Early diagnosis and treatment will give your horse the best chance of recovery.

By Lauren MacLeod

It’s a scene all too familiar to equestrians: A small crowd has gathered at the rail, all focused on a horse and rider trotting around the arena. Amidst scratching heads, various opinions arise. 

“I think it’s his right hind,” says the rider. 

“Nope, definitely right front,” replies the trainer. 

“I think he’s dragging the left hind toe,” offers another bystander. 

The horse in question certainly does not seem sound; however, the source of the issue remains enigmatic to all observers. The lameness seems inconsistent, and no two steps taken by the horse seem alike. Sometimes there is a toe-drag, sometimes he trips and stumbles, and every now and then it seems like he swings a leg out to the side on a turn.

What is going on?

The detection of subtle lamenesses can prove challenging to even the most experienced horsemen. Matters are further complicated when the gait abnormality is inconsistent or intermittent. In some of these cases, efforts to find the source of pain are fruitless. In fact, some of these “mystery lamenesses” are not true pain-related lamenesses at all; they are caused by neurological dysfunction. Sometimes, differentiating between true musculoskeletal pain and neurological disease can be tricky. A major clue that the neurological system could be involved is a random appearance to the gait abnormality. That is, the abnormal step is different each time rather than a consistently shortened stride, dragged toe, or head nod, which are more characteristic of a pain-related lameness. Other potential indicators of neurological disease are significant loss of muscle, excessive stumbling, a staggering or swaying gait, sweating that only occurs in one area of skin, and apparent weakness of one or more parts of the body. More severe neurological disease can manifest as a dull, depressed mental state, complete paralysis of various muscle groups, an inability to rise, or behavioural changes such as aggression or compulsive behaviour. 

Subtle lameness caused by neurological dysfunction can manifest as a random gait abnormality where the step is different each time. Photo: Shutterstock/Osetrik

Form and Function

To understand how injury or disease of the nervous system can result in the clinical signs observed in affected horses, it is first necessary to have a basic understanding of the anatomy and function of the nervous system in the normal horse. The parts of the nervous system are divided into various categories, which reflect their anatomic location in the body and/or their purpose. The following is a brief overview of these classifications.

1) Central vs. Peripheral Nervous System: The central nervous system (CNS) refers to the brain and spinal cord, whereas the peripheral nervous system (PNS) is an umbrella term for all of the nerves in the body. These nerves transmit information between the CNS and the other body parts such as the skin, muscles, and internal organs.

2) Cranial vs. Spinal Nerves: The PNS is further divided into cranial nerves, which arise from the brain, and spinal nerves, which originate from the spinal cord. The cranial nerves mainly provide innervation to the head, but also extend to the internal organs such as the heart and digestive system. The spinal nerves are responsible for innervation of the rest of the body, including the limbs and trunk.

3) Motor vs. Sensory Nerves: All cranial and spinal nerves of the PNS are categorized as either motor or sensory nerves. Motor nerves relay information from the CNS to the body, and cause contraction of muscles, resulting in movement. Sensory nerves gather information from the body and environment, and provide this information to the CNS. For example, if a horse steps on a stone, sensory nerves relay information about this painful event to the CNS, which promptly stimulates the motor nerves to make the horse move his foot away from the stone.

When presented with a horse with suspected neurological disease, a veterinarian must use his or her knowledge of the anatomy of the nervous system in order to pinpoint the area that has been injured. For instance, if a horse shows signs of neurological deficit in both of his hind limbs, it is much more likely that the horse has a problem in his spinal cord (CNS) than two separate injures in the spinal nerves (PNS) of both hind limbs. Once the anatomical location of the injury has been determined, further testing can be conducted to find out which disease is causing the observed problems.

A multitude of potential hazards to the equine nervous system exist. Some causes of neurological disease include physical trauma, genetic and developmental disorders, toxins, parasites, infectious disease, nutritional imbalances, and tumours. Depending on the case, a veterinarian may recommend medical imaging such as radiography, blood testing for infectious diseases or toxins, or collection of cerebrospinal fluid for analysis. Sometimes, the problem can be diagnosed on a physical examination alone. With an accurate diagnosis, a treatment plan and prognosis can be developed for the horse. 

A Tale of Two Neurological Cases:

The following case studies are meant to help illustrate just a few of the different ways in which illness or injury can affect the equine nervous system.

CASE STUDY #1:

Let’s revisit the horse described in the first paragraph of this article. The horse in question is an eight-year-old Arabian gelding named Beau with a long history of success on the Arabian show circuit. However, in the past couple weeks, he has been intermittently stumbling, dragging his toes, and just generally performing poorly, yet no one at the barn can entirely pinpoint what the issue is. There have been no recent changes in his workload, diet, or turnout situation, and he has always been sound and healthy in the past.

An example of decreased in muscle mass, or atrophy, in the left hindquarter muscles. Photo courtesy of Agwest Veterinary Group Ltd.

To get to the bottom of his mysterious lameness problem, Beau’s owner called out her veterinarian to examine him. A physical examination revealed that Beau seemed in overall good health. He was bright and alert, in an ideal body condition, and his heart, lungs, and gastrointestinal tract were all in good working order. However, the veterinarian noticed a subtle abnormality that provided a clue towards the diagnosis: Beau had noticeable loss of muscle, or atrophy, in his left hindquarter region. 

To assess his gait, the veterinarian put Beau through his paces in a full soundness evaluation. The gelding did have an asymmetric gait in his hind limbs, and closer assessment revealed an intermittent lameness in his left hind. As the observers had previously noted, he would occasionally drag his toe or take a wide step with the left hind. Overall, the leg just seemed weak. However, no pain, heat, or swelling was detectable on examination, and no amount of flexion or manipulation of the limb could elicit a pain response in Beau. Based on these findings, a neurological examination was performed, and showed that Beau had difficulty placing his left hind correctly when turning in a tight circle or backing up. Unfortunately, the cause of the gelding’s gait abnormality was now clear: he had neurological dysfunction resulting in ataxia, or incoordination in his left hind.

WHAT SHOULD BE DONE NEXT? There are various diseases of the CNS or PNS that may cause ataxia in the horse. In Beau’s case, the asymmetrical distribution of the ataxia and muscle atrophy hinted at a likely cause - Equine Protozoal Myeloencephalitis, or EPM, which is a neurological disease caused by a protozoan (single-celled) parasite called Sarcocystis neurona. Another parasite, Neospora hughesi, causes identical clinical signs, but is far less common than S. neurona. The parasite is shed in the feces of the opossum, Didelphis virginiana, which has a widespread geographic range throughout North and Central America. In Canada, its range is restricted to southern Ontario and British Columbia. Horses become infected when they are exposed to the feces of an infected opossum, usually through contaminated feed or water. The protozoan parasite then multiplies within the horse’s body and localizes in the CNS. The spinal cord is the most commonly affected part of the nervous system, but brain lesions also occur. Asymmetrical clinical signs are a classic finding in EPM cases, but in some cases, symmetrical neurological deficits are found.

Beau’s veterinarian had a hunch that he was affected by EPM, and therefore needed samples to send to the laboratory to achieve a diagnosis. Blood and cerebrospinal fluid were collected and sent off for analysis. The results confirmed the veterinarian’s clinical suspicion: Beau had high antibody titres for S. neurona in both his blood and cerebrospinal fluid.

The opossum Didelphis virginiana, found in Canada in southern Ontario and British Columbia, is a solitary and nocturnal marsupial about the size of a domestic cat. It spreads the parasite responsible for EPM in its feces. Photo: Wikipedia/Cody Pope

As you get to the front of her stall, you can’t believe what you see. Fiona looks as if she’s had a stroke! The left side of her face is drooping down like it’s paralyzed. You immediately call the vet. She can come right way, and advises you to keep Fiona standing quietly in the stall until she arrives – don’t let her eat or drink anything, and don’t handle her if she seems unsteady on her feet.

This vertical radiograph through the head shows enlargement of the left stylohyoid bone compared to the right. The paralyzed facial nerve lies close to this reactive area. Photo courtesy of Agwest Veterinary Group Ltd.

Fiona appears coordinated and looks stable on her feet, so you grab the halter and go in to see her. She has always been a bit fussy about the halter going over her poll and ears, and today is no different. Occasionally, you’ve had to bridle her from the right side. You wonder if that behaviour is related to what’s happening now. Once haltered, she doesn’t seem to mind it being on, but she is definitely aware of any attempt to touch her left side and moves away to avoid contact. Her left eyelid is sagging down and half closed; her left nostril is a different shape; and her muzzle looks as if it is pulled towards the opposite side of her face. Her left lip is drooping down and there is saliva drooling out of that side of her mouth. Thankfully, there is no blood anywhere. You can’t even see any swellings or lacerations on her face or neck. How could this have happened?

Looking around the stall, everything looks normal. Most of her hay from supper has been eaten, and a good amount of water is gone from her bucket. There are no signs of struggle or casting in the bedding, but she doesn’t always lay down at night. Thinking back over the last few days, there is really nothing that stands out in your mind. You had the show last weekend, then she had two days off, and just the lesson yesterday. There was nothing out of the ordinary except that she was a bit cranky for small circles both directions, and her turn-backs were a challenge again.

Endoscopy of the guttural pouch shows enlargement of the stylohyoid bone. Photo courtesy of Agwest Veterinary Group Ltd.

When your vet arrives, you get the five-star award for all this information! There are a few more things to do on the exam, but you have built a good history that will help with the diagnosis. Temperature, pulse, respiration, and gut sounds are all normal. The veterinarian does a basic neurological exam on Fiona and finds that her vision appears normal; foot and leg placement is 100 percent normal as she stands and walks forward and backward; she can feel light touch over the affected areas of her face but is unable to move the muscles or skin. A diagnosis of facial nerve paralysis is correctly made, but there are two additional procedures that will possibly identify the exact cause. 

After Fiona is admitted back at the hospital, radiographs of the head and endoscopy of the guttural pouches confirm the diagnosis of Temporohyoid Osteoarthropathy or THO. 

A horse receiving acupuncture for right-sided facial nerve paralysis. Photo courtesy of Agwest Veterinary Group Ltd.

HOW DID THIS HAPPEN TO FIONA? The hyoid apparatus is a connection of small, thin bones that suspend the larynx and the tongue inside the skull. It connects to the skull at the temporohyoid joint deep on the bottom aspect of the skull. Chewing, swallowing, vocalization, and head posture all affect the TH joint and, like any joint in the body, this one can become affected with inflammation and get arthritis. The final thin bone in this structure reacts to arthritis by actually increasing size along its length and at the joint itself. Unfortunately, the location of this change is within the guttural pouch where the facial nerve travels. The nerve paralysis develops secondary to the THO. While the final symptoms may occur suddenly, the arthritis and changes take months or years to develop. Looking back at Fiona’s history, are there any telltale signs of head or neck pain that could have been investigated earlier?

TREATING THO: Fortunately, surgical correction of THO provides very good outcomes. A specific hyoid bone is removed from the apparatus and relieves the tension and irritation of the joint in the skull. In time, this will hopefully reduce and prevent further degeneration of the TH joint and reduce the secondary effect of facial nerve paralysis. Additional supportive treatment including anti-inflammatory medications, antibiotics for guttural pouch infection, acupuncture for nerve and muscle stimulation, and laser therapy are all valuable in helping with a full recovery. 

Conclusion

The two cases presented above demonstrate just how varied the clinical signs of equine neurological dysfunction can be. From behavioural changes to gait abnormalities to muscle paralysis, each sign can be attributed to loss of normal function in one or more anatomical locations in the nervous system. While knowledge of equine neurological disease is not nearly as commonplace within the horse community as that of musculoskeletal problems, these disorders can be just as devastating to a horse's performance and quality of life. 

Your veterinarian is a critical player in the prevention and treatment of neurological dysfunction in your horse. He or she will help you determine which vaccinations are necessary for your horse to prevent viral causes of neurological dysfunction, depending on which diseases are prevalent in your area. 

Furthermore, if a horse is showing signs of neurological disease, prompt veterinary assessment and diagnosis is warranted to allow the problem to be treated early in the course of the disease. For many neurological diseases, early treatment leads to better outcomes for the horse. Recovery from neurological disease or injury is slow and often incomplete, but many horses can return to some level of performance with treatment.

More by Dr. Lauren MacLeod

Main photo: Shutterstock/horsemen