Congenital tremors are due to defects of the protein myelin which covers parts of nerve cells (axons). These parts are responsible for transmitting nerve signals throughout the body. As a result of the defects, axons are either naked or surrounded by a disproportionately thin layer of myelin. The defect is confined to the central nervous system, while peripheral nerves are not affected. Congenital problems associated with tremoring include hypomyelination, dysmyelination, and lysosomal storage disease.
Congenital tremors have been reported in many breeds of dogs, including the Chow Chow, Weimaraner, Bernese Mountain Dog, Samoyed, Springer Spaniel, and Dalmatian. Males inherit an X chromosome from their mother and a Y chromosome from their father, while females inherit an X chromosome from each parent. In X-linked inheritance, the mother carries the defective gene on one of her X chromosomes and passes the disorder along to her sons.
Clinical signs appear within the first several weeks of age; however, cases of delayed onset also have been reported. In the recessive hypomyelination disorder of Springer Spaniel, a severe generalized tremor ("shaking pup syndrome") is first seen in male puppies in the second week of life. The puppies are much reduced in weight and size, unable to stand or walk, and do not improve over time. Female carriers of the defective gene may have a milder form of the disease, which resolves within 4-6 weeks of life. As they mature, dogs with this disorder may develop seizures and will also have resting tremor, which worsens with exercise and excitement. In male Springer spaniels, there is no improvement over time. Chow Chows and Weimaraners have an unpredicatble prognosis and some may recover completely. There is no treatment for this disease.
Tremors could also be a sign of hypoglycemia seen in glycogen storage disease, a group of inherited metabolic diseases, including types Ia and Ib, characterized by poor tolerance to fasting, growth retardation and hepatomegaly.5
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