Subacute combined degeneration

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Vitamin B12 is required for methylation and odd-chain fatty-acid metabolism. Deficiency raises methylmalonic acid and homocysteine, destabilises myelin and injures the posterior and lateral columns. The classic combination is loss of proprioception and vibration with pyramidal weakness, but the peripheral nerves, optic nerves and brain can also be affected. Patients may report numb feet, electric sensations, poor balance in darkness, falls, clumsy hands, cognitive difficulty, depression or visual blurring.

A serum result is interpreted with exposure and treatment context. Supplements can normalise measured B12 without reversing tissue deficiency, renal impairment raises methylmalonic acid and folate deficiency raises homocysteine. Nitrous oxide causes functional inactivation, making metabolites particularly valuable. Copper deficiency can closely mimic posterior-lateral column disease, especially after upper gastrointestinal surgery or excess zinc, and HIV, syphilis, inflammatory myelopathy and cervical compression remain differential diagnoses.

Treatment has two purposes: replace cobalamin rapidly and prevent recurrence by identifying the cause. Autoimmune gastritis is assessed with anti-intrinsic-factor antibody when appropriate, but a negative test does not exclude it. Coeliac testing, operation history, diet, medicines and nitrous oxide exposure shape further work. Physiotherapy, falls and occupational assessment proceed while replacement is given. A rising serum B12 after injection is expected and is not toxicity or a reason to stop necessary therapy.

Key points

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.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Autoimmune gastritis

Loss of intrinsic-factor-mediated absorption causes persistent vitamin B12 deficiency and commonly requires lifelong replacement when malabsorption persists.

Gastric or ileal disease

Gastrectomy, bariatric surgery, terminal ileal resection, coeliac disease and other malabsorption states reduce cobalamin uptake substantially.

Dietary deficiency

Low intake can occur with restrictive diets, malnutrition and frailty, often alongside other nutritional deficits in vulnerable people.

Functional cobalamin inactivation

Nitrous oxide oxidises cobalamin, while metformin and acid-suppressing medicines can contribute to deficiency through different mechanisms.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Cobalamin cofactor failure
Vitamin B12 deficiency impairs methylation and fatty-acid metabolism needed for stable neural myelin and cellular function.
2
Dorsal-column demyelination
Posterior cord injury reduces vibration and joint-position signalling, causing sensory ataxia, paraesthesia and a positive Romberg response.
3
Corticospinal involvement
Lateral-column damage adds weakness, spasticity, brisk reflexes and extensor plantar responses despite coexisting peripheral areflexia on examination.
4
Peripheral and cerebral injury
Peripheral nerves, optic pathways and cognition may also be affected, creating a mixed central and peripheral syndrome.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Posterior-column syndromeRed flag

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.

Combined long-tract diseaseRed flag

Legs become weak or stiff with brisk knees and extensor plantars while numb feet and reduced ankle reflexes reveal accompanying peripheral neuropathy.

Neuropsychiatric or optic featuresRed flag

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.

Malabsorption phenotype

Previous gastrectomy, bariatric or ileal surgery, inflammatory bowel or coeliac disease, autoimmune history or long-term metformin or proton-pump inhibitor use increases pre-test probability.

Nitrous oxide exposureRed flag

A younger person with frequent recreational canister use develops rapidly progressive paraesthesia, sensory ataxia and weakness, sometimes with normal or supplemented serum B12.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Total vitamin B12 or active B12First step
Why
Provide the initial biochemical assessment in most people with compatible symptoms and risk factors before replacement.
Interpretation and limitations
Use laboratory and NICE thresholds and record supplements or injections. An indeterminate result with neurological disease prompts MMA testing and treatment consideration rather than reassurance.
02
Serum methylmalonic acid and plasma homocysteine
Why
Demonstrate functional deficiency when the initial result is indeterminate or nitrous oxide inactivation is suspected.
Interpretation and limitations
MMA is more specific but rises with renal impairment; homocysteine also rises in folate deficiency. NICE uses either as the initial route for suspected nitrous-oxide-related deficiency.
03
Full blood count, film, folate and haemolysis profile
Why
Identify megaloblastic haematopoiesis and concurrent folate or severe ineffective erythropoiesis.
Interpretation and limitations
Macrocytosis, oval macrocytes and hypersegmented neutrophils support deficiency but may be absent. Severe disease can raise LDH and bilirubin; normal indices do not exclude neurological B12 injury.
04
Cause assessment
Why
Determine whether replacement is temporary, oral or lifelong and treat the underlying malabsorption or exposure.
Interpretation and limitations
Review diet, medicines, surgery and nitrous oxide; consider anti-intrinsic-factor antibody, coeliac testing and gastroenterology review. A negative intrinsic-factor antibody does not eliminate autoimmune gastritis.
05
MRI spinal cord
Why
Exclude compression and demonstrate posterior-column signal when the presentation is severe, atypical or diagnostically uncertain.
Interpretation and limitations
Long symmetric posterior cervical or thoracic T2 signal can support subacute combined degeneration, but normal MRI does not exclude it and imaging is not required to begin urgent replacement.
06
Copper and alternative myelopathy tests
Why
Identify a close nutritional mimic or another infectious, inflammatory and structural cause when recovery or phenotype is atypical.
Interpretation and limitations
Check copper and zinc particularly after gastrointestinal surgery or supplement use; tailor HIV, syphilis, AQP4/MOG, CSF and neurophysiology to the clinical problem.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Cervical myelopathy

Hand clumsiness, neck disease and focal compression on MRI favour spondylotic cord injury, which can coexist with vitamin deficiency.

Inflammatory myelitis or MS

Acute attacks, enhancing lesions and cerebrospinal-fluid inflammation support immune demyelination rather than a symmetrical nutritional tract syndrome.

Copper or vitamin-E deficiency

Malabsorption, excessive zinc and fat-related disease can produce similar posterior-column, neuropathic and ataxic phenotypes during assessment.

Infectious posterior-column disease

HIV, syphilis and other infections require exposure-led testing when sensory ataxia and systemic context do not fit isolated cobalamin deficiency.

Additional chapter-specific clues

Mimic warningRed flag

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.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Neurological deficiencySample then treat without delayFirst stepProgressive proprioceptive loss, sensory ataxia or pyramidal signs occur with plausible B12 deficiency.

1Take total or active B12, blood count, folate and cause-directed samples immediately, adding MMA or homocysteine for indeterminate results or suspected nitrous oxide exposure.
2Begin replacement before delayed results return when neurological injury is significant, generally using intramuscular hydroxocobalamin through BNF and local neurological loading guidance.
3Stop 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.
4Assess falls, gait, hand function, vision, cognition and rehabilitation needs at baseline so improvement is measured beyond the blood count.

02Find the causeDetermine duration and routeVitamin B12 deficiency is confirmed or strongly suspected after initial treatment starts.

1Review autoimmune disease, gastrointestinal operations, ileal and coeliac disease, diet, metformin and acid suppression, supplements and detailed nitrous oxide exposure.
2Offer lifelong intramuscular treatment for autoimmune gastritis, total gastrectomy or complete terminal ileal resection and individualise other malabsorption or dietary routes under NICE guidance.
3Investigate persistent uncertainty with intrinsic-factor testing, coeliac and gastrointestinal review and copper or other mimic testing rather than simply increasing injections indefinitely.

03Follow recoveryUse symptoms, not serum overshootReplacement has begun and neurological or haematological response is being reviewed.

1Arrange early review according to severity, with NICE generally recommending an initial follow-up by three months and sooner for major neurological disability.
2Track gait, proprioception, power, reflexes, pain, cognition, vision and daily activity; increase intramuscular frequency or revisit alternatives if symptoms worsen or remain functionally important.
3Do not repeat the initial serum diagnostic test routinely during intramuscular treatment or stop because the level is high; continue lifelong therapy when the cause is irreversible.

Key medicines and prescribing safety4 treatments · regimens, roles and cautions

Rapidly supplies vitamin B12 without relying on gastrointestinal absorption and limits ongoing cord and nerve injury.

Intramuscular hydroxocobalamin loading

For neurological involvement, BNF practice commonly uses 1 mg on alternate days until no further improvement.

Take diagnostic samples first when feasible but do not delay urgent therapy; hypersensitivity is rare, potassium can fall during brisk haematopoietic response and acneiform reactions may occur.

Prevents recurrent deficiency and neurological relapse in autoimmune gastritis or irreversible surgical malabsorption.

Intramuscular hydroxocobalamin maintenance

Neurological maintenance is commonly 1 mg every two months when an irreversible cause persists.

Follow current BNF and local frequency and do not use a high post-injection serum B12 as a reason to stop; investigate new symptoms for alternative disease.

Provides effective replacement for selected dietary, medicine-related or some malabsorption causes when adherence and absorption are adequate.

Oral vitamin B12

When oral replacement is chosen for suspected malabsorption, NICE advises at least 1 mg daily.

Significant neurological disease, adherence concern or irreversible major malabsorption may favour intramuscular treatment; check the actual cobalamin type and licensed product.

Corrects a coexisting megaloblastic deficiency and supports haematological recovery.

Folate replacement

Treat confirmed folate deficiency using the current BNF regimen after vitamin B12 has been addressed.

Folate alone can improve anaemia while allowing B12-related neurological damage to progress, so establish and replace B12 first or concurrently.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Permanent gait disability

Delayed treatment permits fixed axonal loss, leaving sensory ataxia, spasticity, falls and need for mobility support.

Peripheral injury and pain

Loss of protective sensation causes burns, ulcers and neuropathic pain, while weakness promotes foot deformity and deconditioning.

Cognitive and visual impairment

Cerebral and optic involvement can leave memory, mood and visual deficits even when blood indices normalise.

Recurrent disease

Failure to identify malabsorption, nitrous oxide exposure or an irreversible cause permits continuing or repeated neurological injury.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Record gait, Romberg, vibration, joint position, power, reflexes, plantars, hand function, vision and cognition before treatment and at each substantive review.
Review symptom trajectory earlier than three months when deficits are severe and investigate compression, copper deficiency or inflammatory disease if recovery is absent or deterioration continues.
Follow blood count and potassium when severe megaloblastic disease is present, recognising that neurological improvement can lag behind reticulocyte and haemoglobin recovery.
Confirm cessation of nitrous oxide and address diet, medicine and malabsorption causes; recurrence is likely if exposure continues despite injections.
Do not routinely repeat the diagnostic serum B12 during intramuscular therapy; monitor clinical response and adherence to the maintenance schedule instead.
Safety-net new falls, weakness, sphincter change, visual loss, confusion or a sensory level for urgent reassessment rather than assuming slow recovery.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Anaemia is optional

Neurological B12 deficiency can precede or occur without macrocytosis, so a normal full blood count must not close the diagnosis.

Nitrous oxide is functional

Oxidation inactivates circulating cobalamin, explaining why metabolites and exposure history can be more informative than a supplemented serum concentration.

Ankles and knees can disagree

Peripheral neuropathy reduces ankle jerks while corticospinal tract injury increases knee reflexes and produces extensor plantars.

Copper copies the pattern

Copper deficiency can produce nearly identical posterior-lateral column and neuropathic findings, particularly after gastric surgery or excess zinc.

Serum rise is expected

A high B12 concentration after injection demonstrates recent replacement and is not in itself toxicity, overdose or evidence that maintenance is unnecessary.

Recovery has a ceiling

Early treatment can reverse much dysfunction, but axonal or cord injury after prolonged deficiency may leave persistent balance, sensory and motor disability.

11Common pitfallsFrequent interpretation and management errors.

01
Excluding deficiency because haemoglobin and mean cell volume are normal.
02
Waiting for MMA, intrinsic-factor antibody or MRI while progressive neurological signs remain untreated.
03
Using serum B12 alone after nitrous oxide exposure or recent self-supplementation.
04
Giving folic acid alone before recognising B12-related cord injury.
05
Failing to test copper and structural mimics when the phenotype or treatment response is atypical.
06
Stopping lifelong replacement for an irreversible cause because the post-injection serum level is above range.