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Case Study: Dealing with Diagnostic Blocking Conundrums

By Laurie Tyrrell-Schroeder Laurie Tyrrell-Schroeder, DVM | Updated on | Blocking, L Meehan, LT Schroeder


Case Study: Dealing with Diagnostic Blocking Conundrums

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Case contributed by Lucy Meehan, BVSc, MSc, CertAVP(VDI), DipECVDI, MRCVS of Langford Vets University of Bristol

This case illustrates two conundrums faced when working up a lameness – partial improvements from diagnostic analgesia (and the decision of when to stop) and when the lameness gets worse before it gets better. 

Signalment

5-year-old WB gelding used for eventing. 

History

Recent LF lameness.

Physical Exam and subjective lameness evaluation findings

  • Mild wastage of the left suprascapular muscle. 
  • Upright, boxy front feet (left>right). 
  • 2/10 left forelimb lameness on straight line.  
  • 3/10 LF lameness lunging to the right. 

Baseline Objective Evaluation Measurement 

On the straight line, a mild and slightly variable left forelimb impact lameness was measured (Vector Sum of 13.4 mm) along with a more variable RH impact lameness (fig. 1). On the lunge, there was mild RF lameness on the left rein and a more moderate LF lameness on the right rein (fig. 2). 

 

Following a LF palmar digital nerve block, the left forelimb lameness worsened to an amplitude of 29.6 mm (fig. 3). Subjectively the horse appeared worse as well, increasing to a grade 4/10. This is a fairly common phenomenon when blocking the distal limb of a lameness that originates higher in the limb.  This is thought to occur due to altered proprioception which reduces protective mechanisms in the horse.  Following a LF abaxial sesamoid block, the left forelimb lameness worsened to an amplitude of 41.4 mm (fig. 4).  Note also that the horse now has a RH push off lameness. This could be compensatory for the worsening LF.  An important consideration in these cases is to repeat the trial, sometimes repeat more than once, to be certain that the lameness has stabilized at the higher amplitude, in order to evaluate change more accurately with subsequent blocks.

Following a LF low four-point block, the horse improved to a VS of 22.8 mm. Should this result be compared to the previous block or to baseline?  It is often helpful to compare to both. If compared against the trial following the ASB, the improvement is about 50% (VS 41.4 mm to 22.8 mm). Also note that the RH pushoff lameness has resolved, further support that this development was compensatory for the LF.   If compared back to the initial baseline, however, the LF lameness was still worse than when the exam began (fig. 5). 

When considering partial improvements, the decision of when to stop is not always black and white. In general, achieving at least 50% improvement in the lameness metric is often clinically significant. However, in this case, because the horse got worse before it got better, and following the low four-point the horse was still more lame than at baseline, the decision was made to continue blocking. 

Following a left forelimb lateral palmar block, the left front lameness improved to a VS of 13.8 mm, essentially back to baseline level of lameness.  A repeated trial showed slightly more improvement, to a VS measurement of 9.1 mm (fig. 6).  While the lameness improved back to baseline, or a little bit better, it did not fully resolve.  This is another clinical conundrum that can be difficult to interpret as well as to explain to the client. 

How do you interpret lameness that worsens during the evaluation, and then improves back to the original baseline? 

While there is no explicitly right or wrong approach to this scenario, it is important to keep in mind that improvement following an acute exacerbation of lameness during the blocking process should not be discounted just because the horse does not improve beyond the baseline level of lameness. There are a few possible explanations as to why this might occur. 

If a baseline lameness is not stable, it may be that it simply worsens due to exercise. A marked improvement after a block later in the evaluation would then certainly be indicative of clinical improvement. 

Another possible explanation is the theory that each block makes a horse a little bit lame in the blocked limb. For instance, if a completely normal horse is blocked, you can often measure (but not always see) a mild lameness in that limb. With each successive block performed, particularly with multiple distal perineural blocks, the potential exists for a continued exacerbation of lameness in that leg; therefore, making it less likely to completely abolish the original lameness once it is finally localized. This most likely explains this case, as the horse was confirmed to have a stable lameness by repeating baseline trials and obtaining similar results at the start of the evaluation.  

The lameness was considered localized, partly to the fetlock region and partly to the proximal metacarpal/carpal region and imaging was obtained. 

Recent radiographs of the fetlock were reviewed, and no abnormalities were found. Radiographs of the carpus were performed and revealed no abnormalities.  

Ultrasound of the proximal metacarpal and fetlock region was performed. Mild loss of fiber pattern in the medial suspensory branch and mild enlargement of the proximal suspensory ligament were noted. 

The ultrasound findings were fairly mild and not necessarily reflective of the degree of lameness. Considerations of the blocking pattern were reviewed.  To rule out cross over of the lateral palmar block, the horse was re-blocked with an intercarpal joint block which was negative.  MRI was then performed of the fetlock and proximal metacarpal and carpal region to determine if additional pathology was present.  MRI showed enlargement of the medial suspensory branch, enlargement and fibrosis of the proximal suspensory ligament, and some evidence of adhesion to the lateral splint bone. 

Diagnosis

Mild LF medial suspensory branch desmitis and mild LF proximal suspensory desmitis.

Case Reflection

When imaging findings do not align with clinical presentation (level of lameness, blocking results), re-evaluate the clinical picture. Consider what other structures may have been blocked with this outcome? Will more sensitive imaging yield additional information?

When a lameness gets worse after blocking the distal limb, this is a good indication you are working on the correct limb, but the foci of pain is higher. 

Stabilize the lameness – at the beginning of an evaluation prior to blocks, and sometimes during the evaluation, if you find the lameness is changing. In this case, a stable lameness was re-confirmed by conducting multiple trials after the horse got worse with an abaxial sesamoid block.  This prevented confusion in the interpretation of the blocks performed later in the evaluation.

When interpreting partial improvements, take into consideration the clinical picture. 50% improvement in the lameness metric (the absolute change of Vector Sum, Diff Max pelvis, or Diff Min pelvis) is often indicative of a clinically significant change.  However, the determination of whether this is enough improvement to stop may depend on the baseline amplitude of lameness, stability of the lameness (stride by stride variability and trial to trial variability), the blocking technique used, the structures involved, and the type of pathology present.  In this particular case, while the low four point did improve the lameness by approximately 50%, the decision to proceed with blocking was made because the residual lameness was still considerably worse than the baseline level of lameness measured.



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