Wobbler Syndrome and the thoroughbred
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By Celia M Marr, Rossdales Equine Hospital and Diagnostic Centre
Wobbler Syndrome, or spinal ataxia, affects around 2% of young thoroughbreds. In Europe, the most common cause relates to narrowing of the cervical vertebral canal in combination with malformation of the cervical vertebrae. Narrowing in medical terminology is “stenosis” and “myelopathy” implies pathology of the nervous tissue, hence the other name often used for this condition is cervical vertebral stenotic myelopathy (CVSM).
Wobbler Syndrome was the topic of this summer’s Gerald Leigh Memorial Lectures, an event held at Palace House, Newmarket. Gerald Leigh was a very successful owner breeder and these annual lectures, now in their second year, honour of Mr Leigh's passion for the thoroughbred horse and its health and welfare. The lectures are attended by vets, breeders and trainers, and this year because of the importance and impact of Wobbler syndrome on thoroughbred health, several individuals involved in thoroughbred insurance were also able to participate.
Blindfolding the horse, exacerbates the ataxia and improves the accuracy of objective ataxia assessment.
Dr Steve Reed, of Rood and Riddle Equine Hospital, Kentucky and international leader in the field of equine neurology gave an overview of Wobbler Syndrome. Affected horses are ataxic, which means that they have lost the unconscious mechanisms which control their limb position and movement. Young horses with CVSM will generally present for acute onset of ataxia or gait abnormalities, however, mild ataxia and clumsiness may often go unnoticed. Trainers often report affected horses are growing rapidly, well-fed, and large for their age. It is common for riders to describe an ataxic horse as weak or clumsy. Sometimes, a horse which has been training normally will suddenly become profoundly affected, losing coordination and walking as though they were drunk, or in the most severe cases stumbling and falling. Neurological deficits are present in all four limbs, but are usually, but not always more noticeable in the hindlimbs than the forelimbs. In horses with significant degenerative joint disease, lateral compression of the spinal cord may lead to asymmetry of the clinical signs.
When the horse is standing still, it may adopt an abnormal wide-based stance or have abnormal limb placement, and delayed positioning reflexes. At the walk, the CVSM horse’s forelimbs and hindlimbs may not be moving on the same track and there can be exaggerated movement of the hind limbs when the horse is circled. Detailed physical examination may reveal abrasions around the heels and inner aspect of the forelimbs due to interference, and short, squared hooves due to toe-dragging. Many young horses affected with CVSM have concurrent signs of developmental orthopaedic disease such as physitis or physeal enlargement of the long bones, joint effusion secondary to osteochondrosis, and flexural limb deformities.
Radiography is generally the first tool which is used to diagnose CVSM. Lateral radiographs of the cervical vertebrae, obtained in the standing horse, reveal some or all of five characteristic bony malformations of the cervical vertebrae: (1) “flare” of the caudal vertebral epiphysis of the vertebral body, (2) abnormal ossification of the articular processes, (3) malalignment between adjacent vertebrae, (4) extension of the dorsal laminae, and (5) degenerative joint disease of the articular processes. Radiographs are also measured to document the ratio between the spinal canal and the adjacent bones and identify sites where the spinal canal is narrowed.
ABOVE L–R: Lateral radiographs can show the vertebral bones have an abnormal shape with flare of the caudal vertebral epiphysis (curved arrow) and extension of the dorsal laminae (straight arrow). Abnormal ossification of the articular processes and enlargement of the joints due to degenerative joint disease (arrows). Measuring the ratio of the spinal canal to the adjacent bone identifies narrowing of the spinal canal. In this case, the narrowing is dramatic due to mal-alignment of adjacent vertebral bones.
Dr Reed also highlighted myelography as the currently most definitive tool to confirm diagnosis of focal spinal cord compression and to identify the location and number of lesions. The experts presenting at the Gerald Leigh Memorial Lectures agreed that myelography is essential if surgical treatment is pursued. However, an important difference between the US and Europe was highlighted by Prof Richard Piercy, of the Royal Veterinary College, University of London. In Europe, protozoal infection is very rare, whereas in US, equine protozoal myeloencephalitis can cause similar clinical signs to CVSM. Protozoal myeloencephalitis is diagnosed by laboratory testing of the cerebral spinal fluid but there is also a need to rule out CVSM. Therefore, spinal fluid analysis and myelography tends to be performed more often in the US. Prof Piercy pointed out that in the absence of this condition, vets in Europe are often more confident to reach a definitive diagnosis of CVSM based on clinical signs and standing lateral radiographs.