Incomplete ossification of the atlas in cats: seven cases detected on computed tomography

Defects of the ventral arch of the atlas were detected on computed tomography in nine cats aged 3…12 months with signs of upper cervical injury including paina and ataxia. Seven cats have an ambulatory tetraparesis, and two cats have a nonambulatory tetraparesis. The bone defects were consistent with the normal location of the ventral arch growth areas of the atlas. In all observed cats, the pattern of ossification abnormalities was similar — the lateral portion of the arch was completely absent in seven cats on the left and in two cats on the right. The structure of the ventral tubercle was traceable in 8 of 9 cats. Also, in 8 of 9 cases an isolated bone fragment was observed lateral to the dens of the axis, the exact origin of which was not determined. This fragment was observed in 6 cases on the right, in two cases on the left, and only in two cases it corresponded to the side of the undeveloped arch. In 7 out of 9 cats, the dorsal arch was not fused; in 2 cats with complete fusion, the dorsal arch was deformed. An dens fracture was visualized in 3 cases, no hypoplasia of the dens was visualized, and one cat have atlantoaxial subluxation. Seven cats received conservative treatment and 2 cats received surgical treatment. Clinical improvement was observed in all cats. Disorder the ossification of the ventral arch of the atlas should be considered as the differential diagnosis in young cats with suspected atlanto-axial instability and trauma of the cervical spine. The authors were unable to find publications describing this atlas developmental abnormality in cats, so the authors believe that this is the first mention of incomplete ossification of the atlas in cats.