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EQUINE VETERINARY EDUCATION


Equine vet. Educ. (2019) 31 (5) 248-249 doi: 10.1111/eve.12820


Clinical Commentary Clinical evaluation of occipitoatlantoaxial malformation


W. F. Gilsenan* Rood and Riddle Equine Hospital, Lexington, Kentucky, USA. *Corresponding author email: gilsenanw@gmail.com


Keywords: horse


unisholz et al. (2019) present an important account of documenting this lesion in a Warmblood mare. In this case report, the clinicians’ thorough and pragmatic approach to diagnostics led to an unexpected but accurate diagnosis. These diagnostics included a complete and objective physical examination and the incorporation of multiple diagnostic imaging modalities. Proper clinical evaluation led to the antemortem diagnosis of a progressive, untreatable and possibly heritable disease. The clinicians’ findings allowed appropriate management of the horse with respect to the


horse’s quality of life and to the owner’s safety, time and finances. The horse described in the report by Br€


unisholz et al.


(2019) was presented by the owner for a suspected hip luxation. This conclusion precipitated from the observation of ‘atypical swaying of the hindlimbs with an increased range of motion of the pelvis while trotting’. Differentiating between musculoskeletal disease and neurological disease is among the greatest challenges that confront a clinician evaluating a horse with gait abnormalities. The examination of the musculoskeletal system and neurological system need to occur in conjunction with one another. As explained by Furr and Reed (2015), horses with musculoskeletal disease can appear to exhibit neurological deficits; the reason for this is that a normal neurological examination is predicated upon the presence of a sound musculoskeletal system. If multiple musculoskeletal lesions are present, affected horses may very well stumble or pivot when asked to perform complicated manoeuvres. The key to discerning disease of these two systems is that horses with lameness will have predictable gait deficits; these horses have an intact proprioceptive system. They will consistently alter their gait in a manner that minimises pain. Horses with spinal ataxia will have gait deficits that are reliably unpredictable (‘irregularly irregular’). There is little rhyme or reason to the manner in which they place their feet when asked to circle, backup, or come to a stop. Bentz (2009) provides an algorithm to approach differentiating these two disease categories. The algorithm reinforces that a complete neurological examination (including mentation,


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Occipitoatlantoaxial malformation (OAAM) is an uncommon cause of spinal ataxia in horses. This congenital disease is primarily considered to be one of Arabian horses. This breed pattern has led to the strong suspicion that the disease is at least in part hereditary in nature. Pedigree analyses (Mayhew et al. 1978; Watson and Mayhew 1986) suggest that the malformation is passed on in an autosomal recessive manner. To date, four types of OAAM have been described and there are no accounts of muscular anomalies relating to this malformation. There are a handful of reports of this malformation in other equine breeds (Wilson et al. 1985; Gonda et al. 2001), but an extensive body of scientific literature studying this disease does not presently exist. Br€


cranial nerve reflexes, and segmental reflexes) is helpful to include, as additional neurological deficits might be identified elsewhere. The preceding case report also highlights well the


potential for congenital diseases to remain occult or to go unnoticed for several years. Congenital abnormalities of the central nervous system are uncommon in horses (Furr 2015). One post-mortem study evaluated 543 horses with neurological disease and identified congenital malformations in only 2.4% of these cases (Laugier et al. 2009). Congenital lesions are, by definition, present from birth. Congenital lesions are not necessarily hereditary in nature but might also precipitate from toxic, metabolic, infectious or other causes. It is difficult to identify a single inciting cause in most cases. It might be that congenital malformations result from a combination of genetic and environmental factors. While congenital abnormalities of the neurological system are uncommon, they are typically apparent at birth or shortly thereafter. Most accounts of OAAM describe the disease in affected foals. The mare in the preceding case report was believed to have been neurologically normal. Thorough physical examination of this mare indicated palpable and visible abnormalities as well as decreased range of motion of the mare’s neck. These findings prompted suspicion of a cervical vertebral anomaly and directed the diagnostic work- up towards imaging of the neck. Not unlike horses affected by cervical vertebral stenotic myelopathy (CVSM), the delayed onset of neurological deficits in this mare might be attributable to progressive narrowing or deforming of the spinal canal as the mare matured and as ossification occurred. The authors observed that the severity of this horse’s ataxia varied with neck position; the onset of neurological deficits appeared to coincide with breaking and training the horse. Presumably, the introduction of exercise and increased neck motion dynamically exacerbated pre-existing narrowing of the bony spinal canal. To the author’s knowledge, this is the first report that


includes computed tomographic and magnetic resonance imaging findings from a horse affected with OAAM. Antemortem CT examination illustrated irregularities in the occiput and affected cervical vertebrae. Cross-sectional imaging also facilitated depiction of dorsolateral myelographic compression of the cervical spinal cord at the atlanto-occipital joint. The utility of computed tomography to identify circumferential spinal cord compression in horses affected with CVSM has been investigated (Moore et al. 1992). The advantage of these modalities is that the spinal cord can be assessed for compression that occurs beyond a simple dorsal– ventral plane. Our comprehension of CT and MRI anatomy of the equine cervical spine is still evolving, but as standing advanced imaging machines and larger bore magnets become more readily available, these diagnostic imaging


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