Both conditions (this horse was ultimately diagnosed with eNAD) plague the central nervous system. | Courtesy Dr. Carrie Finno

By Sarah F. Colmer, VMD, Dipl. ACVIM, and Amy L. Johnson, DVM, Dipl. ACVIM

Horses can develop ataxia (incoordination) for a variety of reasons, including diseases that involve spinal cord compression (e.g., cervical vertebral stenotic myelopathy, aka wobbler syndrome) and infectious causes (e.g., equine protozoal myeloencephalitis or Lyme neuroborreliosis), among others. Similarly, a plethora of factors can cause behavioral changes in horses, from saddle fit and environmental disturbances to pain and stress. A category of disease often comprising both ataxia and behavioral changes that has recently gained attention is neurodegenerative. Two conditions—equine neuroaxonal dystrophy (eNAD) and equine degenerative myeloencephalopathy (EDM)—both plague the central nervous system but differ primarily in where they affect the brain and spinal cord.

While the exact mechanisms remain unknown, veterinarians suspect low vitamin E levels in the blood are associated with the damage that leads to these consequences, which might occur in the horse’s early life. A genetic predilection might also exist; although veterinarians have diagnosed EDM/eNAD in most breeds, certain lines and individuals have been known to produce multiple affected offspring. However, researchers have not identified any associated genes. Most likely, both a genetic predilection and a period of insufficient vitamin E status contribute to disease development, and other factors might also be involved.

Clinical Signs of EDM/eNAD

Physical manifestations of the disorder can arise at any time, often appearing gradually. The most common signs involve ataxia, which can include stumbling and falling, dragging the feet, and other consequences of a general lack of awareness of the limbs in space. This can affect the forelimbs, hind limbs, or all four. In many cases (though not all), the horse might show signs of brain dysfunction, including behavioral changes. Some horses appear duller or less reactive to their environment than normal, while others might become hyperreactive, extremely spooky, and even aggressive toward their handlers or other animals. These signs can be intermittent and unpredictable in some cases and more consistent in others. For some horses these behavioral signs remain unchanged, while others worsen in severity and frequency. Owners who describe these horses to us often refer to them as having “flipped a switch” or as “a completely different horse than the one I used to know.”

Diagnosing EDM/eNAD

No definitive test for EDM/eNAD exists, aside from finding the microscopic changes signifying degeneration in the brain and spinal cord at necropsy. (A test is available from the University of California, Davis, which, when positive, is supportive of but not definitive for a diagnosis.) For this reason EDM/eNAD is a “diagnosis of exclusion,” meaning vets can only confirm the horse does not have other conditions that can cause similar clinical signs. If your horse is ­neurologically abnormal during an evaluation, your veterinarian might pursue radiographs (X rays) or a myelogram (­radiographs obtained under general anesthesia using a contrast injection around the spinal cord to look for compression) to rule out diseases like wobbler syndrome or a spinal tap to rule out EPM and Lyme. If a horse is negative for these diseases, EDM/eNAD might shoot to the top of the list of possible diagnoses. Some horses might have neurologic signs due to less common conditions, such as cancer or trauma, which diagnostics should also confirm or rule out.

Is EDM/eNAD Treatable?

Perhaps even more frustrating than the lack of diagnostics for EDM is the lack of treatment options. As mentioned, vitamin E might play a role in its development, and supplementation with vitamin E, particularly when the horse exhibits low blood levels, might help stabilize clinical signs. However, most persist, and many worsen over the weeks to months following evaluation. Due to the poor prognosis, lack of treatment options, and likelihood of the horse becoming dangerous to itself and handlers, we often recommend euthanasia for moderately or severely affected animals. Following euthanasia, most of these cases receive a final diagnosis of EDM/eNAD, based on microscopic changes to the brain and spinal cord as detected by a skilled pathologist.

Take-Home Message

Veterinarians and researchers are investigating EDM/eNAD to provide answers and explanations for what has become an increasingly common condition in the equine ­population. If you suspect your horse is experiencing neurologic signs, contact your vet, who will start with a physical exam and neurologic evaluation and proceed based on the findings.