Equine neurology is a subject that is somewhat taboo among equine vets. The myriad of different synapses, neuronal pathways and reflexes were taught at vet school but promptly forgotten among the host of other more hands-on and practically relevant day-to-day issues our clients need us to sort out. However, there has been a real growth of interest in this area in recent years, largely due to the advancement of cross-sectional imaging that has enabled us to understand so much more of what is going on with horses suffering from neurological conditions.
There are broadly three types of neurological conditions: those affecting the head, brain and components of the horse’s nervous system related to the head, those affecting the cervical spine and those that affect the horse systemically, usually related to infectious diseases such as equine herpes virus.
The most common neurological symptom that equine vets are presented with are horses that have become ataxic. This may vary from horses that are only mildly ataxic and may have subtle gait abnormalities, right through to horses that look like they have had a bit too much to drink and have lost their normal coordination. Such horses – often termed “wobblers” due to their degree of ataxia – fall broadly into two categories: young horses that are not fully mature and older horses who have become increasingly ataxic or stiff-necked over time. Both types of horses’ symptoms are caused by cervical spinal cord inflammation that is usually the result of either a dynamic or a static compression of the spinal cord by the articulations in the cervical spine.
The horse’s neck consists of seven cervical vertebrae and the first thoracic vertebrae and, as links in a chain, has two paired articular process joints either side of the vertebrae that are synovial in nature at the dorsal aspect of the vertebrae. There is a fibrocartilage disc that lies between the caudal end plate of one and the cranial end plate of the other vertebrae that forms the third joint ventrally in the chain link. The spinal canal forms a bony tube through which the spinal cord passes and in the normal horse protects the spinal cord as it passes down the cervical spine and into the thoracic spine. The neck moves both dorsal-to-ventral and lateral-to-lateral with each joint working as a link in a chain to enable complete flexion in each direction while preventing any pressure on the spinal cord. Narrowing of the spinal column, either due to malformative disease processes such as osteochondrosis or osteoarthritis or due to malpositioning processes in which the vertebrae of the neck are positioned abnormally when the neck is in flexed or extended positions, results in spinal cord inflammation which leads to the clinical signs of ataxia as well as other symptoms of neurological disease in the horse.
Typically, horses with neurological disease related to spinal cord compression present with pelvic or thoracic limb weakness, incoordination or gait abnormalities. The pelvic limbs are typically affected first due to the position of the pelvic limb spinocerebellar tracts more superficially on the dorsolateral and dorsomedial aspects of the spinal cord white matter. As the pressure on the cord increases it affects the deeper forelimb spinocerebellar tracts resulting in forelimb symptoms being seen.
It is important to recognise and differentiate compressive myelopathy symptoms such as those described above from those symptoms that are a result of pressure on the spinal nerve roots that exit the spinal canal through the intervertebral foramen on each side. These horses typically have unilateral symptoms related to forelimb weakness with varying degrees of lameness that may be intermittent in nature and only seen under particular conditions, such as being ridden in varying degrees of neck flexion. Other pointers that may suggest the cervical spine being involved include a thin or pencil neck appearance, a horse that acts like it is mildly sedated in its demeanour and movement, a lack of hindlimb musculature and a history of tripping or “being clumsy”.
The neurological examination will help to confirm the presence of neurological symptoms by testing the normal ability of the horse to recognise abnormal positioning of limbs and perform gait transitions, backing up, turning in circles, walking over and around obstacles and walking up and down hills. Very often a suspicion of neurological disease is seen by performing these very simple tests with the horse having delayed or abnormal responses and an inability to adapt to abnormal positions and movements when asked.
The reason neurological disease in horses related to the cervical spine has become more accessible and better understood has largely been due to the ability to now routinely perform cross-sectional imaging of the neck. Radiographic evaluation of the cervical spine gives some indication of disease relating to narrowing of the spinal canal, enlargement of the articular process joints and abnormal positioning of the cervical vertebrae, but the use of computed tomography (CT) in which multiplanar reconstructions can be performed has enabled the recognition of abnormalities that previously were only possible with post-mortem examination. The presence of osteochondral fragments, narrowing of intervertebral foramen and compression of spinal nerve roots, the collapse of the intervertebral disc and the presence of spinal column bone abnormalities are but a few of them. It has also given us greater confidence in recognising the presence of spinal cord compression on a lateral-to-lateral plane which previously has not been possible. Thus, equine vets are better equipped to provide more complete assessment and diagnosis, and therefore provide a more complete therapeutic plan to manage these cases.
CT is complemented with the introduction of contrast into the spinal column to perform a CT myelogram (Figures 1 to 4). Where there is narrowing of the contrast column as it flows down the spinal column surrounding the spinal cord, spinal cord compression can be assumed. The presence of lateral as well as dorsoventral contrast column narrowing is only evidenced using CT and has given us a greater understanding of horses that otherwise would have been refractory to treatment had the lateral compression not been identified.
CT myelography is performed under general anaesthesia and takes approximately 30 to 45 minutes to complete (Figure 5). It enables the cervical and cranial thoracic spine to be imaged and is now routinely performed in horses of all shapes, size, breed and discipline. It has become the true gold-standard way of evaluating the cervical region and is a key component in enabling us to treat neurological disease more effectively and more accurately.
Horses diagnosed with ataxia due to spinal cord compression have traditionally been considered cases which have no future. There is, however, no reason for this assumption. For over 40 years now the ability to perform surgery to fuse the affected sites has been documented, with successful resolution of neurological horses well documented. There remains in the veterinary community an ignorance of such evidence and a bias against treating such horses that is outdated and unfounded. With thousands of horses being treated with cervical fusion over the past 40 years and reported success rates of between 68 percent and 88 percent of horses returning to being useful riding and competition animals, there is more evidence for success of this procedure than many less-questioned procedures such as upper airway surgery and fracture repair.
The aim of the surgery is to prevent the spinal cord compression occurring at the articulations identified on myelography by fusion of the affected vertebrae. By fusing the affected vertebrae, the spinal column cannot then impinge on the spinal cord in flexion or extension. In addition, the bony remodelling that results in spinal cord impingement regresses over the course of 6 to 12 months with resultant freedom of the spinal cord from ongoing impingement and inflammation. The fusion is performed by placement of a special titanium implant – the partially threaded Seattle Slew Bagby basket – into a drilled-out site that bridges the two affected vertebrae on their ventral aspect. The basket is named after the most famous Triple Crown winner who underwent two cervical arthrodesis fusions as an 18- and 20-year-old which enabled him to continue with a highly successful breeding career for several years before succumbing to an unrelated death.
The surgery is performed under general anaesthesia. Horses typically are hospitalised for seven days post-procedure and are then rehabilitated in their home environment over the course of six months. Rehabilitation of the spinal cord is an important part of the recovery process which involves neck flexion exercises as well as owner-driven proprioceptive exercises such as walking around and over obstacles and up and down hills and with the use of training aids such as long-lines and the Pessoa. The spinal cord can take 12 months to heal from its compressive inflammation and typically horses are progressing into ridden work between 6- and 12-months post-surgery.
For horses with evidence of osteoarthritis affecting the cervical spine, the use of intra-articular medications just as in other joints is now a readily performed technique that is performed under ultrasound guidance. Techniques using both linear and microconvex and abdominal ultrasound probes are well described and medications can provide much relief and enhance performance in horses that have been limited by stiff or painful cervical articulations.
Equine neurological disease specifically as it pertains to the cervical spine can be better understood thanks to the now routinely performed cervical computed tomography. It enables horses that have previously been consigned to retirement or euthanasia to be treated, preventing the need for unwarranted destruction of horses that can be helped. Recognising horses with minor neurological symptoms should enable faster, more accurate diagnosis of sites and therefore provide more effective responses to treatment being given at an earlier stage. Equine veterinary practitioners should be aware of the capability, the affordability and the effectivity of CT and CT myelography in diagnosing disease and the success rates of surgery and medication in treating horses suffering from these clinical symptoms.