2-Year-Old MN Chihuahua

Signalment & History

A 2 year old MN Chihuahua presents to your clinic for neck pain of several months duration despite medical pain management. On physical examination, his vitals are within normal limits. On orthopedic examination, he guards his neck and tries to bite when his neck is palpated; a full range of motion of the cervical region is not performed. On neurologic examination, he is mildly to moderately ataxic in all four limbs.

VD cervical Right lateral cervical


C2 is dorsally displaced relative to C1. The space between the dorsal arch of C1 and dorsal spine of C2 is widened. There is a smoothly marginated, concave defect along the cranial aspect of the C2 dorsal spine. The dens is minimally developed. There are no other abnormalities in these cervical radiographs.


This patient is diagnosed with dens hypoplasia and secondary atlantoaxial subluxation. Differential diagnoses of the cranial defect of the C2 dorsal spine include an anomalous development (incomplete ossification) or pressure necrosis from an external mass.


Atlantoaxial subluxation and dens hypoplasia

  • Atlantoaxial subluxation/instability
    • Congenital or acquired
    • Occurs with:
      • Agenesis or hypoplasia of the dens
      • Absence of the transverse ligament
      • Nonunion of the dens
      • Trauma
    • Signalment: young, small breed dogs
    • Causes compression of the cranial cervical spinal cord
      • Manual flexion of the neck can cause catastrophic damage to the spinal cord!


Bahr A. The Vertebrae. In: Thrall, ed. Textbook of Veterinary Diagnostic Radiology. 5th ed. St. Louis: Elsevier; 2007. p. 179-93.