3 1/2 is a little young for that diagnosis.
Did he take xrays to rule out a spinal cord issue or hip problems?
How did he test her for DM? There is a blood test but it's inconclusive - the test can tell you if she has the markers for DM but it can't tell you if she's affected by it.
There's a simple test you can do at home - take her hind foot and knuckle it under so the foot is resting on the fur ontop her toes, not on her pads. If she leaves it like that, DM. If she moves it back immediately, not.
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I have duct tape and I am not afraid to use it.