THE equine spinal column is composed of both bony and soft tissue structures. The bony elements are known as vertebrae and they are numbered sequentially in accordance with their anatomical location.

The normal spine consists of seven cervical vertebrae (the neck), 18 thoracic vertebrae (from which the ribs originate), six lumbar vertebrae (region just behind the saddle), five sacral vertebrae (the pelvic region) and 15-21 coccygeal vertebrae (the tail).

Each vertebra varies slightly in shape, depending on its position in the spinal column, however they all follow a basic plan analogous to an airplane. Each vertebra consists of a body (fuselage), left and right transverse processes (wings) and a dorsal spinous process (tail fin).

The dorsal spinous processes (DSPs) vary in length, shape and angulation, and are largest in the thoracic and lumbar regions of the spine.

In normal horses, a space exists between adjacent DSPs. This space is occupied by a ligament and is evident on an X-ray.

In some instances, this space is obliterated as one DSP comes into contact with its neighbouring DSP. These are known as impinging DSPs, or more colloquially as “kissing spines” (Figure 1).

This impingement may result in pain, with the thoracic and lumbar DSPs involved most commonly. Impinging DSPs is the most common cause of back pain in horses, with thoroughbred racehorses more commonly affected. Post-mortem studies have detected impinging DSPs in up to 90% of horses, many of whom had shown no evidence of back pain during their careers.

It is not exactly clear why impinging DSPs occur, however it has been theorised that horses with shorter or more dipped backs may potentially be at increased risk due to overcrowding of the vertebrae.

Clinical Signs of Impinging DSPs

Abnormalities noted by riders/trainers, which may indicate back pain, form an extensive list, however, the most common complaints include:

  • Change in temperament
  • Reluctance to be saddled, have the girth tightened or mounted
  • Rearing or bucking
  • Refusing to jump or
  • Change in the horse’s normal gait or lameness

    • DIAGNOSIS

    Before any treatment is prescribed, it is essential to definitively diagnose impinging DSPs as the primary cause of pain or discomfort because, as mentioned previously, this may be an incidental X-ray finding in some horses.

    Typically diagnosis begins with an accurate and thorough history from the owner, along with observation of the horse at rest and at various gaits. If the aforementioned conformational traits are identified, this may be a useful first clue.

    Palpation of the thoracolumbar spine may be resented, particularly in specific regions, and commonly the surrounding muscles may demonstrate spasms and pain.

    Suspicion of impinging DSPs may be confirmed using X-rays and ultrasonographic assessment of the bony and soft tissue structures. The degree of impingement present may range from subtle bony remodeling to cystic bone changes in bone to advanced bony fusion. The severity may be evaluated and graded based on appearance on X-rays.

    Occasionally scintigraphy (bone scan), which uses radioactive isotopes to localise areas of increased inflammation, may be useful to confirm the diagnosis. Local anaesthetic solution can be infused into the region of interest to eliminate pain and further confirm the diagnosis. Given the large size of the horse, MRI or CT scans of the thoracolumbar spine cannot yet be performed, however given rapid advances in modern technology, these modalities may become available in the not too distant future.

    It is important to note that many horses presenting with back pain also have concurrent lameness issues. Therefore, individuals with skills limited solely to the back, are unlikely to be addressing these issues.

    TREATMENT

    Broadly speaking, two forms of treatment are available, namely conservative/medical treatment or surgical treatment.

    Many horses, particularly older horses with quiescent impinging DSPs, may experience an occasional acute painful flare-up. Generally these cases can be managed successfully with short periods of rest and non-steroidal anti-inflammatory drugs (e.g. Phenylbutazone).

    Conversely, the majority of chronic, long-standing cases do not respond to such basic therapy. Unlike most other musculoskeletal injuries, it has been my experience that rest does not improve the pain and discomfort resulting from impinging DSPs and can in fact exacerbate the condition. This is because rest results in further wastage of the core muscles supporting the bony spinal column, permitting progressive impingement.

    Therefore, alteration of the training regime, combined with physiotherapy and exercises that strengthen these core muscles, is essential. Additionally, the long-term use of non-steroidal anti-inflammatory drugs is not recommended due to their deleterious effects on the gastrointestinal tract.

    Potent corticosteroid injections administered directly into the affected interspinous spaces using X-ray or ultrasound guidance are frequently useful. If concurrent painful muscle spasms are also present, I frequently employ mesotherapy to break the pain cycle. This involves performing multiple shallow injections into the deeper layers of the skin with very fine needles.

    This therapy is thought to block pain pathways in the spinal cord and brain. Other alternative conservative treatments commonly used include acupuncture and chiropractic techniques, however there is a paucity of evidence regarding their efficacy in the veterinary literature.

    Surgical intervention is generally reserved for horses that have either become refractory or are non-responsive to medical treatment. Various procedures have evolved over the years; however, they all report a success rate in the region of 80%.

    Older techniques involved surgical removal of either a portion of or the entire offending DSP. These techniques are reasonably invasive and require up to 12 weeks rehabilitation.

    More recently a minimally-invasive procedure where the ligament connecting adjacent DSPs is transected through a small 1cm skin incision has been developed (Figure 2). This procedure can be performed at multiple adjacent sites in the same episode.

    At this stage, this technique appears to be equally effective and when combined with a shorter rehabilitation of just six weeks and fewer reported complications, it may be a more attractive option.

    Turlough McNally MVB Dip ACVS Dip ECVS MRCVS is a member of the Veterinary Ireland Equine Group and is a specialist in equine surgery at Anglesey Lodge Equine Hospital, The Curragh, Co Kildare.

    Email: hq@vetireland.ie

    Telephone: 01-4577976