Hypo-/dysmyelinogenesis ("shaking pup")

What is ... ?: 

Myelin is a fatty substance that coats nerve cells. It serves as an electrical insulator and speeds the conduction of nerve impulses. The formation of myelin begins mid-way through pregnancy and continues for a short period after birth.

In this disorder, there is a lack of ("hypo"), or abnormal ("dys") myelination, primarily in the spinal cord but also in parts of the brain. Most affected is the general proprioceptive system, which is important for the coordination of body movements and positioning. In Golden retrievers, the hypomyelination is in the peripheral nervous system, leading to weakness, poor balance and muscle atrophy, but without tremors.

The condition is most severe in the springer spaniel. In other breeds, puppies often gradually improve, perhaps because of continued slow myelination of cells after birth.
 

How is ... inherited?: 

The condition is an x-linked trait in the springer spaniel ("shaking pup"). In some other breeds it is believed to be autosomal recessive.

What does ... mean to your dog & you?: 

Because the proprioceptive system is most affected, pups with this condition have problems with balance, coordination, and positioning. They have a wide-based stance, and appear clumsy (but without weakness). Pups may have what is described as a rocking horse gait. There are pronounced tremors of the head and legs, which are stronger when the pup is excited or moves, and subside at rest or during sleep. Affected pups are normally alert and responsive to their surroundings. Because it is the peripheral nerve myelin that is reduced in Golden retrievers, there is weakness, poor balance and muscle atrophy, but without tremors.

Signs are first noticed after birth or when the pups first walks. In breeds other than the springer spaniel, the clinical signs slowly disappear so that dogs are normal by 12-18 months.

Because the trait is sex-linked in the springer spaniel, males are most severely affected. By about 2 weeks of age, affected male pups develop a severe tremor of the body, head and legs that decreases during rest and worsens with excitement. They are unable to stand, walk, or eat and generally do not survive unless intensively hand-reared. The disorder is generally fatal by 3 to 6 months of age. Females will be carriers of the trait and may show mild signs as puppies that disappear by 4 to 6 weeks of age.
 

How is ... diagnosed?: 

There are other conditions, primarily affecting the cerebellum, that cause similar signs in newborn puppies. Your veterinarian will do tests to rule out other possible causes. 

How is ... treated?: 

There is no treatment, but affected puppies (other than springer spaniels) tend to improve over time, often to complete recovery by 12 - 18 months of age.

For the veterinarian: 

Primarily proprioceptive fibers are affected, so clinical signs resemble cerebellar disease. In neonates this condition is easily confused with cerebellar hypoplasia. Diagnosis is based on the clinical signs, gradual improvement (except in the springer spaniel), and lack of significant findings on other diagnostic tests.

Breeding advice: 

The mother of any affected springer spaniel pup is a carrier and should not be used again for breeding. Breeding of his sisters should also be avoided as they have a 1 in 2 chance of being a carrier, and passing this severe disorder to any male offspring.

In other breeds, affected animals (even if recovered to normalcy), their parents (carriers of the trait) and siblings (suspect carriers) should not be bred.
FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
 

Resources: 

Cuddon, P.A., Duncan, I.D. 1992. The canine myelin mutants - clinical, electrophysiological, MRI, and pathological studies.  ACVIM- Proceedings of the 10th Annual Vet. Med. Forum. pp 757-759.

Sargan DR. Deafness. In IDID - Inherited diseases in dogs:web-based information for canine inherited disease genetics. 2002-2011. Mamm Genome. 2004 Jun;15(6):503-6. Search this database for more detailed information on inheritance patterns and mutations in specific breeds.

 


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