Page 1084 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Nervous system 1059

VetBooks.ir 10.18 OCCIPITOATLANTOAXIAL
MALFORMATION

Definition/overview
Congenital occipitoatlantoaxial malformation
(OAAM) is a rare problem in horses and appears to
be not one single defect, but rather a spectrum of
cervical spinal abnormalities.

Aetiology/pathophysiology
Abnormalities associated with OAAM include
changes in the occiput, atlas (C1) and axis (C2)
where, broadly, the atlas resembles the occiput
Fig. 10.18 Cerebellar abiotrophy in an Arabian. (occipitalisation) and the axis resembles the atlas
Note the hypermetria of the forelimb. (Photo courtesy (atlantalisation). More specifically, malformations
FT Bain) include abnormalities of the occipital condyles,
fusion of the occiput and the atlas, abnormal
localisation and development of the axis, and
Differential diagnosis hypoplasia of the wings of the atlas and dens of the
Meningitis; equine protozoal myeloencephalitis axis. The lesions may be symmetrical or asymmet-
(EPM); hypoxic–ischaemic encephalopathy; cer- rical. Historically, it was presumed to be inher-
ebellar abscess; parasitic migrans in the cerebellum; ited as an autosomal recessive genetic disorder in
West Nile virus (WNV) encephalitis. Arabian horses and part-bred Arabians. Recently,
a genetic deletional mutation at the HOX gene
Diagnosis cluster (an area highly conserved amongst species
A tentative diagnosis can be made on history and and associated with the development of the axial
clinical signs. Ancillary examinations have not and appendicular skeleton) has been reported in
detected any specific abnormalities. The CSF val- an Arabian foal with OAAM. Variations of OAAM
ues in affected foals are usually normal, although have been reported in other breeds.
increased creatine kinase (CK) has been reported
in a number of cases. Performing genetic testing Clinical presentation
for the TOE1 genotype may support a diagnosis of Clinical signs of OAAM reflect the degree of spinal
cerebellar abiotrophy and is available commercially. cord compression that occurs in the atlas foramen
Recently, authors have reported the use of morpho- but, occasionally, the malformation occurs without
metric analysis of MR images to support an ante- spinal cord compression. At worst, foals are recum-
mortem diagnosis of cerebellar abiotrophy, whereby bent from birth, and if supported to stand show
affected individuals had significantly reduced cere- spastic tetraparesis and readily fall, especially if
bellar size (relative to the whole brain), and increased the neck is extended to try to suckle from the mare
cerebellar CSF spaces (relative to the cerebellum) in (Fig. 10.19). Movement of the head is sometimes
comparison with normal horses. associated with an audible ‘click’ or crepitus. In some
affected foals, the signs are not apparent for weeks
Management/prognosis or months after birth, and are steadily progressive.
There is no effective treatment and these animals are It is thought in these cases that this chronology is
unsafe for riding. On recognition, the owner should associated with failure of the malformed atlas and
be counselled with regard to the inherited nature of axis to expand to fit the enlarging spinal cord. More
the disorder and discouraged from future breeding rarely, signs are not recognised until horses are a few
of the parents or affected animal. years of age. A common characteristic of OAAM is