eNAD and EDM are clinically indistinguishable. | Shelley PaulsonVitamin Eresponsive neurologic conditions such as equine neuroaxonal dystrophy (eNAD) and equine degenerative myeloencephalopathy (EDM) can affect horses long before obvious clinical signs appear. Both disorders can be linked to inadequate vitamin E intake during critical periods and can compromise balance, coordination, and performance.During a table topic session at the 2025 American Association of Equine Practitioners Convention, held Dec. 610, in Denver, Colorado, two veterinary equine internal medicine specialists discussed eNAD and EDM and how vitamin E plays a role in these diseases. Carrie Finno, DVM, PhD, Dipl. ACVIM, professor of veterinary genetics and the Gregory L. Ferraro endowed director at the University of California, Davis, School of Veterinary Medicine, and Amy Johnson, DVM, Dipl. ACVIM, professor of large animal internal medicine and neurology and section chief of internal medicine and ophthalmology at the University of Pennsylvania School of Veterinary Medicines New Bolton Center, in Kennett Square led the conversation.Johnson started out by reviewing some basic facts regarding eNAD/EDM, noting the two conditions are clinically indistinguishable, but affect different parts of the central nervous system. Affected horses suffer loss of neuromuscular function due to premature death of neurons, similar to Alzheimers and Parkinsons disease, she said. Diagnosing eNAD/EDM in HorsesJohnson presented the case of Sheldon, a 10-year-old successful hunter that had been purchased at 4 years old. Over the years his owners noticed a slow, progressive decline in performance, primarily presenting as bad behavior. He began stopping at fences, but this progressed to refusals that became more explosive with bucking, spinning, and spooking.His veterinarian noticed slowly progressive ataxia (incoordination) and altered demeanor.Johnson described this as a typical presentation for eNAD/EDM, yet achieving a diagnosis in a live horse like this is difficult. If it was simple, we would run vitamin E levels and supplement horses with low levels, said Johnson. But 80% of horses with clinical signs of eNAD/EDM have adequate vitamin E levels at the time of presentation, meaning that a normal vitamin E level doesnt exclude the possibility of disease.Bedford et al. (2013) previously found abnormal type I muscle fibers on tail head muscle (sacrocaudalis dorsalis medialis) biopsy, which prompted Johnson to explore the tail head as a proxy for an eNAD/EDM brain biopsy.Tail head muscle biopsies are more effective than just measuring blood vitamin E, but we still only see 44% of horses with eNAD/EDM and tail head muscle changes, which is nowhere close to 100%, said Johnson. The retina is another place veterinarians can look to make a diagnosis of eNAD/EDM. Accumulation of pigment at the tapetal/nontapetal junction (in the back of the eye) indicates a horse is vitamin Edeficient, said Finno. This may not work as well for younger horses as they dont have time to accumulate pigment.New Information on Testing for Vitamin E in HorsesEven though vitamin E levels can be normal in horses with eNAD/EDM, both Johnson and Finno recommended testing suspect cases.They agreed that either serum or plasma vitamin E testing is appropriate but encouraged veterinarians to pick one type of sample and stick to it. Most importantly, veterinarians must discard hemolyzed samples (where the red blood cells have broken down) to avoid inaccurate results.Their other tips included:Storing serum upright without letting the sample touch the red stopperKeeping the sample refrigerated or frozen (do not repeatedly freeze/thaw) and protected from lightConsistently using the same laboratory, especially for follow-up evaluationsNot taking a few representative samples from a larger herdEven if a horse has an adequate blood level of vitamin E, if they have clinical signs of neuromuscular disease, those levels are obviously not adequate and supplemental vitamin E is needed, said Johnson.Tips for Supplementing Horses Using Vitamin EVeterinarians consider natural alpha-tocopherol the clinically relevant form of vitamin E. Synthetic forms, advertised as all-rac or dl-alpha-tocopherol, show low bioavailability in horses. Finno and Johnson recommended starting horses with neuromuscular disease on liquid alpha-tocopherol because it is twice as bioavailable as pellets or powders.Finno recommended supplementing with 10 international units per kilogram (IU/kg), the equivalent of 5,000 IU for an average 500 kg horse. With the liquid formulation at this level, vitamin E levels will increase within one week, said Finno. With the powder/pelleted natural form of vitamin E, it will take approximately eight weeks to increase vitamin E blood levels. In some cases with severe deficiencies and clinical signs, Johnson recommended starting with 10,000 IU alpha-tocopherol per day and then decreasing the dose when blood levels return to the target range. However, both Johnson and Finno encourage veterinarians to recheck vitamin E levels within a few weeks.Some horses do not respond appropriately (clinical improvement) to vitamin E supplementation. Finno said these horses could have a low-grade inflammatory bowel disease that decreases absorption of vitamin E. Owners should work closely with their veterinarians to investigate causes of malabsorption.Take-Home MessageEquine neuroaxonal dystrophy and equine degenerative myeloencephalopathy can quietly undermine a horses performance long before obvious neurologic deficits emerge, and normal blood vitamin E levels do not rule them out in a horse. Early recognition of clinical signs, careful handling and interpretation of vitamin E test results, and timely supplementation with bioavailable alpha-tocopherol remain key tools for managing suspect cases and supporting equine neurologic health.