Study of asymmetry: Is it normal, or are we missing signs of lameness in horses?

Is asymmetry just a normal part of being a horse, or are we as owners failing to pick up the smaller signs of lameness?

These intriguing questions are raised by the findings of a just-published study.

Movement imbalances in a straight line trot were found by researchers to be remarkably widespread among 222 warmblood horses considered by their owners to be sound.

The Swedish researchers found that asymmetry was actually more common than symmetry in the horses used in the study, with 72.5 percent of them identified as having asymmetries based on measurements taken by body-mounted technology.

“It is not known to what extent these asymmetries are related to pain or to mechanical abnormalities,” Marie Rhodin and her colleagues reported in the open-access peer-reviewed journal PLOS ONE.

They said that one of the most important questions to be addressed was how objective asymmetry scores can be translated into pain, orthopedic abnormalities, or any type of unsoundness issues.

The research team noted that recent studies evaluating horses in training and considered sound by their owners had identified a large proportion with motion asymmetries.

The authors set out to objectively investigate the presence of motion asymmetries in the 222 riding horses in training by identifying the side and quantifying the degree and type of forelimb and hind limb asymmetries found during straight line trotting and on the lunge.

The study, centered at the Swedish University of Agricultural Sciences, used body-mounted accelerometers on the horses, all of whom were considered by their owners to be free from perceived performance issues and sound.

Previously reported symmetry thresholds were used for the study.

Thresholds for symmetry were exceeded in 161 horses – that’s 72.5 percent of the animals – for at least one variable while trotting in a straight line, the authors reported.

Of the 161 asymmetric horses, 57 were mares, 3 were stallions and 101 were geldings, with an age distribution of 3 to 25 years. The median age was 11 years.

The study team described the percentage of horses with asymmetry issues as remarkably high, given that all owners had judged their animals as free from lameness.

“This raises the question whether these asymmetries are caused by pain and/or pathology or could simply be a biological variation.

“Asymmetries are generally caused by differences in loading and force production between limbs. However, we do not know whether such asymmetries always are related to pain.”

This also meant there was a limited understanding around the extent to which asymmetries were related to an underlying problem, and whether their presence always posed a welfare issue.

“Only full clinical lameness examinations with diagnostic analgesia or analgesic testing by systemically administered analgesic drugs could possibly answer this question.”

They said studies over time on motion asymmetry and incidences of different orthopedic disorders were needed to answer this and other significant questions.

“It may be speculated why owners and riders do not notice these asymmetries or which factors influence the decision of referring their horse to a veterinarian.

“One reason for not noticing that the horse moves asymmetrically could be that the horse is ridden on a soft surface, while asymmetry was measured on a hard surface, where disease processes might get provoked by increased forces, resulting in increased pain.”

The study team described the strong effects of lungeing on the symmetry of the horses in the study. The findings confirmed that circle-dependent effects influenced motion symmetry.

“Since circular movement induces asymmetries, especially in the pelvis, the initial asymmetry observed on the straight may be masked with the asymmetric limb on the outside of the circle.

“Therefore when evaluating horses on the lunge, both an increase or a decrease in symmetry could be a sign of positive diagnostic analgesia or of effective treatment depending on the lame limb being on the inside or the outside of the circle.

“The high correlation between straight line asymmetries and the sum of both lungeing directions indicates that the asymmetries generally are amplified by the circle-dependent asymmetries in one direction and attenuated when changing direction.”

The study team, in conclusion, said it was not known to what extent the identified asymmetries were caused by pain, may be related to poor performance, or were a result of inherent natural unevenness. It was possible that poor or asymmetric training may also play a role in some cases.

“Increased knowledge about the relationship between pain and movement asymmetry is needed to avoid excessive lameness investigations and unnecessary treatment or euthanasia.

“One of the most important questions to be addressed before biomechanical objective asymmetry scores can be translated into pathology, is to which extent pain, dysfunction or orthopedic abnormality and motion asymmetry are synonymous conditions.”

The study team comprised Rhodin , Agneta Egenvall and Pia Haubro Andersen, all from the Department of Clinical Sciences at the Swedish University of Agricultural Sciences in Uppsala; and Thilo Pfau, from the Department of Clinical Science and Services at the Royal Veterinary College, part of the University of London.

Rhodin M, Egenvall A, Haubro Andersen P, Pfau T (2017) Head and pelvic movement asymmetries at trot in riding horses in training and perceived as free from lameness by the owner. PLoS ONE 12(4): e0176253. https://doi.org/10.1371/journal.pone.0176253

The study, published under a Creative Commons License, can be read here

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