A six–month–old Chihuahua presented emergently for severe neck pain and reluctance to walk after playing vigorously at home. No neurologic deficits were reported at the time of examination. A ventrodorsal and lateral projection of the cervical spine are available for evaluation.
Radiographs demonstrate a complete absence or aplasia of the dens along with caudodorsal angulation of the axis (C2) relative to the atlas (C1). Normally the large spinous process of the axis remains in close proximity or overlaps the dorsal aspect of the atlas. The articular processes of C3–C4 appears fused relative to the other articular processes. This likely reflects a concurrent congenital or developmental abnormality.
Atlantoaxial subluxation has been proposed to occur secondary to fracture or malformation of the dens, fractures of the atlas or axis, or rupture or stretching of the atlantoaxial ligaments. Absence of the transverse ligament of the atlas, which normally secures the dens along the ventral aspect of the vertebral canal, has also been implicated. The dens is known to develop from two centers of ossification. It has been suggested that ischemia with resorption of bone may result in hypoplasia or aplasia of the dens, as opposed to a failure of the bone to ossify. Hereditary factors may be involved in some lines of miniature and toy breeds (Yorkshire Terrier, Chihuahua, Pomeranian and Toy Poodles). Atlantoaxial subluxation is also reported as a congenital disorder in Rottweilers and the Doberman Pinscher.
Clinical signs vary according to severity of subluxation and can range from cervical pain and rigidity, to spastic paraparesis or tetraparesis. Clinical signs may develop slowly and episodically, or acutely. Hemorrhage and edema of spinal cord may extend cranially to brainstem resulting in cranial nerve deficits. Survey radiographs without anesthesia are usually sufficient for diagnosis. Anesthetized animals do not maintain cervical muscle tension and are susceptible to cervical flexion and resultant spinal cord injury. If survey radiographs are equivocal, gentle flexion of the cervical spine using fluoroscopy may demonstrate the vertebral subluxation and instability. This technique is best performed by a neurologist. CT and MRI examination may be useful to assess spinal cord injury/compression.