Evidence of occipitoatlantoaxial malformations independent of the HOXD3 mutation in an Arabian horse.

Occipitoatlantoaxial malformations (OAAM) are developmental disorders occurring at the occiput, atlas and axis. OAAM type 1 is a familial disorder affecting Arabian horses. Currently, a single genetic mutation in the HOXD3 gene region can be tested to confirm the diagnosis. This case describes a 3-year-old Arabian gelding affected with OAAM. Clinical examination revealed a proximal cervical deformation on the right side and severe stiffness of the neck in the sagittal plane without associated pain. Neurological examination demonstrated a generalized proprioceptive ataxia. Radiographs revealed severe cranio-cervical vertebral malformations. This horse tested negative for the genetic mutation in the HOXD3 gene. Considering the poor prognosis and a generalized grade 3/5 proprioceptive ataxia, euthanasia was elected. This case highlights the phenotypic variability of familial OAAM type 1, showing that this kind of cranial cervical vertebral malformation may be diagnosed or detected in adult horses. Further investigations are needed to better understand the genetic heterogeneity of OAAM.