Vitamin B12 deficiency causes macrocytic anaemia. The key causes of a low B12 are:
- Pernicious anaemia
- Insufficient dietary B12 (particularly a vegan diet, as B12 is mostly found in animal products)
- Medications that reduce B12 absorption (e.g., proton pump inhibitors and metformin)
Pernicious anaemia is an autoimmune condition involving antibodies against the parietal cells or intrinsic factor.
Pathophysiology of Pernicious Anaemia
The parietal cells of the stomach produce a protein called intrinsic factor. Intrinsic factor is essential for the absorption of vitamin B12 in the distal ileum. In pernicious anaemia, autoantibodies target either the parietal cells or intrinsic factor, resulting in a lack of intrinsic factor and a lack of absorption of vitamin B12.
Vitamin B12 deficiency can cause neurological symptoms:
- Peripheral neuropathy, with numbness or paraesthesia (pins and needles)
- Loss of vibration sense
- Loss of proprioception
- Visual changes
- Mood and cognitive changes
TOM TIP: For your exams, remember to test for vitamin B12 deficiency and pernicious anaemia in patients presenting with peripheral neuropathy, particularly with pins and needles.
Antibodies
Autoantibodies used to diagnose pernicious anaemia are:
- Intrinsic factor antibodies (the first-line investigation)
- Gastric parietal cell antibodies (less helpful)
Management
NICE CKS (April 2023) recommend the below regimes. Check the latest guidelines before treating patients.
Intramuscular hydroxocobalamin is initially given to all patients with B12 deficiency, depending on symptoms:
- No neurological symptoms – 3 times weekly for two weeks
- Neurological symptoms – alternate days until there is no further improvement in symptoms
Maintenance depends on the cause:
- Pernicious anaemia – 2-3 monthly injections for life
- Diet-related – oral cyanocobalamin or twice-yearly injections
Where there is B12 and folate deficiency together, it is essential to treat the B12 deficiency first before correcting the folate deficiency. Giving patients folic acid when they have a B12 deficiency can lead to subacute combined degeneration of the cord, with demyelination in the spinal cord and severe neurological problems.
Last updated August 2023