Up to now there is no way to stop this process. The disease is not mortal. The disease has a simple recessive heredity. This is very treacherous disease. That can happen if both parent dogs are carriers of PRA genes.
Some veterinary surgeons have special training to diagnose eye diseases.
Though they cannot determine if your dog is a carrier. Your regular veterinary surgeon can recommend you an eye specialist. For ordinary eye-test the dog should be at least 18 month. For ERG test not less than 2 years old. The test will last for the whole lifetime of the dog.