Just briefly: People know I don't have the sort of life which leads to watching YouTube videos. This is a long one. I watched it all when I should have been at comments on the blog or answering emails (oops).
Diagnosing and Treating Vitamin B12 Deficiency
I have a (severely coeliac) friend who has recently, and rather belatedly, been diagnosed with catastrophic neurological B12 deficiency. The biggest problem with getting a diagnosis in most cases is folic acid supplementation. Folate eliminates the anaemia associated with the term "pernicious anaemia". Your haematology is normal. Your myelin falls to pieces. Your doctor diagnoses all sorts of things except the one which might avoid neurological melt down.
As a vet I use serum low B12/folate as surrogate markers for GI malabsorption problems and, in these days of crapinabag, we do a lot of testing. In my patients B12 deficiency is very, very common and we probably miss a fair number of "atypical" B12 deficiencies. We treat the ones we see. It's a good idea.
BTW I've never seen a low folate in a cat or dog. They do happen, but B12 is the very common one.
EDIT WTF, Paula had a low folate, normal B12 result from a dog today. Upper GI signs and vomiting. Well, there you go! Bed time now. END EDIT
BTW two, myelin failure as a loss of insulation is self explanatory. But the schwann cells also supply the lactate for normal neurological energy generation. Loss of lactate is a metabolic catastrophe for nerve cells.