Propulsive circling, always in the same direction, is usually a sign of a brain tumor. Dogs do not get strokes, the way that people do. I do not know why your vet would suggest this.
The loss of proprioception in the left limbs can be from a brain tumor or from a lesion in the spinal cord in the neck. However, dogs with neck spinal cord problems usually do not show circling.
I am attaching a client information handout that I use in my practice on brain tumors. I hope that you find it useful.
Click Here I hope that this information is of help to you, and I wish you the best of luck with your dog. Please let me know if I can be of further assistance.
Dr. K
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Dr. K