Sometimes some Australian Shepherds will have involuntary muscle tremors in the hindquarters. In a few cases the forequarters will also be involved. This may be a sign of hypomyelinogenesis, a condition in which myelin, the fatty insulation around the nerves, has failed to develop fully. Young puppies may also be unusually clumsy or have head tremors. These more severe signs usually go away, probably as the myelin more fully develops. However some dogs will continue to have tremors throughout life. Causes of the disease may be infection, dietary issues, or heredity. Too little is known about this usually benign condition in dogs to say for sure what causes it, though the fact it is so common in Aussies indicates there may be a hereditary component.