This syndrome is defined by limb ataxia and weakness caused by developmental anomalies or degenerative conditions of the cervical vertebrae. Cervical Stenotic Myelopathy (CSM) is an important condition of several breeds, including Thoroughbreds, Warmbloods and Tennessee Walking Horses. It is less commonly seen in Quarter Horses and Quarter Horse-based breeds. Standardbreds and Arabians are rarely affected. The prevalence in Thoroughbreds is 1.3%. Males are more likely to be affected than females.
There are actually two overlapping ‘sub’ syndromes:
Type I CSM – narrowing (stenosis) of the vertebral canal between the first and sixth cervical vertebrae (C1-C6). The narrowing is exacerbated by flexion of the neck, termed dynamic stenosis. This type is mainly seen in horses 6 to 18 months of age.
Type II CSM – remodelling of the articular process joints (APJ) of the caudal cervical vertebrae (usually C5 to T1) leads to bony and soft tissue impingement into the vertebral canal. The impingement is present in all positions of the neck, termed static stenosis. Affected horses are typically 18 months to 6 year of age at the onset of signs.
Signs
In most horses the neurologic signs slowly develop, but in some the signs may develop suddenly. Once the abnormal gait is identified the signs often progress for several months before they stabilize, and occasionally regress. Complete recovery is rare. Horses with type II CSM are often painful to deep palpation of the caudal neck and are reluctant to flex laterally. In some animals impingement of the caudal cervical spinal sensory nerves can cause forelimb lameness.
The classical neurologic signs include ataxia, weakness and dysmetria. These are caused by compression of the spinal cord leading to damage of upper motor neurons and proprioceptive tracts running within the white matter of the cord. The tracts to the rear limbs run more superficially in the cord in the cervical spine, and therefore compression at these sites causes more severe signs in the back legs. As you move caudally in the cervical area the tracts to the front limbs begin to emerge, and consequently the severity of signs between the thoracic and pelvic limbs with caudal type II lesion will become similar, or even worse in the forelimbs. Dynamic stenosis is usually associated with symmetric signs (left side = right side), whereas static stenosis may cause asymmetric signs due to asymmetry of the joint arthrosis.
Subtle signs under saddle may include rider complaints of a loose rear end, most noticeable as the horse gallops around turns. There may be reluctance to change leads, cross-cantering or cross-galloping, or even stumbling. As signs progress the horse will often become abnormal at the walk.
Diagnosis
Plain radiography is a critical diagnostic step in confirming clinical suspicion of CSM. Plain lateral films should be obtained from the back of the skull to T1. There are a number of different abnormalities that are consistent with type I CSM. These are apparent on straight lateral projections. Oblique projections (latero-50°-ventral to latero-dorsal) enable more accurate description of remodelling and other changes in the APJs. These are particularly valuable in the caudal cervical spine. Various measurements are used to predict cord compression, including both intravertebral and intervertebral sagittal ratios. Suspicions can be confirmed using contrast myelography with the neck in neutral, flexed and extended positions, although there may be problems in interpretation in some cases.
Treatment
Type II CSM may be treated with intra-articular (into APJs) corticosteroids. This would only be helpful if there was a soft tissue component to the arthrosis. It will not reduce the bony deformation. It is more useful to treat if the joints are causing compression or impingement of the spinal nerves, rather than direct compression of the spinal cord. Young horses diagnosed with type I CSM may improve with a ‘paced’ diet. This is for animals diagnosed before 12 months of age, ideally before 6 months of age. There are a number of surgical procedures that have been described. On average the expected improvement with any of these procedures is no more than 1 or 2 grades, at best.
Tags: Neurological diseases