RESUMO
From a clinical, radiological and morphological study of 9 horses with congenital malformations of the occiput, atlas and axis, and from a study of 2 reported cases, 3 diseases were defined: A. Familial occipitalisation of the atlas with atlantalisation of the axis in Arabian horses (7 cases in this report and the case reported by Leipold, et al., 1974). These horses had congenital atlantooccipital fusion, hypoplasia of the atlas and dens, malformation of the axis and modification of the atlantoaxial joint. B. Congenital asymmetrical occipitoatlantoaxial malformation (2 cases in this report). A Standardbred and a Morgan horse had atlantooccipital fusion, a wedge shaped vertebral piece attached to the caudal end of the axis and sigmoid scoliosis of the cervical vertebrae. C. Asymmetrical atlantooccipital fusion (the case reported by Schmaltz, 1915). This horse of an unknown breed had asymmetrical fusion between the atlas and occiput and cervical scoliosis. The clinical syndromes shown by horses with these malformations were variable but were broadly classified as: 1. Foal dead at birth, seen in one foal with A. 2. Tetraparesis at birth, seen in 5 foals with A. These foals were born with signs varying from tetraparesis to tetraplegia. 3. Progressive ataxia, seen in 2 foals with A. Clinical signs were due to a progressive focal cervical compressive myelopathy. 4. Congenital cervical scoliosis/deviated head, seen in the 2 horses with B and the horse with C. These horses had no signs of spinal cord or brain disease. The diagnoses were made clinically by palpation of the occipitoatlantoaxial region and were confirmed radiographically and/or by post mortem examination in all except one case. Pedigree analysis showed the familial nature of the particular occipitoatlantoaxial malformation seen in horses of only the Arabian breed.
