New Advancements in Diagnosing Equine Neurologic Disease
Fortunately, technology is on the vet’s side. Rapid advancements in imaging options and diagnostic testing have made neurologic horses easier to assess and treat. Earlier this year Krista Estell, DVM, Dipl. ACVIM, a clinical assistant professor of equine medicine at the Virginia-Maryland College of Veterinary Medicine’s Marion duPont Scott Equine Medical Center, in Leesburg, Virginia, reviewed how some new technologies are helping veterinarians diagnose and manage equine neurologic disease.
To best understand neurologic issues, one must first understand how the structures should function.
“The spinal cord is a fairly soft cylinder of tissue that connects the brain to the rest of the body,” Estell said. “It carries messages to and from the brain and integrates sensory perception and motor control.”
The cord is cushioned by cerebrospinal fluid (CSF) and threaded through the vertebral column, both of which protect the all-important structure, she said. Nerves that help transmit messages from the brain to other parts of the horse’s body enter and exit the spinal cord between each vertebra.
Veterinarians’ first step when evaluating a horse with potential neurologic issues is a neurologic exam. While it’s not a new development, it remains an important way for veterinarians to gather information and decide next steps.
“Everyone will do it a little differently,” Estell said, but veterinarians should ultimately arrive at a similar conclusion.
She said she starts her exams by evaluating the horse’s stance and mental status (how reactive is he to external stimuli, for example). Next, she palpates and flexes the horse’s neck and back before checking the cranial and spinal nerves and evaluating the horse’s reflexes. Then she watches the horse’s gait and limb movements under normal conditions (i.e., walking or jogging in a straight line) as well as special circumstances (such as up and down a slope or in a tight circle) to determine if motor function and sensory perception are compromised.
Estell said veterinarians use the modified Mayhew scale to grade horses with neurologic issues:
- Grade 0 – The horse appears normal
- Grade 1 – The horse is inconsistently abnormal under special circumstances
- Grade 2 – The horse is consistently abnormal under special circumstances
- Grade 3 – The horse is abnormal all the time
- Grade 4 – The horse is extremely ataxic and may fall
- Grade 5 – The horse is down and unable to rise
A quick note on spinal anatomy: Horses have (from nose to tail) cervical, thoracic, lumbar, sacral, and caudal vertebrae. Estell said C1 through C6 and C6 through T2—essentially from the poll to just before the withers—are most associated with neurologic issues and often cause the most obvious clinical signs.
“Think of it as a river,” she said, “everything downstream of the affected spinal cord segment will be affected.”
Common Issues and Scientific Advancements
Veterinary technology is making it easier for veterinarians to diagnose and evaluate neurologic ailments, and Estell focused on three in particular: traumatic injury, cervical stenotic myelopathy (CSM, aka wobbler syndrome), and equine protozoal myeloencephalitis (EPM).
Traumatic Injury—Most commonly caused by flipping over backward, other types of falls, and collisions, traumatic brain and spinal cord injuries account for 22-24% of all neurologic issues diagnosed in horses, Estell said.
The physical and neurologic exams are important parts of evaluating horses with potential traumatic brain and spinal cord injuries, Estell said, while imaging can reveal exactly what’s causing the issue, such as fractures of the skull and neck vertebrae. However, imaging this region has been historically challenging.
Radiographs of the skull and neck, for instance, can be difficult to capture and read properly, Estell said; the skull is complex and made up of thick bone, and multiple structures within it could obstruct views (such as the teeth and sinuses) while the vertebrae, which are also structurally complex, are surrounded by thick muscle and soft tissue, which sometimes take a high-energy X ray machine to make an accurate diagnosis.
“There’s a lot we’ve been missing,” Estell said.
While traditional computed tomography (CT) machines produced more useful images, the horse must be anesthetized and placed in a recumbent position. And for a horse with a potential neck or skull fracture, one bad movement while recovering from general anesthesia could prove disastrous.
Thanks to technologic advancements, said Estell, newer CT machines can reveal what’s happening in a standing horse’s head and neck with no need for general anesthesia.
“The head and neck fit right into the machine when the horse is sedated,” Estell said. “We can get all the way down to C6 in all horses, even to C7 in some smaller ones.”
CSM—Estell said CSM is essentially a “catch-all” term for spinal cord compression. When diagnosed in young horses, it’s most commonly caused by congenital malformation or malarticulation. If it’s diagnosed in older horses, it’s commonly the result of articular facet joint osteoarthritis (which can cause issues even without associated spinal cord compression), she said.
Radiographs alone don’t give much information about spinal cord compression; they can reveal areas where the spinal cord canal is narrower than usual, but that doesn’t necessarily mean the cord itself is compressed.
The gold standard for diagnosing spinal cord compression has been the contrast myelogram, which involves collecting radiographs under general anesthesia after a contrast agent is injected into the fluid around the spinal cord. Estell said horses are diagnosed with CSM when the contrast column is narrowed more than 50%. This method has a high specificity (ability to correctly identify those without compression) of 89% but a “quite poor” sensitivity (ability to correctly identify those with compression) of 53%, she said.
Veterinarians can now use CT, however, to get a 360-degree view of the spinal cord and the surrounding bony and soft tissue structures, she said. This allows veterinarians to get more information about areas of even subtle compression that might not have been obvious on X ray or a myelogram.
This is important, Estell said, because “you don’t want to miss these cases. These horses aren’t safe to be ridden.”
EPM—Finally, Estell turned her attention to EPM, one of the most common neurologic diseases in horses. Two protozoa (most commonly Sarcocystis neurona and sometimes Neospora hughesi) can cause EPM, which results in asymmetric muscle wasting and varying degrees of ataxia (incoordination), proprioceptive deficits, and weakness, Estell said.
Diagnosing EPM has long challenged veterinarians, she said. In Maryland and Virginia, for instance, 80 to 90% of horses will be exposed to S. neurona at some point in their lives and, thus, could have antibodies against it flowing through their blood. But only a small percentage of exposed horses develop clinical disease, she explained. As such, blood-testing a neurologic horse in an area like Maryland or Virginia for EPM will likely produce a positive result, even if the horse doesn’t have an active EPM infection.
A more accurate way to diagnose EPM is to compare antibody levels in blood and CSF, she said. A ratio greater than 100 suggests the horse is actively producing antibodies against the protozoa. Collecting spinal fluid remains a challenge, she added, but ultrasound guidance and new access points (between C1 and C2) are making it easier and more reliable.
Recently, Estell and colleagues conducted a study in which they found that horses with an active EPM infection have increased concentrations of sCD14 (a proinflammatory marker) in their blood and CSF compared to healthy controls. This, she said, could prove to be a useful supplementary diagnostic marker for EPM.
While neurologic issues are nothing new, veterinarians now have more advanced technologies allowing them to better diagnose and evaluate these problems. From modern imaging to new diagnostic tests, these tools are providing veterinarians with more accurate information with which to plan the next steps for their patients.
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