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SUMMARY 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: 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 atlanto‐occipital fusion, hypoplasia of the atlas and dens, malformation of the axis and modification of the atlantoaxial joint. Congenital asymmetrical occipitoatlantoaxial malformation (2 cases in this report). A Standardred 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. 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: Foal dead at birth, seen in one foal with A. Tetraparesis at birth, seen in 5 foals with A. These foals were born with signs varying from tetraparesis to tetraplegia. Progressive ataxia, seen in 2 foals with A. Clinical signs were due to a progressive focal cervical compressive myelopathy. 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. RÉSUMÉ Une étude clinique, radiologique et morphologique de 9 chevaux présentant des malformations congénitales de l'occiput, de l'atlas et de l'axis a été faite. On y ajoute deux cas décrits par ailleurs. On en déduit trois entités morbides différentes: L'occipitalisation familiale de l'atlas et de l'atlantisation de l'axis telle qu'elle existe chez le cheval arabe (7 des neufs cas exposés dans cet article auquel il convient d'ajouter le cas cité par LEIPOLD en 1974).Ces chevaux présentaient une fusion occipito‐atloidienne, une hypophasie de l'atlas et une malformation de l'axis avec modification de l'articulation atloido‐axiale. Une malformation congénitale asymétrique occipito‐atloidienne (2 cas rapportés par cet article).Un Standardbred et un Morgan montraient une fusion occipito atloidienne, une pièce vertébrale en coin à l'extrémité caudale de l'axis et une scoliose sigmoide des vertèbres cervicales. La fusion occipito atloidienne (cas rapporté par SCHMALTZ en 1925). Ce cheval de race inconnue présentait une fusion occipito atloidienne et une scoliose cervicale. Les signes cliniques accompagnant ces malformations étaient variables: foal mort né (un cas appartenant au groupe A). paré...

The shape of the foramen magnum and the variation in the ossification pattern of the supra‐occipital bone were studied in 48 skulls, including 12 from neonatal and 33 from immature and mature beagles of known age. The shape varied markedly, and a dorsal notch, dorsomedial to the nuchal tubercles and present in over half of the beagles, developed as a result of incomplete ossification of the ventromedial part of the supra‐occipital bone. In a pomeranian and a chihuahua a membrane of dure mater and connective tissue, which covered the prominent dorsal notch, prevented prolapse of the brain. The spinal cord passed through a smaller oval opening in the ventral half of the foramen magnum. As there was no apparent impairment of function or anomalies associated with the dorsal notch and since a prominent dorsal notch was common in small breeds with brachycephalic skulls, it was concluded that the dorsal notch is a variation in the normal morphological pattern.

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Skeleton preparations of 200 Beagles of known age, ranging from embryos at 28 days of age to 10 year old adults were studied. Preparations of gross specimens were examined with the aid of Alizarin red staining, binocular dissection and histological sections. Ossification centers for the components of the atlas and axis were identified and followed to their fusion. The developmental ossification sequence for these bones is described and compared with that of other similar studies in other species. vertebrae in pups (HARE, 1961 a and b) and one other, more general, radiographic study of the axial skeleton in pups (BOCCADORO et af., 1972), there appear to have been n o studies on the development of the occipiro-atlas-axis complex in the dog.The functional interrelationships in the occipito-atlas-axis complex can sometimes be seen when there is a structural abnormality, or a dysfunction, in one part of the complex, which may lead to functional abnormalities in another part. Atlanto-axial subluxation in the dog demonstrates this functional interdependence. Bone fractures, torn or lax ligaments, and an abnormally shortened dens may each lead to atlanto-axial subluxation; the ::-This study is part of a thesis written by the senior author in partial fulfilmenr of the requirements for the Ph. D. degree

A clinical, radiological, and morphological study of a congenital occipito-atlanto-axial malformation in a 13-week-old male Saint Bernard dog that became suddenly tetraplegic at 8 weeks is described. The dog was recumbent, had generalized muscle atrophy, but was alert and responsive. Pain was elicited when the head-neck junction and the cervical vertebrae were palpated, and a bony abnormality was palpated at the occiput and atlas. Clinical signs of upper motor neuron and general proprioceptive deficits in all four limbs were compatible with a focal lesion in the cervical spinal cord. Plain radiographs of the head and neck revealed malformation of the occipital bones, atlas, and axis, unilateral atlanto-occipital fusion, and atlanto-axial subluxation. At necropsy the right half of the atlantal neural arch was fused to the right exoccipital bone. On the axis, the dens was small, malformed, and deviated to the left; the transverse processes were enlarged; and the spinous process was small with a cleft caudally. The spinal cord was severely compressed at the level of the atlanto-axial articulation, and histological examination revealed extensive loss of neuronal cell bodies, axons, myelin, and the central canal. Reactive astrogliosis was also extensive. After a discussion of normal and abnormal development of the vertebral column and its joints, it was concluded that a failure of normal joint development at about 30 days of gestation in the dog could lead to congenital occipito-atlanto-axial malformation.

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