Cervical Vertebral Malformation (CVM) is a malformation of the vertebrae of the neck region resulting in compression of the spinal cord and stumbling and incoordination. It is also known as Cervical Stenotic Myelopathy (CSM) or Cervical Compressive Myelopathy or "Wobblers" syndrome.
There are two recognized forms:
Dynamic stenosis - Defined as deformation of the vertebral column with compression of the spinal cord with the neck in flexion or extension. Affected animals are typically younger (6-18 months).
Static stenosis - Bony and soft tissue impingement into the vertebral canal resulting from degenerative joint disease of the articular facets. Affected animals are typically older (greater than 18 months-3 years).
There has been no clear evidence that CVM is hereditary, however wobbler to wobbler matings have resulted in animals with a propensity for rapid growth. Dietary factors such as high protein and caloric intake may play a role in the development of the disease, by giving rise to osteochondrosis (OCD) of the articular facets and vertebral physes (growth plates). This OCD results in multiple vertebral abnormalities, one of which is a disparity in longitudinal growth of the vertebral body and dorsal lamina. This abnormality allows for subluxation of the vertebra upon flexion and subsequent compression of the spinal cord.
The condition is seen mostly in Thoroughbreds, Standardbreds and Quarter Horses. Males are most commonly affected, and usually are well grown. They may have a previous history of developmental orthopedic disease, such as OCD of other joints and physitis. Many horses may have subtle neurologic disease that goes unnoticed until the horse experiences a traumatic incident such as a fall, which worsens its neurologic status.
Clinical Signs: Common signs include ataxia or incoordination, tripping or stumbling while walking, and weakness characterized by toe dragging, most often in the hind limbs. The forelimbs are usually less severely affected (1 grade less). The horse may adopt a base wide or base narrow stance in an effort to support itself. Reluctance to flex the neck laterally is a common finding. These findings are often seen upon neurologic examination.
Diagnosis: A presumptive diagnosis can sometimes be made by cervical (neck) radiographs or x-rays if narrowing of the vertebral canal is observed. Vertebral canal to body ratios can be measured to support a diagnosis of vertebral canal narrowing. A ratio of less than 52% from the 2nd cervical vertebrae to the 6th cervical vertebrae predicts spinal cord compression with a 95% sensitivity and specificity. Other supportive findings include malformation, degenerative joint disease or OCD of the articular facet joints. A confirmatory diagnosis can be obtained by performing a contrast myelographic study. Confirmation of the affected sites is necessary if surgical decompression is being considered.
Medical Treatment: Younger horses under 1 year of age can be given a restricted diet, referred to as the "Paced Diet." It reduces energy and protein intake and increases trace mineral intake. If still suckling, the foal should be weaned. Free choice grass hay plus an appropriate quantity (according to body weight) of a high nutrient/low caloric supplement should be fed.
Surgical Treatment: Surgical decompression of the cervical spinal cord and fusion of the involved vertebral bodies is performed. A stainless steel basket is implanted with cortical bone to promote fusion of the vertebrae. This realignment and fusion of the affected vertebrae results in relief of the dynamic compression or stenosis. Static stenosis may improve over months as atrophy of the degenerate intervertebral joints occurs. Improvement in neurologic status may be seen in 44-90% of horses with dynamic compression. 12-62% of horses may return to previous athletic function. Generally there is a 60% chance that the affected horse will improve at least 1 neurologic grade. That being said, the neurologic grade at presentation should be taken into consideration when formulating a prognosis for return to athletic function.