Analgesic and anti-hyperalgesic effects of epidural morphine in an equine LPS-induced acute synovitis model.

The research article explores the pain-relieving and hyperalgesia-inhibiting effects of using epidural morphine on horses suffering from acute synovitis, which is an inflammation of a joint caused by an attack on the synovial membrane.

Research Context

• The study is significant because, despite the wide usage of epidural morphine in veterinary medicine, there has been no prior exploration on its anti-inflammatory and anti-hyperalgesic effects on horses suffering from joint disease, specifically acute synovitis.

Research Methodology and Evaluation

• A cross-over study was conducted where synovitis was induced in horses in the talocrural joint, a joint in the hind limb, using intra-articular lipopolysaccharide (LPS).
• The effects of epidural morphine were evaluated by looking at a range of variables spanning from physiological, kinematic, to behavioural.
• Among other parameters, the study evaluated ranges of motion (ROM) of the metatarsophalangeal and talocrural joints, clinical lameness scores (degree of difficulty the horse faces while moving), mechanical nociceptive thresholds (MNTs, pain thresholds), and synovial fluid inflammatory markers.

Findings and Conclusions

• The injection of LPS was found to cause a transient synovitis, mathematically translating into clinical lameness, decreased ranges of motion in the talocrural and metatarsophalangeal joints, less limb loading when at rest, and increased composite pain scores.
• Administration of epidural morphine led to a significant improvement in clinical lameness, an increased range of motion, and better loading on the inflamed limb at rest.
• Edurial morphine however did not have effects on synovial fluid inflammatory markers, indicating it did not have peripheral anti-inflammatory effects.
• Morphine was found to prevent a decrease in MNTs and inhibited the development of hyperalgesia near the dorsal aspect of inflamed talocrural joints. Hyperalgesia refers to heightened pain sensitivity.
• Therefore, the study concluded that epidural morphine provides analgesic, pain-blocking, and anti-hyperalgesic, excessive pain preventing, effects in horses with acute synovitis, without showing any peripheral anti-inflammatory effects.