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Horses with lameness issues often adapt their movement to ease discomfort, which can produce compensatory movement asymmetries. This means a primary lameness in a forelimb can cause asymmetries in the hindlimb and vice versa.
Sleip now flags straight-line analysis results where a suspected compensatory pattern has been detected. We asked Sleip’s equine biomechanics expert, Filipe Braganca, to explain what this means.
What is a compensatory pattern?
When a horse experiences pain in a limb, its primary method of pain relief is to reduce weight-bearing on that limb. This strategy creates a telltale sign when horses are trotted on a straight line:
- Forelimb lameness: A head nod occurs due to an asymmetry in the head's vertical movement as the horse compensates for the sore forelimb.
- Hindlimb lameness: The horse exhibits an asymmetry in pelvic movement as it tries to alleviate pain in the hindlimb.
However, diagnosing lameness is not as straightforward as that. In many cases, the horse may further adapt its movement to manage the pain caused by the primary lameness. This creates an additional layer of compensation.
How can we identify compensatory asymmetries to ensure that we are treating the root cause?
The compensatory asymmetries follow certain patterns as the horse tries to redistribute its weight and lessen the load on the painful limb. A key diagnostic tool for veterinarians in recognising these patterns and identifying primary lameness is the "law of sides" principle.
Can you explain the law of sides?
This is a well-researched principle. Simply put, when a horse experiences lameness or pain in one of its forelimbs, it tends to get a compensatory asymmetry from the diagonal hindlimb. In cases of hindlimb lameness, it tends to display a compensatory asymmetry from the ipsilateral (same-sided) forelimb.
Pelvic asymmetry in forelimb lameness
When a horse experiences lameness or pain in one of its forelimbs, it tends to compensate by changing the movement of the diagonal hindlimb. This often results in a push-off asymmetry from the diagonal hindlimb (C) and an impact asymmetry on the same-sided hindlimb (C). As a result, the horse's pelvis moves unevenly.
Head nod in hindlimb lameness
When a horse experiences hindlimb lameness, the resulting head nod may be misinterpreted as lameness in the forelimb on the same side (C). This head nod occurs as the horse tries to alleviate weight from the painful hindlimb by using its head as a counterweight. In instances of primary hindlimb lameness, the compensatory pattern observed in the head movement can be pretty pronounced. Clinicians should be mindful of working up the wrong limb.
Unusual patterns
Although the law of sides explains most of the compensatory patterns studied, there are still unusual patterns that are not fully understood. One of these patterns is the “head push-off pattern”. In this case, horses show a marked push-off asymmetry of the head without an impact asymmetry and with no further asymmetries on the pelvis. This unusual pattern might be associated with pain but it is still unknown if this represents a weight-bearing lameness, where horses try to unload the affected limb. Further assessment of the horse with a thorough clinical exam and assessment of motion under different conditions, such as longeing, often provides further insight into the origin of this asymmetry.
What are telltale signs on a straight-line trot-up that indicate that there may be a compensatory asymmetry at play?
If you notice multiple asymmetries across various body parts, such as an asymmetry in the head combined with one in the pelvis, or vice versa, it generally calls for extra caution in clinical interpretation. An asymmetry should always be interpreted in a broader clinical context and preferably against a baseline measurement of the horse's natural movement pattern at sound. It’s important to remember that asymmetry is not synonymous with weight-bearing or pain-induced lameness.
How common are these compensatory asymmetries?
They are fairly common. In a recent study (Reed et al., 2020), of 1,224 horses presented for lameness evaluation, 56.6% of horses showed combined head and pelvis asymmetries, which were attributed in the majority of cases to compensatory asymmetries.
How can Sleip flag compensatory patterns?
The new feature assesses the relationship between measurements of impact and push-off asymmetries in both the head and pelvis. Following pre-defined rules based on the principles of the law of sides, Sleip categorises a recording as exhibiting asymmetry patterns with potential compensatory components.
Initially, the app flags the three most common known compensatory patterns. Going forward, Sleip has the potential to recognise additional patterns based on aggregate data to enhance the collective understanding of compensatory mechanisms further.
Clinical case examples
Primary front limb lameness with a compensatory hind asymmetry
Guus, a young Haflinger gelding used for recreational riding, presented with a gradual onset of low-grade lameness. Despite a period of rest, the lameness persisted without improvement.
Guus’s analysis results indicate a primary forelimb lameness accompanied by a typical compensatory diagonal push-off hindlimb asymmetry.
A diagnostic nerve block, an infiltration of the insertion of the suspensory ligament
of the right front was performed. Following the block, both the primary lameness in the right forelimb and the compensatory asymmetry in the left hindlimb showed significant improvement. This confirmed that the left hindlimb asymmetry was indeed a compensatory response. A lesion of the suspensory ligament was confirmed by ultrasound.
In Guus's case, the diagonal hindlimb asymmetry could have been mistaken for a primary hindlimb issue if not carefully evaluated in conjunction with the forelimb lameness.
Primary right hind lameness with compensatory head asymmetry
Hoho is a 12-year-old Dutch Warmblood owned by a riding school. He presented with a progressively worsening lameness on the right hind limb (RH). Notably, this lameness exhibited increasing asymmetry over time.
Hoho demonstrates a typical pattern of primary hindlimb lameness with a compensatory asymmetry in the ipsilateral forelimb.
This case illustrates that compensatory asymmetries, observed here in Hoho's right forelimb, can sometimes be as pronounced—or even more pronounced—than the primary lameness in the hindlimb. This can easily result in incorrect diagnostic procedures, such as blocking the wrong limb if the clinician interprets the head nod as a primary forelimb lameness rather than a secondary compensatory response.
Key takeaways
When evaluating lameness, especially in cases involving subtle or low-grade presentations, it's crucial to consider the possibility of compensatory asymmetries. These can confound the origin of the primary issue, leading to potential misdiagnosis.
