Etiology

• Equine neuroaxonal dystrophy (eNAD) which causes specific damage to neurons in the brainstem and spinal cord is the underlying basis for EDM.
• EDM and eNAD are not distinguishable clinically but EDM is the more advanced form.
• Many breeds are affected but an increased incidence in closely related horses is highly suggestive of a genetic basis to the disease. An autosomal dominant mode of inheritance with incomplete penetrance has been identified. An actual mutation has not been identified yet but research is ongoing.
• An environmental trigger is required in animals that are genetically predisposed to initiate disease. Research has shown this is dietary vitamin E deficiency. Vitamin E Vitamin E is a biological antioxidant which is essential for normal neurological function by decreasing oxidative stress and free radical damage. The deficiency must occur during specific developmental stages of the animal to cause clinical signs.
• Other risk factors suggested in earlier reports included prior use of insecticides and exposure to wood preservatives both of which contain antioxidants, although this has not been confirmed in later studies.

Predisposing factors

Specific

• Access to dirt lots, use of heated pelleted feed and sunbaked forages with low vitamin E contents.
• Lack of access to fresh green grass.
• Familial tendency.

Pathophysiology

• Progressive diffuse spinal cord disease → tetraparesis and ataxia.
• Diffuse degenerative disease of the caudal brainstem and spinal cord that affects proprioceptive relay nuclei as well as dorsal and ventral spinocerebellar tracts.
• The histologic lesions in eNAD are confined to the cuneate and gracile nuclei of the caudal medulla oblongata, while in EDM, the lesions are more widespread and include demyelination within the ascending tracts of the spinal cord. Immunohistochemical calretinin staining has defined afferent proprioceptive tracts as the primary tracts damaged in eNAD/EDM horses, and isolated soma within the dorsal root ganglia as the source of dystrophic axons.

Timecourse

• Once signs of EDM appear the disease is usually slowly progressive, although some cases may stabilize with maturity.
• No significant improvement/remission is seen with age.

Epidemiology

• The disease may occur in individuals or may affect a group of animals at the same time (usually related).