The Companion Animal Welfare Council in the UK has produced a comprehensive report on "Welfare Aspects of Modifications, through Selective Breeding or Biotechnological Methods, to the Form, Function, or Behaviour of Companion Animals". What that translates to is a consideration, from an animal welfare perspective, of breeder responsibilities in producing animals for a pet or show market -- eg should clubs do more to protect breeds from known genetic probelms by more strictly controlling access to regsitration. Sweden for example will not give full registration to cavalier puppies whose parents have not been properly tested for cardiac problems, according to the MVD breeding protocol.

Two of the issues considered and used as examples is syringomyelia and MVD in cavaliers. These are considered particularly relevant to ths study because cavaliers are by far the most popular toy breed in Britain and one of the most popular breeds overall.

The report can be downloaded here. It is a fairly large pdf:

Here's the appendix on cavaliers:

Syringomelia and mitral valve disease in Cavalier King Charles Spaniels

There were, we understand, around 700 litters of King Charles Spaniels registered in
2004 in the Breed Supplement for this breed from breeders who were members of the
breed Club and about 1800 litters from breeders who were outwith the breed club
(comprising a total of 10,733 puppies). This breed is the most popular of the toy
breeds (only 3,877 puppies of the next most popular toy breed, the Yorkshire Terrier,
were registered that year). It has been predicted that about 50% of King Charles
Spaniels could have heart murmurs as a result of mitral valve disease by 5 years of
age. It has been suggested to us that it is perhaps now time to ensure that only litters
from dams and sires that are certified to have had no heart murmurs at the time of
mating are registered.

Syringomelia, a painful and progressive neurological condition caused by obstruction
of the flow of cerebrospinal fluid (CSF) has also been found to affect over 50% of
Cavalier King Charles Spaniels. In affected dogs, the CSF flow is compromised by
pressure from the cerebellum at the foramen magnum which is caused by the shape of
the skull and, in particular, of the occipital bone in this (and sometimes other) small
breeds. The symptoms include signs of head and neck pain, which can be very severe,
fore and hind limb weakness and ataxia. There is a method available for screening (by
magnetic resonance imaging) prior to using dogs for breeding (see Rusbridge &
Knowler, 2005).
Info on the Companion Animal Welfare Council, which is chaired by Lord Soulsby of Swaffham Prior:

CAWC was launched on the 26th April 1999 and was set up to conduct independent studies into the welfare, care and treatment of companion animals and their role within society. The Council offers advice to members of the public, any other institution or organisation and the Government by publishing the results of their studies.