Degenerative Myelopathy
A progressive, incurable spinal cord disease in dogs caused by SOD1 gene mutations. It begins with hind limb weakness and progresses to paralysis over 6-36 months, primarily affecting older large-breed dogs.
Degenerative myelopathy (DM) is a progressive neurodegenerative disease of the spinal cord in dogs. It is analogous to amyotrophic lateral sclerosis (ALS) in humans. DM causes progressive demyelination and axonal degeneration in the thoracolumbar spinal cord, resulting in worsening hind limb weakness, incoordination, and eventually paralysis.
Genetic Basis: The SOD1 Mutation
DM is associated with a mutation in the SOD1 (superoxide dismutase 1) gene. The most common variant, SOD1:c.118G>A, follows an autosomal recessive inheritance pattern — dogs must carry two copies of the mutation (homozygous) to be at risk. Not all homozygous dogs develop clinical disease; penetrance is incomplete, and other genetic and environmental modifiers influence whether and when disease manifests.
A second SOD1 mutation (SOD1:c.52A>T) has been identified primarily in Bernese Mountain Dogs and can compound risk.
Breeds at Highest Risk
DM has been documented in over 100 breeds, but prevalence is highest in:
- German Shepherds (the breed in which DM was first characterized)
- Pembroke Welsh Corgis
- Boxers
- Chesapeake Bay Retrievers
- Rhodesian Ridgebacks
- Bernese Mountain Dogs
Clinical Progression
DM typically presents in dogs aged 8 years or older. The progression follows a predictable pattern:
Stage 1 (months 1-6): Hind limb ataxia (wobbling), knuckling of the paws, scuffing of toenails on one or both hind feet. Dogs remain ambulatory.
Stage 2 (months 6-12): Progressive hind limb weakness and loss of proprioception (awareness of limb position). Difficulty rising, crossing legs when standing.
Stage 3 (months 12-24): Loss of voluntary hind limb movement. Paraparesis progresses to paraplegia. Urinary and fecal incontinence may develop.
Stage 4 (months 24-36): Disease ascends to forelimbs. Respiratory muscle involvement can occur in advanced cases.
Diagnosis
There is no definitive antemortem test for DM. Diagnosis is made by exclusion: MRI or CT to rule out intervertebral disc disease, spinal tumors, and other compressive myelopathies; combined with SOD1 genetic testing, signalment, and compatible clinical presentation. Definitive diagnosis requires histopathological examination of spinal cord tissue post-mortem.
Management
No treatment reverses or halts DM. Management focuses on:
- Physical rehabilitation: swimming, assisted walking, range-of-motion exercises to maintain muscle mass and slow functional decline
- Mobility aids: harnesses, slings, wheelchairs to maintain quality of life as hind limb function deteriorates
- Pressure sore prevention: padded bedding, regular repositioning for non-ambulatory dogs
- Quality-of-life assessment: ongoing evaluation of pain, function, and dignity to guide end-of-life decisions
Genetic Testing and Breeding Decisions
DNA panel testing for SOD1 mutations is available through multiple laboratories. Results classify dogs as clear (no copies), carrier (one copy), or at-risk (two copies). Responsible breeding programs use these results to avoid producing at-risk puppies while maintaining genetic diversity within breeds.