Equine degenerative myeloencephalopathy (EDM) is an idiopathic, diffuse, degenerative disease which primarily causes damage to the horse’s spinal cord. EDM was identified as the second most common cause (24% of cases) of spinal cord damage in horses at Cornell University in 1978 and ranked second in the causes of spinal ataxia at the University of Montreal from 1985 to 1988. EDM is considered to be an advanced form of neuraxonal dystrophy (NAD). EDM appears to have a genetic basis and frequently affects Lusitano, Appaloosa, Standardbred, Arabian and Paso Fino horse breeds.

Research has shown that horses can develop EDM and equine motor neuron disease (EMND) concurrently, in young related horses, and in association with an underlying vitamin E deficiency. EDM and cervical vertebral compressive myelopathy (CVCM) or “Wobblers”, both share clinical signs of a general proprioceptive symmetric ataxia, abnormal base-wide stance while at rest, and proprioceptive deficits in all limbs. Horses will usually start to show signs of EDM when they are 6 to 12 months of age.

Horses with mild cases of EDM may present as performance-related problems. At first the condition produces subtle signs, being nothing more than a "clumsy" foal but slowly progress as clinical signs are usually slow and insidious. Ataxia signs will become more apparent and worsen over time. Paralysis and spastic muscular movements will become more evident, until late stages where the horse is unable to get up from laying down without assistance. The only way to get a definite diagnosis that a horse has EDM is by conducting postportem examination shortly after death.