Edward MacKillop, DVM, DACVIM (Neurology)
Pittsburgh Veterinary Specialty and Emergency Center
Degenerative compression of the caudal cervical spinal cord in large and giant breed dogs has been referred to as Wobbler Syndrome. This term primarily describes unsteadiness and incoordination in the pelvic limbs which is often the first clinical sign. Spinal hyperesthesia, neck stiffness, thoracic limb reflex deficits and atrophy are also seen clinically. Other terms that have been applied to this condition include cervical vertebral instability, cervical spondylomyelopathy, spondylolisthesis, disk-associated Wobbler syndrome (DAWS), cervical spondylotic myelopathy, and cervical vertebral malformation/malarticulation. A consensus has not been reached regarding the most appropriate term; however, this author uses cervical spondylomyelopathy (CSM) with osseous and disk-associated subtypes.
Osseous CSM is primarily a syndrome of vertebral canal stenosis. Secondarily, it is a syndrome of a degenerative joint disease. Osteoarthritis develops due to malarticulation or overt malformation of the articular processes causing medial encroachment of the vertebral canal. Compression is situated dorsolaterally and is a combination of both bone (osteophytosis) and soft tissue (synovial hyperplasia or cysts). Hypertrophy and cartilaginous or osseous metaplasia of the ligamentum flavum causes dorsal compression of the spinal cord. The most severe compression is usually found at C5-6 and C6-7 but can be present at any level of the cervical or cranial thoracic vertebral column.
Signs are usually slowly progressive although sudden onset may be seen due to decompensation or concussive injury from sudden movement. Osseous CSM is classically seen in giant breed dogs such as the Great Dane, Mastiff and Bernese mountain dog but may also occur in large breeds. Clinical signs develop between 6 months and 2 years of age although more mild cases may not present until later in life. High protein diet during developmental years has been associated with a more severe disease phenotype and earlier onset of signs. Magnetic resonance imaging (MRI) of the cervical spine typically provides a diagnosis although computed tomography (CT) shows the osseous abnormalities more clearly.
Disk-associated CSM is (DA-CSM) a condition that has been most extensively described in the Doberman Pinscher. Other breeds with DA-CSM include the Weimaraner, Standard Poodle, German Shepherd, Dalmation and Labrador. This disorder is still incompletely understood and while its pathogenesis is considered multifactorial, intervertebral disk protrusion is responsible for most of the clinical signs. Hypertrophy of the dorsal longitudinal ligament and ligamentum flavum , mild vertebral malformation and relative stenosis of the vertebral canal may also contribute to spinal cord compression. Vertebral column conformation has been suggested in the pathogenesis of DA-CSM. Signs may be chronic and progressive or acute in onset. An abnormal gait is a common clinic sign and may vary from only pelvic limb deficits (UMN paresis and ataxia) to tetraplegia. Neck pain may be the only clinical sign or seen in combination with tetraparesis and ataxia. Thoracic limb gait is typically short-strided in contrast to spastic and ataxic pelvic limb movements. Most Dobermans with DA-CSM present with clinical signs between four and eight years of age
MRI of the cervical spine is the “gold standard” in diagnosing DA-CSM. Disk degeneration and protrusion are seen in the caudal cervical region. Approximately 25-50% of dogs will have more than one clinically significant lesion. Linear traction to spine may not change the protrusion (static compression) or may lead to significant resolution of cord compression (dynamic compression). Spinal cord hyperintensity on T2-weighted MR sequences correlates with gliosis, syrinx, focal myelomalacia or edema. Signal change within the cord is thought to be of prognostic value. Increasing intensity of T2 signal suggests more permanent damage to the cord and a lower chance of recovery. MRI should be interpreted cautiously since abnormalities have been noted in 25-30% of normal Doberman Pinschers.
Cervical spondylomyelopathy may be treated either medically or surgically. Controversy exists in whether surgical intervention changes the long term progression of disease. Da Costa and colleagues studied a group of dogs with DA-CSM and found no significant difference in the long term survival between medically and surgically managed dogs; however this study was retrospective and only included dogs available for follow-up >6 months after the diagnosis. De Decker reported prospectively on medical management of DA-CSM and found that only 38% (8/21) of dogs showed stabilization or improvement in clinical signs with restricted activity and corticosteroids. All dogs that failed medical therapy had deteriorated by 1 month after diagnosis. Over 24 procedures have been described to treat cervical spondylomyelopathy in dogs. Techniques can be broadly divided into decompression (ventral slot or dorsal laminectomy), distraction with fusion or disk arthroplasty. Ventral slot is ideal for large, static protrusions at a single level or at two non-continuous levels. Dorsal laminectomy is usually recommended for osseous CSM since compression is dorsolaterally situated. Distraction with fusion is thought to be most effective for dynamic protrusion and can be accomplished in a number of ways. Ventrally placed locking plates appear to provide rigid stability and are usually applied in conjunction with a vertebral distractor (e.g. cage or cortical ring allograft). Vertebral pins or screws with PMMA also provide a stable construct for fusion and do not require specialized equipment. Recently, disk arthroplasty or placement of an “artificial disk” following discectomy has been studied in dogs. Case numbers are limited but early reports show this to be a safe and potentially effective procedure. Adjacent segment disease or domino lesions are seen in up to 20% of dogs surgically treated for DA-CSM and disk arthroplasty is intended to minimize this phenomenon by allowing for movement between vertebrae.