Small wounds leading to synovial infections
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Article by Peter Milner
Most experienced trainers will know from bitter experience that a seemingly tiny wound can have a big impact if a horse is unlucky enough to sustain a penetrating injury right over a critical structure like a joint capsule or tendon sheath. Collectively, joints and tendon sheaths are called synovial structures, and synovial infection is a serious, potentially career-ending and sometimes life-threatening problem.
A team of veterinary researchers from Liverpool University Veterinary School, published a study in Equine Veterinary Journal that examined factors influencing outcome and survival. This article was first published in European Trainer (issue 50 - summer 2015) but is being republished due to popular demand.
What is synovial infection?
Infection involving a synovial cavity, such as a joint or tendon sheath, is a common and potentially serious injury for the horse. The most prevalent cause is a wound, although a smaller proportion of cases result following an injection into a joint or tendon sheath, or after elective orthopaedic surgery to the area. Additionally, infection can occur via the bloodstream, particularly in foals that have not received enough colostrum. Left untreated, the horse will remain in pain, and ongoing infection and inflammation can result in permanent damage. This can ultimately result in euthanasia on welfare grounds.
What factors are important for horse survival?
When a synovial infection occurs there is a huge inflammatory response, leading to swelling and pain. The horse usually shows severe lameness but following a good clinical examination, the cause is often quickly identified. Prompt veterinary recognition of involvement of a joint or tendon sheath and aggressive treatment (involving flushing the affected synovial cavity and the correct use of systemic and local antibiotics) will often result in a good outcome for the horse. Flushing removes inflammatory debris including destructive enzymes and free radicals, and it eliminates contaminating bacteria in most cases. This is performed most effectively by arthroscopic guidance (“keyhole” surgery) under general anaesthesia. Using a “scope” to do this is considered superior to flushing through needles because arthroscopy allows the inside of the problem area to be inspected, foreign material (for example, dirt or splinters of wood) to be removed, and any concurrent damage (such as damage to the cartilage or a cut into a tendon) to be evaluated. In addition, targeted high volume lavage is best achieved via arthroscopy.
Survival following arthroscopic treatment of synovial sepsis is good – approximately 80-90% of adult horses undergoing a flush are discharged from hospital. In foals, however, the figure is much lower, at around 55%, and this likely due to complicating factors such as concurrent sepsis involving multiple organs. Our study, recently published in Equine Veterinary Journal, investigated what factors might be involved in determining survival to hospital discharge in 214 horses undergoing arthroscopic treatment for synovial sepsis. We used statistical modelling to evaluate the interactions with different factors at three key time points during the management of the condition at Liverpool Veterinary School, one of the leading UK referral veterinary hospitals. Information collected on admission to the hospital included when the horse was last seen to be normal, the cause of the infection, the degree of lameness present, and the level of white blood cells and protein in synovial fluid collected from the infected joint or tendon sheath. These lab tests are an important method which veterinarians use to determine how severe the infection is. Additional data collected included whether the surgery was performed out-of-normal working hours, if foreign material was present, the amount of inflammation present in the area, and whether any additional cartilage or tendon damage was found at surgery. Post-operative information gathered included what the levels of white blood cells and protein were in the synovial fluid after surgery and whether the horse needed further surgical treatment.
All horses in this study were greater than six months old and the majority had sustained a wound that communicated with a joint or tendon sheath. Eighty-six per cent of the 214 horses admitted to the hospital survived to hospital discharge. Of the 31 horses that did not survive, 27 were euthanised due to persistent infection or lameness.
An angry, protein-soup
A high level of protein in the synovial fluid of the affected joint or tendon sheath on admission and levels that remained high after surgery were strongly associated with a poor outcome and loss of the horse. Protein concentrations are normally fairly low in a normal joint or tendon sheath, but protein leaks into the synovial cavity from surrounding blood vessels when inflamed. Protein is also produced by cells in the synovial cavity when they are activated in response to a severe insult such as infection. Protein clots trap bacteria in the joint, making it harder to remove infection. The protein soup also includes lots of inflammatory mediators such as enzymes and signalling molecules, and these cause further inflammation, tissue damage, and sensitise pain receptors in the synovial cavity magnifying the inflammatory response and increasing the pain experienced by the horse. Unchecked, this angry, inflamed environment can result in cartilage degeneration, bone damage, and adhesion (scar) formation. This fits well with another observation from this study linking the presence of moderate or severe synovial inflammation at surgery as a negative factor for survival.
Small wounds can lead to big trouble
Interestingly, horses presenting with an obvious wound (as opposed to a small penetrating injury or no visible wound) were more likely to survive to hospital discharge. This may be due to the injury being noticed earlier and hence prompting earlier veterinary intervention. Alternatively, open wounds may allow drainage of inflammatory synovial fluid and lessen the detrimental effects of increased pressure within the joint as well as reducing ongoing exposure to inflammatory mediators. This finding highlights the fact that trainers should act promptly when faced with a wound – it is easy to underestimate just how much damage may be going on under the surface.
Horses undergoing surgical treatment of a joint or tendon sheath infection out-of-hours (for example in the middle of the night) were three times less likely to survive to hospital. Often, horses with a synovial infection arrive stressed and painful and not in an ideal state for having an anaesthetic. Early identification of an infection and appropriate management is important but stabilisation of the horse and preparation for surgery appear to outweigh any perceived benefits of undertaking immediate surgery. This is borne out by the finding that time from initial injury to treatment was not associated with outcome and is in agreement with previous findings from other researchers. It is important to reiterate that prompt recognition and treatment of a horse with an infection in a synovial cavity is essential but that surgical management within 12-24 hours of diagnosis, so that the horse is in the best condition for undergoing anaesthesia, does not affect outcome.
Do horses return to work after a synovial infection?
The big question that owners and trainers want to know is whether the horse will regain full function of the joint or tendon sheath after having an infection. Figures for return to function following surgical (arthroscopic) treatment for a synovial infection vary between 54-81%. Various factors appear to relate to outcome but when looking at a predominately thoroughbred racing population, the statistic for return to training appears to be at the higher end of this range. Factors associated with failure to return to athletic performance include the presence of thickened inflammatory tissue (known as pannus) at the time of surgery and that may relate to the development of fibrous adhesions and scar tissue within joint or tendon sheath longer-term. Some structures are particularly likely to compromise future function, and horses with an infection of the navicular bursa in the foot following a nail penetration generally do worse.
Take home message
Horses sustaining an infection to a joint or tendon sheath have a good chance of the infection clearing up and surviving the injury, with the likelihood of racing as high as around 80%. Our key message for trainers from this study is that it is essential that they recognise early when an infection involves one of these structures and have a veterinarian fully evaluate the injury. Aggressive treatment is important and involves flushing the synovial cavity using a “scope” under anaesthesia to remove as much inflammatory and infective debris as possible.
