Clinical signs: Onset of signs in dogs with the congenital form of the disease usually occurs in young animals (<2 years of age), though problems can develop at any age. Signs can develop acutely or gradually, and waxing and waning of signs is often reported – presumably a reflection of instability at the atlantoaxial junction causing repeated injury to the spinal cord. Signs include neck pain (variably present), ataxia, tetraparesis, and postural-reaction and conscious pro-prioceptive deficits with normal to increased muscle tone and myotatic reflexes in all four legs. In severe cases, animals can present with tetraplegia and difficulty in breathing and they may die acutely as a result of respiratory failure.
Pathogenesis: The atlas (first cervical vertebra) and axis (second cervical vertebra) are bound together by ligaments that run from the dens of the axis to the atlas and the skull, over the dens binding it to the floor of the atlas (the transverse ligament) and between the dorsal lamina of the atlas and the dorsal spinous process of the axis (). The dens is a bony projection from the cranial aspect of the body of the axis and develops from a separate growth plate. Subluxation of the atlantoaxial junction () is a relatively common problem and usually results from a failure of ligamentous support. Toy and small breeds such as Chihuahua and Yorkshire Terrier are at highest risk of the problem as a result of failure of development of the dens (congenital absence or hypoplasia of the dens). Dorsal angulation of the dens can also occur. Fracture of the dens and rupture of the soft tissues maintaining the stability of the atlantoaxial junction can occur in any dog or cat as a result of trauma ().
Diagnosis: Atlantoaxial subluxation can be diagnosed from survey radiographs of the cervical spine but extreme care must be taken when restraining and moving dogs in which this disease is suspected. If the animal is sedated or anaesthetized, the head and neck should be supported in slight extension to avoid further spinal cord injury. On lateral radiographs an increased space can be seen between the dorsal lamina of the atlas and the dorsal spinous process of the axis. In severe cases, malalignment of the bodies of the atlas and axis is clearly visible (). The presence and size of the dens can be evaluated most accurately on ventrodorsal (VD) views. If there is no evidence of subluxation on the lateral views, the neck can be carefully flexed to see if there is instability (the space between the dorsal lamina of the atlas and the dorsal spinous process of the axis should be evaluated). It is preferable to do this with fluoroscopy so that the movement can be monitored, to prevent accidental iatrogenic subluxation.
Treatment and prognosis:
Conservative management: Dogs with mild signs can be treated conservatively by placing an external splint for at least 6 weeks. The splint must immobilize the atlantoaxial junction and so must extend over the head cranial to the ears and go back to the level of the chest (). The aim is to stabilize the junction while the ligamentous structures heal. The splint should be checked daily by the owner for signs of pressure sores and checked weekly by the veterinarian, with regular bandage changes if necessary. While often effective in the short term, the long-term efficacy of this approach is not known and dogs treated in this way will always be at risk of repeated injury.
Surgical management: This is recommended for dogs with neurological deficits, though it can be associated with high perioperative morbidity and mortality. Dorsal and ventral approaches to the atlantoaxial junction have been described; dorsal approaches are associated with a greater risk of causing spinal cord injury during surgery and a higher incidence of implant failure. Using ventral approaches, subluxation is reduced and the atlantoaxial articular surfaces are curetted to promote bony fusion. The two bones are fused using transarticular screws or Kirschner wires and a cancellous bone graft is placed over the junction (). In the case of a traumatic injury or poor bone purchase, screws or Kirschner wires are placed in the body of the atlas and axis and the junction is stabilized with polymethylmethacrylate cement. A neck splint is placed post-operatively while fusion occurs. This is a problematic area to repair surgically; bone quality is often poor, the bones are small, movement of the vertebrae may cause additional injury to the spinal cord, and the pharynx and larynx can be damaged during retraction. There is a risk of respiratory arrest and death in the perioperative period as a result of additional spinal cord injury, or inflammation of the upper airways secondary to retraction.
This is a serious disease but dogs with mild deficits treated surgically have an excellent prognosis if they survive the 48-hour perioperative period. Although reported surgical success rates range from 50% to 90%, the majority report a mortality rate in the region of 20%, with most deaths occurring either during or immediately after surgery. As with all spinal cord diseases, prognosis is worse in animals with severe and chronic neurological deficits. It has also been shown that prognosis is better in young dogs (< 24 months) ().
Dermoid sinuses are inherited developmental defects in which there is a failure of separation of the skin and neural tube, leading to the formation of a sinus that extends to the supraspinous ligament or the subarachnoid space.
Also known as cartilaginous exostosis and multiple cartilaginous exostoses, this is a disease of growing dogs in which bony protruberances, capped by cartilage, develop from the region near the growth plate of bones of endochondral origin. Vertebral lesions commonly arise, causing severe spinal cord compression.
Clinical signs: Neurological signs are caused by progressive expansion of the cavities and relate to their location. A prominent sign in affected Cavalier King Charles Spaniels is persistent flank scratching with apparent pain in the neck, facial and shoulder region. Torticollis and scoliosis have also both been reported in association with the problem and are thought to result from denervation of local epaxial spinal musculature due to destruction or compression of spinal cord grey matter ().
Pathogenesis: Syringomyelia is a fluid-filled cavity within the spinal cord; hydromyelia is simply dilation of the central canal. As it can be difficult to differentiate between the two conditions based on imaging studies, the term syringohydromyelia is often used. Both conditions can be a secondary long-term complication of any spinal cord disease, especially those in which a large volume of spinal cord tissue becomes necrotic. Any disease that causes obstruction of normal cerebrospinal fluid flow within the spinal cord can result in syringohydromyelia and as such it can be seen in association with neoplasia, in particular intra-axial tumours, and in feline infectious peritionitis (FIP), associated with ependymitis. Cervical syringohydromyelia also occurs as a component of congenital anomalies such as Chiari-type malformations in the Cavalier King Charles Spaniel () and other small dog breeds such as Pomeranians. The author has seen several Pugs with thoracolumbar syringohydromyelia.
Diagnosis: If the cavities connect with the subarachnoid space, myelography may be diagnostic, but they are most reliably seen on MR images ().
Treatment and prognosis: Treatment may be conservative with anti-inflammatory doses of prednisolone and physical therapy, or surgical if there is an underlying cause that can be addressed, such as occipital dysplasia. There is relatively little information available on prognosis, reflecting the recent recognition of this disease with the advent of MRI. Prognosis depends on the severity of signs and whether an underlying cause can be addressed. Successful outcomes have been reported with surgical management of Chiah-like malformations.
Vertebral and spinal cord anomalies
Atlantoaxial subluxation is the most common consequence of a vertebral anomaly that causes tetraparesis and this condition was addressed earlier in this chapter. Other vertebral anomalies include: hemivertebrae; transitional, block and butterfly vertebrae; congenital spinal stenosis (often a component of cervical stenotic myelopathy); and vertebral facet dysplasia.
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