Equis ISSN 2398-2977

Spine: occipitoatlantoaxial malformation

Synonym(s): OAAM

Contributor(s): Graham Munroe

Introduction

  • Dysgenesis of one or more of the occiput, atlas and axis resulting in compression of the cervical spine.
  • Cause: heritable in Arabians   Arab  .
  • Signs: stillbirth, palpable abnormality, reduced range of movement, ataxia, visible malformation of the cervical spine, abnormality of head and neck carriage.
  • Diagnosis: radiography   Spine: radiography  .
  • Treatment: surgery (rarely undertaken); do not breed from affected individuals or their parents.
  • Prognosis: guarded.

Pathogenesis

Etiology

  • Dysgenesis of one or more of the occiput and axis   →    compression of the cervical spine:
    • Fusion of the atlas to the occiput.
    • Hypoplasia of the atlas.
    • Hypoplasia of the dens.
    • Alteration of the joint surface(s).
  • May have a simple recessive mode of inheritance in the Arabian.

Specific

  • Heritable problem in Arabians   Arab  , but asymmetric and symmetric malformations have been reported in other breeds.

Pathophysiology

  • Unstable atlantoaxial joint   →    ventral luxation of the axis on the atlas   →    compression of cervical spinal cord   →    ataxia.
  • Brainstem malformation can be associated with the bony malformations.
  • Scoliosis may be associated with extra bone formation on the caudal axis.
  • Severe parturient compression of the medulla oblongata and spinal cord by the unstable bones may   →    stillbirth.
  • Unstable atlantoaxial joint may   →    chronic accumulation of fibrous tissue   →    spinal cord compression   →    delayed onset of progressive ataxia and tetraparesis in yearlings/weanlings ('Wobblers').

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Hahn C (2006) The wobbly horse: differential diagnoses. In Pract 28 (1), 8-13 VetMedResource.
  • Walmsley J P (2005) Surgical treatment of cervical spinal cord compression in horses: a European experience. Equine Vet Educ 17 (1), 39-43 VetMedResource.
  • Gonda C, Crisman M & Moon M (2001) Occipitoatlantoaxial malformation in a Quarter Horse foal. Equine Vet Educ 13 (6), 289-291 VetMedResource.
  • de Lahunta A et al (1989) Occipitatlantoaxial malformation with duplication of the atlas and axis in a half Arabian foal. Cornell Vet 79 (2), 185-193 PubMed.
  • Watson A G et al (1986) Familial congenital occipitoatlantoaxial malformation (OAAM) in Arabian horse. Spine 11 (4), 334-339 PubMed.
  • Wilson W D et al (1985) Occipitoatlantoaxial malformation in two non-Arabial horses. JAVMA 187 (1), 36-40 PubMed.
  • Mayhew I G et al (1978) Congenital occipitoatlantoaxial malformations in the horse. Equine Vet J 10 (2), 103-113 PubMed.

Other sources of information

  • Hahn C N, Mayhew I G & Mackay R J (1999) Diseases of the Spinal Cord. In: Equine Medicine & Surgery. 5th edn. Eds: Colahan P T, Mayhew I G, Merritt A M & Moore J N. Mosby, USA. pp 945-946. ISBN: 0 8151 1743 4.


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