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Basics ⬇

Outline

BASICS

Overview!!navigator!!

NAD/EDM is a degenerative neurologic disease of young growing equids resulting in severe and progressive ataxia.

Pathophysiology

  • NAD/EDM is associated with temporal deficiency of α-tocopherol in genetically susceptible individuals. Although the pathogenesis of NAD/EDM remains unclear, it has been shown that low serum α-tocopherol concentrations have a causative role in the development of NAD/EDM and that prophylactic administration of α-tocopherol to foals at risk of NAD/EDM results in a decreased incidence of the disease
  • In predisposed animals, inadequate amounts of α-tocopherol result in lipid peroxidation of cellular membranes. Histopathologic lesions of NAD/DEM include oxidative injuries leading to axonal degeneration of the spinal cord and the brainstem nuclei

Genetics

NAD/EDM is an inherited polygenic trait or a dominant disorder with variable expression depending on breeds and families. No specific gene has been clearly involved in the pathogeny of the disease yet.

Geographic Distribution

The disease is reported in North America and Europe.

Signalment!!navigator!!

  • Breed—common in Arabians, Quarter Horse-associated breeds, Thoroughbreds, Standardbreds, and Paso Finos
  • Age—the onset of clinical signs can vary from birth to 12 years of age but very rarely occurs after 2 years

Signs!!navigator!!

  • Clinical signs are a progressive and symmetric ataxia of all 4 limbs
  • The disease progresses from mild proprioceptive deficits to severe general ataxia
  • A history of progressive lameness or traumatic injury is not uncommon
  • Pelvic limbs are usually more severely affected than the thoracic limbs
  • Severe cases can show hyporeflexia over the neck and trunk (local cervical, thoracolaryngeal, and cutaneous trunci responses)
  • There has been no evidence of cranial nerve deficits or muscle atrophy in horses with NAD/EDM

Causes and Risk Factors!!navigator!!

  • The disease is caused by a deficiency in α-tocopherol in genetically susceptible individuals
  • Risk factors include being born from a mare with low serum α-tocopherol, having siblings diagnosed with NAD/EDM, and having limited access to pasture

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Equine cervical stenotic myelopathy—common in growing horses; ruled out with radiographs, myelogram, or advanced imaging
  • Trauma—common in young horses; ruled out with radiographs, cervical ultrasonography, or advanced imaging
  • Cervical vertebral malformation and occipital–atlantoaxial malformation—reported in young Arabians; ruled out with radiographs, myelogram, or advanced imaging
  • Equine protozoal myelopathy—common in horses <4 years of age; ruled out with immunodiagnostic testing of serum and CSF to identify intrathecal antibody production against Sarcocystis neurona or Neospora hughesi
  • Equine herpesvirus myeloencephalopathy—more common in older horses; ruled out with epidemiologic data and virus identification from nasopharyngeal swab and buffy coat samples

Laboratory Tests!!navigator!!

  • There is no antemortem diagnostic test for NAD/EDM. Low serum α-tocopherol values (<2 mg/L) are suggestive of the disease but poorly specific
  • CSF analysis is unremarkable

Imaging!!navigator!!

Imaging techniques are helpful to rule out differential diagnoses.

Other Diagnostic Procedures!!navigator!!

Thorough and repeated neurologic examination.

Pathologic Findings!!navigator!!

Diagnosis is made at necropsy—axonal degeneration and myelin loss of ascending and descending fibers in the thoracic spinal cord, nucleus thoracicus, and lateral cuneate nuclei of the rostral medulla oblongata.

Treatment ⬆ ⬇

TREATMENT

Access to pasture and properly cured hay containing high amounts of α-tocopherol.

Medications ⬆ ⬇

MEDICATIONS

Treatment focuses on prophylaxis in affected farms with limited improvement of individuals already showing clinical signs; however, signs of improvement can be seen within 4 weeks of treatment.

Contraindications/Possible Interactions

Toxic levels of α-tocopherol in horses have not been documented.

Follow-up ⬆ ⬇

FOLLOW-UP

  • Horses on α-tocopherol therapy should be monitored for serum α-tocopherol levels to ensure adequate absorption. If levels do not increase within 30 days, malabsorption of the fat-soluble vitamins should be investigated
  • Repeated neurologic examinations should be performed to assess the horse's evolution

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Abbreviations!!navigator!!

  • CSF = cerebrospinal fluid
  • NAD/EDM = neuroaxonal dystrophy/equine degenerative myeloencephalopathy

Suggested Reading ⬆ ⬇

Dill SG, Correa MT, Erb HN, et al. Factors associated with the development of equine degenerative myeloencephalopathy. Am J Vet Res 1990;51:1300–1305.

Finno CJ, Higgins RJ, Aleman M, et al. Equine degenerative myeloencephalopathy in Lusitano horses. J Vet Intern Med 2011;25:1436–1446.

Author(s) ⬆

Author: François-René Bertin

Consulting Editor: Caroline N. Hahn

Acknowledgment: The author acknowledges the prior contribution of Caroline N. Hahn.