 236-2). The cause of the compression appears to be a combination of vertebral malformation and osteoarthritic-osteoarthrotic changes at the level of the articular facets. Even though most giant-breed dogs have osseous compression, occasionally these compressions are complicated by disk protrusion, especially in older dogs. Ligamentous compression (by the ligamentum flavum) may be involved in the pathophysiology of the disease in giant- and large-breed dogs, but pure ligamentous compression as the single source of compression appears uncommon.

Critical to understanding the development of clinical signs in CSM-affected large-breed dogs is the concept of the dynamic lesion. Dynamic spinal cord compressions are present in both disk-associated and osseous forms of CSM. A dynamic lesion is one that worsens or improves with changes in the position of the cervical spine. Continuous flexion and extension of the cervical spine can lead to spinal cord elongation causing axial strain and stress within the spinal cord, and this has been proposed as a key mechanism of spinal cord injury in cervical spondylotic myelopathy in humans. This is very different from instability, which has been defined as loss of the ability of the cervical spine to maintain its normal pattern of displacement under physiologic loads and thereby prevent damage to the spinal cord or nerve roots. Instability has not yet been proven in dogs with CSM, and it appears that with the severe disk and osseous degenerative changes, a restricted rather than an excessive motion occurs at the affected sites.

The most common location of compressive lesions in both large- and giant-breed dogs is at C5-6 and C6-7. The lesion is located at one of these sites in 90% of affected large-breed dogs. In giant-breed dogs, the C4-5 site also is commonly affected. Approximately 50% of large-breed dogs have a single site of spinal cord compression, and 50% have two or more sites of similar severity. In giant-breed dogs approximately 20% of dogs have a single site of compression, whereas 80% have multiple compressive lesions. A computed tomographic (CT)–myelographic study identified lesions affecting the T1-T2 and T2 regions in 14% of giant-breed dogs and the C7-T1 region in 22% of all dogs (da Costa et al, 2012). These lesions were not the primary site of compression but were part of the multiple compressions seen in giant-breed dogs. For this reason, it is important to include the cranial thoracic region in imaging studies of dogs suspected of having CSM.

Diagnosis

As indicated earlier, CSM is primarily a disease of large- and giant-breed dogs. It is seen occasionally in small-breed dogs, but it is unclear whether small dogs also have vertebral canal stenosis. The majority of large-breed dogs (weimaraners, dalmatians) and Dobermans with CSM are older than 3 years of age at presentation (mean age, ~7 years). Great Danes and giant breeds (Mastiffs, rottweilers, Bernese and Swiss mountain dogs) with CSM usually are seen at a younger age. The mean age of giant-breed dogs with CSM is 3.8 years, and the disease may be seen in dogs just a few months old.