I am just catching up on this thread now. I would guess that you have already talked to the cardiologist, but all the things you are describing (in your original post) are actually really common with advanced level heart disease. there really isn't much you can do about the issues with incontinence–the best approach is to give the diuretics at least an hour before bedtime and then take him out a couple of times before to try and empty out his bladder. Some dogs have a much harder time with this than others, and it doesn't help that by the time many have MVD at symptomatic level, the dogs are elderly, they can often have weak bladders anyway. You can get special incontinent pants for elderly dogs, and those might help keep him more comfortable by keeping his bedding dry and so on.
The hind leg weakness though is very typical of this horrible heart condition. Again, I don't think there's very much you can do.
There is more information on symptoms here:
http://cavalierhealth.org/mitral_valve_disease.htm#Symptoms
A small excerpt:
Cavaliers with murmurs of between Grade 3 and Grade 6 may display episodic weakness of the hindquarters, ataxia, or collapse, which is called presyncope, or combined with loss of consciousness, which is called syncope, due to a sudden decline in blood flow to the brain. See
Syncope for a discussion of this disorder and its causes.
My Lucy was very wobbly on her hind legs in the final months of her MVD. She also had collapsing episodes, which were pretty much always directly related to some moment of sudden excitement or exertion. I was quite careful about not putting her into situations where this would happen in as much as I could.
It's great that you have had a good response to getting that fluid out of Sydney's abdomen and hopefully he will remain far more comfortable and happy now for a good long while.
I hope you are able to manage the side effects that can come from the other drugs. It is very hard sometimes to know which direction to take and sometimes it becomes an issue of managing for as long as you can keep a very much loved companion comfortable, and recognizing the point at which they need their rest. Closely working with your cardiologist will help you to know where you are in this process and what options remain available. As MVD progresses, sadly the options typically begin to run out. We are lucky in so many ways that there are a good range of medications that can keep them comfortable for many months after congestive heart failure sets in –But at the same time these can have problematical side effects too
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Lucy didn't have side effects of this type with her medications, but it totally broke my heart every day to see her heart disease progressing, because in every other way she was very active and healthy and except for the curse of this disease in this breed, she should easily have lived for several more years. Every day I could see her confusion at not being able to do things she expected to be able to do. It would have been easier for me if she had been less robust in every other way because this just underlined the tragedy of this condition. By contrast, I walk an elderly neighbor's very elderly collie who has had a heart murmur for several years now and it doesn't affect his quality of life at all. He is approaching 16 years of age–which is wonderful for him, but sometimes just emphasized to me how relatively young we lose our Cavaliers to this condition.
It is very hard when they get to this quite advanced stage–all I can say is, many of us have been there and know exactly what you are going through and understand how heartbreaking and stressful this is.