When there are no obvious external lesions that could explain a horse's lameness, flexion tests and local anesthesia (or nerve blocks) are two of the main tools that veterinarians use to determine the source of pain.
However, researchers are learning that nerve blocks may not always provide the most accurate information.
In a flexion test, a veterinarian flexes an individual joint and then observes the horse's movement as it trots forward. If the lameness is worse following flexion, then the specific joint (or soft tissue region) flexed is highly suspect.
The next step is to use peri-neural (around local nerves) or intra-articular anesthesia starting from the ground up. If the horse is no longer lame when trotted, that's further confirmation that the suspected region has been desensitized by the nerve or joint block — and that area is the problem area.
Veterinarians have previously thought that anesthesia of the stifle (knee) joint specifically identified stifle joint pain, such as that which accompanies arthritis.
But that result wasn't true in a recent clinical case. WCVM equine surgeon Dr. David Wilson and former surgical resident Dr. Stacy Anderson examined a horse in which the pain was originating from the hock. Despite this, the horse's pain was resolved by intra-articular anesthesia (blocking) of the animal's stifle joint.
"That was the first time that happened to me," says Wilson, whose veterinary career spans more than three decades. "If [we] block the stifle without blocking the hock joints off [first], we would have assumed the problem was in the stifle."
In the summer of 2015, Wilson and Dr. James Carmalt, an equine surgeon and researcher at the WCVM, conducted a study using six research horses to determine what proportion of horses lose sensation to their distal limbs following injection of local anesthetic into their stifle joints. Their project was the first time that researchers had ever evaluated a switch in the conventional order for intra-articular anesthesia — blocking the stifle joint first before the distal limb.
If a larger proportion of horses lose sensation in the distal limb when the stifle joint is anesthetized, then it could mean that veterinarians performing intra-articular anesthesia during lameness exams should work "from the bottom up" to ensure that they make an accurate diagnosis.
Wilson says this desensitization of the horse's lower leg could be a result of close contact between the stifle joint capsule and the peroneal and tibial nerves passing by on their way to supply the lower limb. When a veterinarian administers local anesthetic into the joint, it may diffuse — like a drop of cream in coffee — out of the joint capsule to reach the nerves a short distance away.
The local anesthetic could also distend the joint to the point that it comes into contact with the adjacent nerves. In either case, the anesthetic can interact with the peroneal and tibial nerves and block nerve conduction and sensation to the lower limb.
Wilson says the situation is similar to a dentist using local anesthetic to "freeze" a person's mouth. The drug can spread from the area of administration and inadvertently block multiple nerves at once.
"They put the local [anesthetic] in, and they are pretty accurate about where they put it … but by the time the procedure is over, your tongue is anesthetized and your lower lip is hanging," Wilson says.
Once the WCVM surgeons have the opportunity to analyze their findings, their results may help to establish standard guidelines that veterinarians can follow when they're performing intra-articular anesthesia nerve blocks in equine hind limbs.
Ultimately, their efforts may help to prevent misdiagnosing hind-limb lameness issues in horses.
Kirsten Henderson of Lloydminster, Alta., is a second-year veterinary student who was part of the WCVM's Undergraduate Summer Research and Leadership program in 2015. Kirsten's story is part of a series of stories written by WCVM summer research students.