Not Degenerative Myelopathy?

My boxer's hips and legs have gotten very thin and he's falling a lot. I assumed degenerative myelopathy (DM), but before he falls he seems light headed or seizing. It's more frequent when he puts his head down low, and even when seated. His heart beats are irregular. He breathes heavily soon after he falls and has been "honking" once in awhile.
Is it possible it's not DM? Could a heart problem cause his back lags too atrophy?

He's not in pain. He does not drag his paws, and back legs are equally weak. His last day is scheduled this Friday, so I want to be sure I'm not making a mistake.

He is 12.5 yrs old, and this has been progressing for a few months.

Thanks!
-Kerrio

What does your vet think? It could be possibility of a cardiac or neurological problem, but those are not good either.
A DM test is only like $40 if you want to rule that out.

My dog has been having issues with his hind legs as well. He is going on 11 years old in January and he is a Lab/Greyhound mix. We are hoping it is not DM and if it has to be something we are hoping its arthritis. (Of course we don't want him to be sick but if it has to be something at least arthritis is treatable.) Please bring your dog to the vet and see what they say. I will be interested in what you find out.