This is what Bateson mentioned in his report about Syringomyelia.
6.17 Strong evidence suggests, however, that certain specific phenotypes have a negative impact on welfare by creating pain or suffering; or by impeding the normal behaviour of the animal. Prominence has been rightly given to syringomyelia in Cavalier King Charles Spaniels. In this case the brain continues to grow after the skull has ossified with the result that the canal between the ventricles of the brain and the spinal cord is occluded. The eventual result is evident pain in the dogs and fitting. However, prior to that the dog may not manifest obvious clinical signs but become quiet and inactive. I was given details of one case in which the dog, termed by its owner as lazy, was given a pain-killer by the veterinary surgeon. The dog perked up after the analgesic was administered and immediately became more playful. The expression of its face was also reported to have changed. Before and after analgesia expressions are shown in the photographs.
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