Hypomyelination in Dogs

Hypomyelination is a congenital condition caused by insufficient myelin production in the body. A fatty substance that covers the axons (the portions of the nerve cells that transfer impulses to other cells of the body), myelin serves an important function for the nerve cells: as an insulator, protecting the nerve from outside influences, and as an aid for forwarding the process of cellular transmission of nervous system actions. This condition affects the central nervous system (CNS), with related tremors that are most apparent when a dog is active.

CNS hypomyelination has been reported in some breeds more than others, including Welsh springer spaniels, Samoyeds, chow chows, weimaraners, Bernese mountain dogs, and dalmatians. In springer spaniels and Samoyed breeds, male puppies have a higher rate of diagnosis, with symptoms appearing within days of birth; females of these breeds remain largely asymptomatic carriers of the disorder. There are no gender specific differences in other breeds. Golden retrievers (both genders) have a higher rate of peripheral nervous system hypomyelination, with symptoms appearing in puppies younger than eight weeks of age.

Symptoms

Central nervous system:

• Clinical signs appear within days of birth
• Body tremors that worsen with activity and decline during rest
• Symptoms generally improve by one year of age in most breeds, with the exception of springer spaniels and Samoyeds, which are affected for life

Peripheral Nervous System:

• Clinical signs appear at 5–7 weeks of age
• Weakness
• Incoordination of the rear limbs (ataxia)
• Muscle wasting
• Hyporeflexia (below normal or absent reflexes)
• Symptoms do not resolve with age

Causes

• A genetic recessive has been proven for central nervous system disease in springer spaniels
• Cause is unknown for other breeds, but viral or toxic sources are considered, especially since symptoms often resolve
• Peripheral nervous system disease origin is undetermined, but is suspected to be genetically based

Diagnosis

Your veterinarian will perform a thorough physical exam on your pet, taking into account the background history of symptoms and your dog's genetic background. Standard tests include a blood profile, including a chemical blood profile, a complete blood count, and a urinalysis.

Diagnosis is based on the clinical signs that your dog is presenting, but for a conclusive diagnosis, your veterinarian will take a sample/biopsy of the nerve to analyze for sufficient myelin at the axon of the nerve. Your doctor may also choose to perform a brain biopsy. Other techniques include electromyography, which measures the electrical activity and potential of the muscle cells. In this case, the finding is usually normal to mild spontaneous activity. Motor nerve conduction velocity can be used to assess the ability of the motor and sensory nerves to conduct electricity. With hypomyelination, there is usually slowed conduction or only a small amount of potential.