Experiences with a new surgical technique for ‘Wobblers’ horses

By Lynn Pezzanite

Wobbler syndrome, also known as cervical vertebral compressive myelopathy (CVCM),  is the most common cause of neurological disease in horses and affects many breeds. Although numerous spinal surgeries are performed on humans, this is the only condition of the spinal cord for which surgery in horses is often performed. 

Wobbler syndrome involves compression of the spinal cord due to narrowing or abnormal development of the spine in the neck, which results in neurologic deficits—specifically ataxia. Ataxia is a term used by veterinarians to describe incoordination and inability of an animal to properly place their legs and maintain balance when they are standing and walking. It is easy, therefore, to see why horsemen describe CVCM horses as “wobblers.” CVCM has been described in many breeds, and it was estimated to affect up to 3% of thoroughbreds in one UK study. There is a high prevalence in young male horses, and these horses comprise 75 to 80% of cases. The condition negatively affects athletic performance, and up to 2/3 of horses diagnosed with CVCM are euthanised due to severity of the ataxia or perceived poor response to therapy and subsequent loss of use of the horse. Treatment recommendations are controversial due to the fear that horses cannot recover function when diagnosed with this condition, as well as concerns regarding the cost of treatment, its invasiveness and complications associated with current surgical procedures. Also, at the current time, it is still very unlikely a veterinarian can accurately predict the degree of improvement and prognosis for a specific horse undergoing treatment. Furthermore, veterinarians do not always agree amongst themselves how severe the ataxia is, which makes it even more difficult to measure improvement following treatment and compare treatments. Despite these concerns, there are many horses that do improve and return to athletic use after neck spinal surgery. 

What are the current options for spinal surgery?

The goal of spinal surgery for CVCM is to remove the ability of two vertebral bodies to move by fusing the two adjacent bones together. The result is that over time, the two bones and joints will change in configuration, the fused bones shrink and more space becomes available for the spinal cord. By removing the compression of the spinal cord, neurological function improves. Current surgical treatments for CVCM include methods for ventral interbody fusion: kerf cut cylinders and ventrally placed locking compression plate and dorsal laminectomy (the top portion of the vertebral body is removed entirely to reduce any compression on the spinal cord). Fusion with using the kerf cut cylinder remains the most commonly performed surgical procedure for cervical stabilisation, but this does not provide stability when the spine is in extension. Locking compression plate technologies are difficult to apply due to the shape of the vertebral body and limited flexibility in placement of the fusion construct and the associated screws. Despite great advancements in equine surgery over the past years, these surgical methods for equine cervical stabilisation require specialised equipment and extensive surgeon experience and still have a high risk of complications, including implant migration or failure and vertebral fracture with a high chance of associated horse fatality. 

The goal of spinal surgery for CVCM is to remove the ability of two vertebral bodies to move by fusing the two adjacent bones together

Recent developments in spinal surgery

Because CVCM is relatively common and there is huge interest in returning affected horses to athletic function, there is a demand to develop surgical techniques that are less technically challenging while reducing complications associated with surgery to safely return horses affected by CVCM to their intended use. Overall, there remains room for improvement in surgical treatment of CVCM to both increase biomechanical stability and reduce complications associated with implant placement.

A new technique for spinal surgery

In a recent pilot study by our group at the PreClinical Surgical Research Laboratory at Colorado State University (Fort Collins, CO, USA), a new technique using advanced surgical implants known as pedicle screws and connecting rods with an interbody fusion device (IFD) were evaluated as an alternative to current techniques for cervical fusion in horses. The idea to use these novel implants came from human surgery, where interbody fusion devices are considered the standard technique for lumbar spine fusion in people, resulting in improved success rates in neurologic function and return to activity. The IFD device was evaluated initially in four horses, showing that the construct integrated with surrounding bone within eight months and did not result in any severe complications, such as implant failure, migration or fracture (as has been reported with other techniques). In addition, we noted that the polyaxial pedicle screw head allowed for increased screw placement options compared to previously described techniques. In particular, this is an improvement compared to the locking compression plate technology, which is limited by the conformation of the ventral keel of the cervical vertebrae. The results obtained in this pilot study prompted further investigation of polyaxial pedicle screw and rod technology in equine patients clinically affected by CVCM. 

The Colorado team’s results

We found 10 horses at the Colorado State University Veterinary Teaching Hospital that were diagnosed with Wobbler syndrome based on examination and diagnostic imaging including x-rays, myelogram, and CT scan. The owners of the horses approved to have them undergo this new surgery with placement of the IFD and polyaxial pedicle screw and rod construct. The 10 horses were closely followed, and clinical outcomes and owner reports were recorded and described in our recent publication in Equine Veterinary Journal. 

The breeds of horses treated included warmbloods, Tennessee Walkers, Arabians and quarter horses. No horses in this case population were intended as racehorses. The median age of horses at the time of surgery was two years (24 months, range 12-168). Male horses were overrepresented as is typical for CVCM, with four geldings, four stallions and two mares treated. Preoperative grade of ataxia ranged from 1 to 3 out of 5 based on the Modified Mayhew neurological grading scale. Surgical fusion was performed at one site in three horses and two sites in seven horses. In 6 out of 8 horses with ≥1-year follow-up, ataxia improved by 1–3 grades, with an average improvement of 1.25 grades. In four horses, ataxia improved to grade 0 (normal) or 1 (mild ataxia). In two horses, the gait was unaffected, but neck comfort improved according to owner follow-up. There were no fatal complications associated with the placement of implants. Complications encountered included swelling around the incision site (seroma), pain and fever. Although we found more serious complications including screw breakage in two horses, a vertebral fracture in one horse, and implant infection in one horse, none of these horses required additional surgical procedures to remove the implants. Two horses were euthanised within the first year after surgery. In one horse with severe neurological deficits preoperatively, surgery did not result in improvement of signs; and the horse was euthanised at six weeks postoperatively. The second horse developed upper respiratory tract obstruction immediately following general anesthesia and was euthanised at the time. 

Long-term follow-up with owners was performed by phone and survey consultation. All eight owners for which at least one year follow-up after surgery was available, reported that their horse’s clinical signs and quality of life were improved, and for all horses the level of exercise was increased since surgery. Five horses were being ridden at the time of follow-up, and one additional juvenile horse was beginning training. All four horses that had been ridden before surgery had improved under saddle. Overall, owner satisfaction with the procedure was reported as excellent in five cases or good in two cases, with one owner not responding to the question. All eight owners reported that they were overall positive about the procedure and would recommend this surgery to other horse owners in the future.

This new surgical technique to treat horses with Wobbler syndrome resulted in at least one grade of gait improvement in 6/10 cases and 6/8 cases for which ≥1-year follow-up was available, which is a similar result when compared to other methods. Advantages of this surgical procedure over others to treat this syndrome in horses include that this technique requires less bone removal from the vertebral column and that the implant itself (polyaxial screw head) may be more easily applied to the vertebral body, as its shape can be varied and so can be tailored to each individual horse. Importantly, this technique offers greater stability in two planes (tension and compression), which is not provided by other techniques such as the Bagby basket or kerf cut cylinder. There were no fatal complications related to implant placement in this procedure. This is in contrast to other techniques such as the basket or kerf cut cylinder, where euthanasia of the horse is the more typical outcome if the implant fails and vertebral fracture occurs due to the extent of damage that usually results in spinal cord injury with subsequent severe neurologic signs. In summary, this technique may represent a safer alternative to current techniques of ventral interbody fusion while achieving similar outcomes in performance. Polyaxial pedicle screw and rod systems for cervical fusion should be considered as an alternative to minimise fatal complications associated with surgery while achieving one to three grades of improvement in neurological signs in horses with Wobbler syndrome. However, this study was performed in a small number of horses, so continued study of this method remains critical, as well as further development and optimisation of other surgical techniques that may result in lower frequency of complications and greater neurologic improvement.

Pezzanite, et al, Outcomes after cervical vertebral interbody fusion using an interbody fusion device and polyaxial pedicle screw and rod construct in 10 horses (2015-2019) https://beva.onlinelibrary.wiley.com/doi/10.1111/evj.13449