Immune Mediated Polyarthritis (IMP) is a disease that is rarely heard of amongst Cavalier owners as there are many more conditions that are far more prevalent in the breed such as heart problems and SM/CM.
IMP can have a fairly initial devastating impact upon a Cavalier as it is not like normal arthritis in that normal arthritis would tend to come on slowly with age. IMP is an aggressive condition that can completely immobilise overnight.
It should be noted that IMP can present with similar symptoms to SM/CM such as instability on one or more legs, pain upon movement, sensitivity to touching around the neck area. Because of these symptoms, it may not be the first consideration of your vet as they are much more used to seeing other hereditary problems.