Magnetic resonance imaging characteristics of atlanto-axial subluxation in 42 dogs: Analysis of joint cavity size, subluxation distance, and craniocervical junction anomalies

Abstract

Background: Atlanto-axial (AA) subluxation can be a complex syndrome in dogs. Accurate identification and
assessment of this condition are key to providing treatment and resolution.
Aim: The purpose of this retrospective study was to describe the magnetic resonance imaging (MRI) characteristics of
AA subluxation and associated neurologic deficits.
Methods: A multicenter review of dogs with a diagnosis of AA subluxation was conducted, evaluating signalment,
neurologic grade, duration of signs, and MRI characteristics. MRI characteristics included degree of spinal cord
compression and joint subluxation, integrity of odontoid ligaments, presence of a dens, spinal cord signal intensity, and
presence of syringohydromyelia, hydrocephalus, and Chiari-like malformation. A control population with normal AA
joints was also evaluated. MR images of 42 dogs with AA subluxation were compared to 26 age and breed-matched
control dogs.
Results: Affected dogs had a median age of 27 months and a median weight of 2.7 kg, and the most commonly affected
breed was the Yorkshire terrier (47.5%). Spinal cord signal hyperintensity, increased AA joint size, and cross-sectional
cord compression at the level of the dens and mid-body C2 were associated with AA subluxation. No associations were
found between cord compression, the appearance of the dens, or cord signal intensity and neurologic grade. Affected
dogs did not have a higher incidence of Chiari-like malformation, syringohydromyelia, or hydrocephalus than control
dogs, and their neurologic grade was not associated with MRI findings. Lack of dens and/or odontoid ligaments was
associated with larger subluxations.
Conclusion: Dogs with clinical signs of AA subluxation were significantly more likely to have intramedullary
hyperintensity at the level of compression (p = 0.0004), an increased AA joint cavity size (p = 0.0005), and increased
spinal cord compression at the level of dens and mid-body C2 (p ≤ 0.05). The authors suggest an AA joint cavity size
>1.4 mm and a subluxation distance >2.5 mm as cutoffs for MRI diagnosis of AA subluxation in dogs. No differences
were noted between dogs with AA subluxation and control dogs regarding syringohydromyelia, hydrocephalus, and
Chiari-like malformation.