Congenital occipitoatlantoaxial malformations in the horse.

The research investigated various types of congenital malformations in the occiput, atlas, and axis of horses. The findings classified three diseases and characterized their clinical syndromes. The study confirmed that certain malformations are exclusive to the Arabian horse breed.

Study Overview:

The researchers conducted a clinical, radiological, and morphological study on 9 horses presenting with congenital malformations in their occiput, atlas, and axis – key structural components in the upper cervical spine of horses. They also took into account 2 previously reported cases.

Disease Classification:

• From the study, they identified three types of diseases:
• Disease A involves familial occipitalisation of the atlas with atlantalisation of the axis that was evident in Arabian horses. These horses displayed congenital atlantooccipital fusion, hypoplasia of the atlas and dens, malformation of the axis, and modification of the atlantoaxial joint.
• Disease B is congenital asymmetrical occipitoatlantoaxial malformation. One Standardbred and one Morgan horse displayed atlantooccipital fusion, a vertebral piece of a wedge shape connected to the rear end of the axis and sigmoid scoliosis of the cervical vertebrae.
• Disease C involves asymmetrical atlantooccipital fusion. An unidentified breed of horse showed asymmetrical fusion between the atlas and occiput and cervical scoliosis.

Clinical Syndromes:

• The clinical manifestations shown by these horses were varied but were primarily grouped into four types:
• Neonatal mortality was observed in one foal affected by Disease A.
• Five foals with Disease A exhibited tetraparesis at birth, showing signs varying from partial to total paralysis.
• A couple of other foals with Disease A exhibited progressive ataxia, due to a growing focal cervical compressive myelopathy – a type of spinal cord injury.
• Lastly, congenital cervical scoliosis or deviated head posture was observed in the two horses with Disease B and one horse with Disease C. These three horses showed no signs of disease in their spinal cord or brain.

Diagnosis and Pedigree Analysis:

• The diagnoses were determined clinically by examining the occipitoatlantoaxial region, and further confirmed either radiographically or by post-mortem examination in all but one case.
• A pedigree analysis was conducted, which revealed the genetic nature of the particular occipitoatlantoaxial malformation seen solely in horses of the Arabian breed.