## Overview Occipitoatlantoaxial malformation (OAAM) is a congenital developmental defect in which the occipital bone, atlas, and axis are abnormally formed and often fused. The malformation narrows the spinal canal and compresses the upper cervical spinal cord. It is most frequently recognized in Arabian horses and is inherited as an autosomal recessive trait in at least one form (OAAM1) linked to a deletion near the HOXD3 gene. Clinical signs are present at birth or appear in the first months of life. ## Causes OAAM1 is caused by a 2.7 kb deletion near HOXD3. Other genetic or developmental causes exist in Arabians and other breeds (Miniature Horses, Quarter Horses, Friesians). The precise embryologic mechanism involves failure of normal segmentation of the cranial cervical sclerotomes. ## Diagnosis Foals show abnormal head and neck carriage (often extended), neck stiffness, progressive ataxia, and sometimes an audible click with neck movement. Radiographs reveal occipitalization of the atlas, fusion, and spinal-canal stenosis. Genetic testing identifies carriers and affected animals for the OAAM1 variant. ## Treatment No corrective treatment exists. Supportive care and restricted activity may be tried in mildly affected animals, but most foals with significant neurologic deficits are euthanized. ## Recovery and Prognosis Prognosis is poor for athletic use or normal quality of life when neurologic signs are present. Mildly affected horses occasionally survive but remain neurologically compromised. ## Prevention Genetic testing of Arabian breeding stock and avoidance of carrier-to-carrier matings prevent OAAM1. Testing is recommended for Arabians and Arabian crosses.
abnormal head and neck carriage from birth; neck stiffness or extended posture; progressive ataxia; reluctance to move the neck; occasional clicking sound
## Overview Occipitoatlantoaxial malformation (OAAM) is a congenital developmental defect in which the occipital bone, atlas, and axis are abnormally formed and often fused. The malformation narrows the spinal canal and compresses the upper cervical spinal cord. It is most frequently recognized in Arabian horses and is inherited as an autosomal recessive trait in at least one form (OAAM1) linked to a deletion near the HOXD3 gene. Clinical signs are present at birth or appear in the first months of life. ## Causes OAAM1 is caused by a 2.7 kb deletion near HOXD3. Other genetic or developmental causes exist in Arabians and other breeds (Miniature Horses, Quarter Horses, Friesians). The precise embryologic mechanism involves failure of normal segmentation of the cranial cervical sclerotomes. ## Diagnosis Foals show abnormal head and neck carriage (often extended), neck stiffness, progressive ataxia, and sometimes an audible click with neck movement. Radiographs reveal occipitalization of the atlas, fusion, and spinal-canal stenosis. Genetic testing identifies carriers and affected animals for the OAAM1 variant. ## Treatment No corrective treatment exists. Supportive care and restricted activity may be tried in mildly affected animals, but most foals with significant neurologic deficits are euthanized. ## Recovery and Prognosis Prognosis is poor for athletic use or normal quality of life when neurologic signs are present. Mildly affected horses occasionally survive but remain neurologically compromised. ## Prevention Genetic testing of Arabian breeding stock and avoidance of carrier-to-carrier matings prevent OAAM1. Testing is recommended for Arabians and Arabian crosses.
abnormal head and neck carriage from birth; neck stiffness or extended posture; progressive ataxia; reluctance to move the neck; occasional clicking sound