Synopsis
Recognise vitamin B12-related posterior and lateral column injury even without anaemia, take cause-defining samples promptly and start replacement before neurological disability becomes fixed.
- Subacute combined degeneration is spinal-cord injury from functional vitamin B12 deficiency, predominantly affecting dorsal columns and lateral corticospinal tracts, often with peripheral neuropathy.
- Symptoms include symmetrical paraesthesia, impaired vibration and joint position, sensory ataxia, a positive Romberg sign, gait disturbance, weakness and later spasticity or extensor plantar responses.
- Peripheral nerve involvement can reduce ankle reflexes while cord involvement produces brisk knees or plantar responses, creating a mixed central and peripheral examination.
- Do not exclude B12 deficiency because haemoglobin, mean cell volume or serum folate is normal; neurological disease can occur without anaemia or macrocytosis.
- Common causes are autoimmune gastritis, total or partial gastrectomy, terminal ileal disease or resection, bariatric surgery, coeliac disease, dietary deficiency, metformin and acid-suppressing medicines.
- Recreational nitrous oxide oxidises and inactivates cobalamin, so serum B12 can be misleadingly normal; NICE recommends methylmalonic acid or homocysteine as the initial test when this cause is suspected.
- Obtain total or active B12 and cause-directed samples before replacement when feasible, but do not wait for delayed confirmatory tests if progressive neurological injury is suspected.
- Intramuscular hydroxocobalamin is generally favoured for significant neurological disease or malabsorption, with loading and maintenance following the BNF and local pathway.
- Autoimmune gastritis, total gastrectomy and complete terminal ileal resection are irreversible causes for which NICE recommends lifelong intramuscular replacement.
- Haematological improvement can precede neurological recovery, which may continue for months and may be incomplete after long-standing cord injury; rehabilitation and cause control remain essential.
Key red flags
Vibration and joint position are impaired in the feet, stance worsens with eyes closed and gait becomes stamping or broad-based from sensory ataxia despite relatively preserved pain and temperature.
Legs become weak or stiff with brisk knees and extensor plantars while numb feet and reduced ankle reflexes reveal accompanying peripheral neuropathy.
Cognitive slowing, mood or psychotic symptoms, optic neuropathy and visual-field change can accompany myelopathy and should not be dismissed because the blood count is normal.
A younger person with frequent recreational canister use develops rapidly progressive paraesthesia, sensory ataxia and weakness, sometimes with normal or supplemented serum B12.
A clear sensory level, severe neck pain, asymmetry, sphincter loss, fever or rapid focal progression prompts urgent MRI for compression or inflammation alongside nutritional testing.
Investigation priorities
Provide the initial biochemical assessment in most people with compatible symptoms and risk factors before replacement.
Management branches
Progressive proprioceptive loss, sensory ataxia or pyramidal signs occur with plausible B12 deficiency.
- Take total or active B12, blood count, folate and cause-directed samples immediately, adding MMA or homocysteine for indeterminate results or suspected nitrous oxide exposure.
- Begin replacement before delayed results return when neurological injury is significant, generally using intramuscular hydroxocobalamin through BNF and local neurological loading guidance.
- Stop nitrous oxide and correct dietary or medicine contributors, and arrange urgent MRI or neurological review when asymmetry, a level, sphincter change or rapid deterioration suggests another cord lesion.
- Assess falls, gait, hand function, vision, cognition and rehabilitation needs at baseline so improvement is measured beyond the blood count.