Also known as: Atlantoaxial Instability — Atlantoaxial Subluxation
Atlantoaxial instability is an uncommon condition of dogs in which there is abnormal movement or instability in the cervical spine or neck, between the atlas and axis. This instability allows abnormal bending between the two bones, which causes pressure or compression of the spinal cord. The severity of the injury to the spinal cord depends both on the amount of pressure, and the duration of the condition.
The atlantoaxial joint is normally stabilised by a projection off the axis called the dens, which fits into the atlas, as well as several ligaments between the two bones.
There are two causes for the instability – trauma and developmental malformations.
- Traumatic instability occurs after forceful flexion of the head, causing either fracture of the dens or another part of the axis, and/or tearing of the fibrous attachments. This can occur in any breed, at any age.
- Developmental malformations can predispose the instability to occur with very small amounts of trauma. Abnormalities include: a missing or malformed dens, lack of normal attachment between the two vertebrae. This occurs most frequently in small breed dogs such as the Yorkshire Terrier, Chihuahua, Poodle, Pomeranian and Pekinese during their first year of life.
Congenital luxations usually occur within the first year of life, and clinical signs vary from sudden onset to insidious.
Signs can range from cervical pain to severe paresis or paralysis. Motor deficits may affect the hindlimbs only or all four limbs.
Severely affected animals may be non-ambulatory and tetraparetic.
Flexing the neck causes severe pain and can cause further neurological injury, so should be avoided.
Further displacement of the dens into the spinal canal can lead to diaphram paralysis, respiratory failure and death.
Atlantoaxial luxation should be suspected in any small breed dog with neck pain, paresis or paralysis.
Lateral and ventrodorsal radiographs usually reveal displacement of the axis. On the lateral view, an increased space is seen between the arch of the atlas and the spinous process of the axis.
Oblique views may be necessary to visualise a hypoplastic or aplastic dens.
Flexed radiographs are not necessary and should be avoided as severe spinal cord compression can occur.
CT is also useful to evaluate the vertebrae and any deformities.
A CSF anaylsis may be performed to exclude inflammation or infection as a possible cause of the symptoms.
Conservative management can be attempted in animals with mild clinical signs.
- Strict cage rest for 4-6 weeks
- Neck brace for 6 weeks to allow fibrous scar tissue to stabilise the joint
- Steroids and pain relief should be used
The dog will have to be protected from trauma for the rest of its life.
Problems include: continued instability, deterioration of clinical signs, bandage sores from the neck brace.
Surgical management is the treatment of choice due to the possibility of recurrence and further spinal cord damage with conservative management. Goals of surgery are to relieve pressure on the spinal cord and stabilise the joint permanently. If the dens is fractured or malformed, it may have to be removed at surgery.
Dorsal and ventral approaches exist, but the ventral approach is preferred as it enables arthrodesis of the atlantoaxial joint to be performed. A cancellous bone graft is placed in the joint and pins or screws are placed across the joints.
A neck brace is recommended for 4-6 weeks post-operatively.
Complications include improper pin placement and pin migration. Repeat radiographs are performed to assess repair and pin position.
The prognosis depends on the degree of spinal cord trauma and neurological deficits present before treatment.
Animals with mild clinical signs have a good prognosis whereas prognosis for dogs with paralysis is more guarded, although recovery is possible with decompression and stabilisation.
Greater success is seen in younger dogs and dogs with more acute problems.
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|Small Animal Emergency and Critical Care Medicine Q&A 15|
Hosgood, G. (1998) Small animal paediatric medicine and surgery Elsevier Health Sciences
Lister, S. (2009) Atlantoaxial instability ACVS Publication
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