I presume that you have googled and read about this. I can only find references to this in humans. There is nothing in dvm360.com and there is nothing in any of my (extensive collection of) veterinary textbooks. However, I do not have a veterinary neurology specialty textbook. I did do a google book search in a veterinary neurology text but found nothing. Human references:
http://www.nebraskamed.com/neuro/surgery/vertebral-basilar-insufficiency
http://www.ucdmc.ucdavis.edu/vascular/diseases/vertebrobasilar.html
In humans, it is generally caused by atherosclerosis, but dogs don’t have atherosclerosis – so in canines it would likely be caused by stenosis (narrowing of the artery which can decrease blood supply probably exacerbated by certain things like exercise or posture - in humans they talk about "beauty chair" episodes caused when your head is tilted back into the shampoo basin) or possibly by clots or embolism. That is the purpose of aspirin therapy – to keep the blood thin and prevent clots and help the blood flow through the narrowed part of the artery. What she is experiencing is called a “drop attack.” I never like to use human references because they often don’t extrapolate to canines, especially in this instance but I can’t find canine references.
It would be good if neurologist could differentiate between stenosis and lesion or both. If there is truly a lesion, what is the prognosis? Is this progressive?
I don't understand his reference to "blood loss" - I presume he means "loss of blood flow" rather than loss of blood as in free flowing blood escaping the artery. Aspirin therapy would make a bleeding disorder worse but would help restricted blood flow because it is a blood thinner.
I did read a good bit about peripheral vascular disease in canines but this didn’t fit. Fibrocartilaginous embolism (FCE) is well described in vet texts; this is similar but is an ischemic accident caused by embolism of disk material rather than by a clot or narrowed artery. Similar presentation, painless, no definitive treatment.
I could find a lot on spinal cord stroke but that is different.
http://www.medlink.com/medlinkcontent.asp
"Transient spinal ischemia. Transient ischemia of the cervical cord may cause "drop attacks." The clinical picture in the lumbar region ("spinal cord claudication") may be of weakness of the lower extremities precipitated by effort and relieved by rest. The pedal pulses should remain palpable. Episodes are typically painless."
Discussion of spinal cord infarcts
http://cal.vet.upenn.edu/projects/saortho/chapter_64/64mast.htm
http://emedicine.medscape.com/article/1164217-overview
I would ask the neurologist for reference material that you can read. If you do aspirin therapy, be really careful to give it with a meal and to watch for black, tarry stools that could be a sign of gastric ulceration.
Pat
I edited a bit this morning to clarify. I wrote this at midnight after reading for a couple of hours. This apparently isn't very common or I would have found more references - or perhaps it isn't diagnosed much because most dog owners wouldn't do an MRI for such transient episodes. Are you going to talk to the neurologist again? I really hope he can give you some reference material to read.