There's an interesting article in this week's New Scientist about something called Functional Disorders - newscientist.com/article/mg... (But it's for subscribers only).
Functional neurological disorders are one of the most common reasons for seeing a neurologist. They normally present as an unusual set of symptoms with no obvious cause. Reading through the list of symptoms that they have encountered in Sheffield Neurology Psychotherapy Service sth.nhs.uk/services/a-z-of-... I was struck at how many of them I'd read on these very pages.
If you have symptoms that just aren't normally associated with a B12 deficiency and, more importantly, aren't responding to B12 treatment then it might be worth asking to see a neurologist to get an opinion on the possibility of it being an FND.
If one looks on the b12 association site one will find a list longer than on the one you directed readers to. Damage to the myelin sheath caused by b12 deficiency causes a vast number of ‘functional disorders ‘ - it is just the case that doctors for the most part do not connect the two which is why so many people report on this site of being taken down long complicated medical routes to find other causes other than b12 deficiency.
From personal experience, I am very cynical about diagnoses of Functional Neurological Disorders. I was stuck with this label for years and I strongly believe it prevented me from getting a diagnosis of B12 deficiency earlier, despite having typical symptoms of B12 deficiency affecting multiple body systems.
I still think there are things humans do not understand about the way the body works and suspect that some conditions that are currently termed "Functional Neurological Disorders" have as yet undiscovered physical causes eg disturbances in brain chemistry.
As I queried the possibility of B12 deficiency with some neurologists I saw, I feel there is no excuse for most of them accepting a prior diagnosis of FND without doing their own research on B12 deficiency. Each could have considered a trial of B12 injections.
I was told to look up the website you mention. If I had accepted this diagnosis I think I would eventually have ended up with dementia and spinal damage.
If any of my remaining symptoms are FND, I suspect the way I was treated by medical profession for many years was the cause.
Another associated term is MUPS, medically unexplained symptoms
I wholeheartedly agree with the problem of misdiagnosis (as you suggest, of FND, or mental disorders, or whatever) can and does result in the accumulation of permanent damage. Whether that is to the nervous system or elsewhere.
In the context of my home forum, Thyroid UK, I am utterly convinced that the combination of failure to look, examine, test and diagnose at sensible levels does cause such permanent damage. The more alternatives are put in the way of B12 (or thyroid) diagnosis, the worse the impact both individually and across the population. Go and have some anti-depressants, a course of CBT, whatever else is "fashionable".
In both B12 and thyroid, failure to go beyond the one test (serum B12 or TSH) can easily mean that even serious levels of disease go unidentified.
Wow - thank you for opening my eyes to that: I was thinking that it was good to have someone who/a possible diagnosis which accepted the symptoms as real, just unexplained - but I'd overlooked that it can be another excuse along the lines of the CFS/ME diagnosis to stop looking for the reason and therefore not try to find a cure when the problem just needs finding.
Thank you for your additional insight - like you, I believe there are answers to problems, even if they haven't been found yet.
It is plain that some people may have missed part of my post.
"If you have symptoms that just aren't normally associated with a B12 deficiency and, more importantly, aren't responding to B12 treatment then it might be worth asking to see a neurologist to get an opinion on the possibility of it being an FND."
I hear you. I have symptoms of B12 and for sure a B12D. Injections are helping some, but the remaining ones could indicate irriversable nerve damage. Or be related to the >2 cm gallstone that was recently found (no clues about that yet). Or an FND. Thanks for posting, fbirder...!
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Functional vitB12 deficiency - anyone else?
Functional B12 Deficiency?
Could this be a thyroid or vitamin deficiency?
Functional B12 Deficiency 
