A 42-year-old man from rural Bihar presents with a 3-month history of progressive weakness, numbness in his feet, and difficulty walking. On examination, he has diminished vibration and proprioception sense in the lower limbs, absent ankle reflexes, and a positive Romberg sign. His diet consists mainly of polished rice with minimal vegetables or legumes. Blood glucose is 98 mg/dL. Serum B12 and folate levels are normal. Which vitamin deficiency is most likely responsible for his neurological findings?

Explanation

## Clinical Presentation Analysis **Key Point:** The patient presents with a classic triad of thiamine deficiency: peripheral neuropathy (numbness, weakness), loss of proprioception/vibration sense (dorsal column involvement), and gait ataxia (positive Romberg sign). This constellation is termed **dry beriberi** when neurological manifestations predominate. ## Pathophysiology of Thiamine Deficiency Thiamine (vitamin B1) functions as **thiamine pyrophosphate (TPP)**, a critical cofactor for: 1. **Pyruvate dehydrogenase** — links glycolysis to TCA cycle 2. **α-ketoglutarate dehydrogenase** — TCA cycle enzyme 3. **Transketolase** — pentose phosphate pathway 4. **Branched-chain amino acid metabolism** Deficiency impairs energy metabolism in high-demand tissues (nervous system, heart), leading to neuronal degeneration. ## Neurological Manifestations of Thiamine Deficiency | Feature | Dry Beriberi | Wet Beriberi | |---------|--------------|---------------| | **Primary system** | Nervous system | Cardiovascular | | **Key findings** | Peripheral neuropathy, ataxia, Wernicke-Korsakoff syndrome | Dilated cardiomyopathy, high-output heart failure, edema | | **Mechanism** | Impaired ATP production in neurons | Impaired myocardial energy metabolism | | **Sensory loss** | Vibration, proprioception (dorsal columns) | Absent | **High-Yield:** The **dorsal column involvement** (vibration and proprioception loss) with **absent ankle reflexes** is pathognomonic for thiamine deficiency neuropathy — it reflects demyelination of large myelinated fibers in peripheral nerves and posterior columns. ## Risk Factors in This Patient **Clinical Pearl:** Polished rice diet is a major risk factor in rural India. Thiamine is found in the bran and germ layers, which are removed during polishing. Combined with minimal vegetable/legume intake (other thiamine sources), this creates a perfect storm for deficiency. ## Why Normal B12 and Folate Rule Out Other Causes - **B12 deficiency** → subacute combined degeneration (also affects dorsal columns BUT also lateral corticospinal tracts with hyperreflexia, not hyporeflexia) - **Folate deficiency** → megaloblastic anemia with neuropsychiatric features, not isolated peripheral neuropathy **Mnemonic: THIAMINE DEFICIENCY SITES — "PATh"** - **P**eripheral nerves (demyelination) - **A**taxia (cerebellar involvement) - **Th**iamine = B1 ## Diagnostic Confirmation - **Erythrocyte transketolase activity** — reduced in thiamine deficiency (TPP is the cofactor) - **Thiamine pyrophosphate effect** — >25% increase in transketolase activity after TPP addition confirms deficiency - **Plasma thiamine levels** — <2 µg/dL is diagnostic **Warning:** Do NOT confuse thiamine deficiency neuropathy with diabetic neuropathy (glucose is normal here) or B12 deficiency (B12 level is normal, and hyperreflexia would be expected in B12 deficiency).