- Neurological symptoms may be the only manifestation of B12 deficiency
- Symptoms can include lower limb sensory symptoms, suggestive of either a spinal cord lesion (subacute combined degeneration or peripheral neuropathy) or visual (optic neuropathy). See below about Nitrous Oxide use
- People with B12 related sensory symptoms may especially have problems with proprioception and vibration loss
- Sensory symptoms are common in the normal population and modestly reduced B12 levels (e.g. 120 to 180pg/mL) are often ‘false positives’ (i.e. there is no connection between the symptoms and result)
- People with mild or intermittent lower limb sensory symptoms, retained ankle reflexes or normal vibration sense at the toes are unlikely to have a B12 deficiency related neurological syndrome. Most such people, especially if symptoms are variable and intermittent, can be managed conservatively
- For people with mild sensory symptoms not responding to IM treatment it is unlikely that the symptoms were due to B12 deficiency. Consider alternative explanations and discontinue alternate day IM treatment after 2 weeks. Maintenance may then be via the oral route unless there is a specific indication to give IM such as pernicious anaemia / malabsorption
- B12 related neurological syndromes may occur with normal haematological indices (thus a normal B12 result may be a ‘false negative’ result) although this is rare
- If there is combined B12 and folate deficiency always start B12 intramuscular injections 24 hours ahead of folate replacement.
- B12 neurological symptoms typically take several months to improve and may not recover completely even with replacement. We suggest waiting at least 3 months for a clinical response
Nitrous Oxide use
People presenting with lower limb sensory symptoms, suggestive of either a spinal cord lesion (subacute combined degeneration or peripheral neuropathy) or visual (optic neuropathy) should be asked about Nitrous Oxide (NO) use. B12 levels will be NORMAL if checked in cases of Nitrous Oxide misuse.
Subacute degeneration of the cord is a medical emergency – please arrange same day emergency assessment in secondary care.
C.M & A.K. 01-08-25
Who to refer:
Symptoms suggestive of either a spinal cord lesion (subacute combined degeneration or peripheral neuropathy) or visual (optic neuropathy) need same day specialist emergency assessment.
Please seek specialist advice if symptoms are severe, there is diagnostic uncertainty, and a neurology opinion is required to help determine the cause.
