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# Thiamine and B12 deficiency in the CNS

- Lesion

Lack of thiamine damages memory and eye-movement centres; lack of B12 damages the back and side columns of the spinal cord.

Updated Nov 12, 2026

## Key points

- Wernicke triad: confusion, ophthalmoplegia (nystagmus, gaze palsies), ataxia; not all three are always present.
- Lesions: mammillary bodies, walls of the third ventricle, thalamus, periaqueductal grey and floor of the fourth ventricle; acute petechial hemorrhage, later atrophy with hemosiderin.
- Korsakoff syndrome: anterograde and retrograde amnesia with confabulation, linked to thalamic (dorsomedial) damage.
- Glucose given before thiamine to a deficient patient can precipitate Wernicke encephalopathy.
- B12 deficiency: swelling and vacuolation of myelin in the dorsal (position, vibration) and lateral (corticospinal) columns, starting in the thoracic cord.
- B12 deficiency also causes neuropathy, cognitive change and optic neuropathy; pernicious anemia is a common cause.

## What it is

Two vitamin deficiencies cause classic CNS lesions. Thiamine (B1) deficiency, common in alcohol use disorder, malnutrition and prolonged vomiting, causes Wernicke encephalopathy: confusion, ophthalmoplegia and ataxia, with hemorrhagic lesions in the mammillary bodies and around the third ventricle and aqueduct. If untreated it leaves Korsakoff amnesia. Vitamin B12 deficiency causes subacute combined degeneration of the dorsal and lateral columns of the spinal cord, often with megaloblastic anemia.

## Step by step

- Thiamine stores, small to begin with, run out within weeks of poor intake.
- Thiamine-dependent enzymes of glucose metabolism fail in active brain regions.
- Energy failure and lactic acidosis injure neurons and vessels around the third ventricle.
- Petechial bleeds and necrosis form in the mammillary bodies.
- Untreated lesions leave permanent amnesia (Korsakoff).

## Thiamine vs vitamin B12 deficiency in the CNS

| Feature | Thiamine (B1) | Vitamin B12 |
| --- | --- | --- |
| Main causes | Alcohol, malnutrition, hyperemesis, bariatric surgery | Pernicious anemia, gastrectomy, strict vegan diet, metformin, ileal disease |
| [Biochemical role](https://durgical.com/en/conditions/wernicke-encephalopathy/) | Cofactor of pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, transketolase | Methionine synthase and methylmalonyl-CoA mutase |
| [Lesion](https://durgical.com/en/conditions/subacute-combined-degeneration/) | Mammillary bodies, periventricular grey matter | Dorsal and lateral spinal columns |
| Clinical | Confusion, ophthalmoplegia, ataxia; amnesia | Lost position and vibration sense, spastic weakness, Romberg sign |
| [Blood](https://durgical.com/en/conditions/pernicious-anaemia/) | Usually normal | Megaloblastic anemia, high methylmalonic acid and homocysteine |

## Clinical notes

- Thiamine is given early in any patient at risk with confusion, before glucose.
- B12 replacement stops progression of subacute combined degeneration; recovery depends on how long the deficit lasted.

## Memory aid

Thiamine guards the memory bodies (mammillary); B12 guards the back and side columns of the cord.

## Test yourself

Think of the answer, then tap the question to see it.

## Which structures show lesions in Wernicke encephalopathy?

The mammillary bodies, walls of the third ventricle and periaqueductal grey matter, with acute petechial hemorrhage.

## Which spinal cord tracts degenerate in vitamin B12 deficiency?

The dorsal columns and lateral corticospinal tracts (subacute combined degeneration).

## Why should thiamine be given before glucose in a malnourished, confused patient?

Glucose metabolism uses up the remaining thiamine and can precipitate or worsen Wernicke encephalopathy.

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## Elsewhere in DurgiPedia

- [Absorption of iron, calcium and vitamin B12](https://durgical.com/en/physiology/absorption-of-iron-calcium-and-vitamin-b12/)
- [Spinal cord syndromes: complete, Brown-Séquard, central, anterior](https://durgical.com/en/clinical-exam/spinal-cord-syndromes-complete-brown-sequard-central-anterior/)

## Connections

## Related 5

- [Metabolic and toxic brain injury](https://durgical.com/en/general-pathology/metabolic-and-toxic-brain-injury/)
- [Vitamin deficiencies: what each one does to tissue](https://durgical.com/en/general-pathology/vitamin-deficiencies-what-each-one-does-to-tissue/)
- [Alcohol: how it injures the liver, brain, heart and fetus](https://durgical.com/en/general-pathology/alcohol-how-it-injures-the-liver-brain-heart-and-fetus/)
- [Severe acute malnutrition: marasmus and kwashiorkor](https://durgical.com/en/general-pathology/severe-acute-malnutrition-marasmus-and-kwashiorkor/)
- [Megaloblastic anaemias: B12 and folate](https://durgical.com/en/general-pathology/megaloblastic-anaemias-b12-and-folate/)

## Related conditions 6

- [Wernicke encephalopathy (thiamine deficiency)](https://durgical.com/en/conditions/wernicke-encephalopathy/)
- [Subacute combined degeneration (vitamin B12)](https://durgical.com/en/conditions/subacute-combined-degeneration/)
- [Pernicious anaemia](https://durgical.com/en/conditions/pernicious-anaemia/)
- [Beriberi (thiamine deficiency)](https://durgical.com/en/conditions/beriberi/)
- [Alcohol use disorder](https://durgical.com/en/conditions/alcohol-use-disorder/)
- [Hyperemesis gravidarum](https://durgical.com/en/conditions/hyperemesis-gravidarum/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Wernicke-Korsakoff syndrome: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000771.htm) (MedlinePlus, US National Library of Medicine, 2026); [Vitamin B12 deficiency anemia: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000574.htm) (MedlinePlus, US National Library of Medicine, 2026).

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/thiamine-and-b12-deficiency-in-the-cns/