This is an extraordinary thorough review of B12 deficiency and B12 insufficiency, with an actual description of the difference, including severity of symptoms.
One interesting factoid is quoted here:
"In 1900, Russell noted characteristics of B12 deficiency that are overlooked with surprising frequency today: “some of the most typical cases presented no anemia throughout the course, others only late in the disease, while in other cases anemia was an obtrusive symptom … and preceded the nervous symptoms by many months” [Citation7]."
So this particular piece of information goes back to 1900, but still hasn't filtered down to most clinicians.
This is an insanely thorough review (266 references). Yet, when it comes to treatment, eg, injections every other day until no further improvement, the source cited is NICE.
Also, it appears that the most effective diagnostic tool for clinicians is the one suggested in "The Many Faces of Cobalamin Vitamin B12 Deficiency": if no other reasonable explanation of neurological symptoms is apparent, then make a detailed listing of symptoms, including an estimate of severity, and inject B12 every other day, checking after two or three months for any symptom improvement. Improvement validates diagnosis.
In my view, restoration of loss of balance, loss of sensation, etc should be the treatment goal, and there is no reason to reduce frequency of injections from every other day until those and other functions are restored, because there is no evidence that those functions cannot be restored with adequate treatment, and there is no harm in continuing treatment.