A 42-year-old woman from rural Maharashtra presents with a 3-month history of progressive weakness, paresthesias in both feet, and difficulty walking. On examination, she has diminished vibration and proprioception sense in the lower limbs, brisk reflexes, and a positive Romberg sign. Her diet consists mainly of polished rice with minimal vegetables or legumes. Blood glucose is 98 mg/dL. MRI spine is normal. Which vitamin deficiency is most likely responsible for her neurological syndrome?

Explanation

## Clinical Presentation Analysis The patient presents with a **subacute-to-chronic sensorimotor peripheral neuropathy** with prominent **dorsal column signs** (vibration/proprioception loss, positive Romberg) and hyperreflexia. This constellation is classic for **B12 deficiency**, which causes **subacute combined degeneration (SCD)** of the spinal cord. ## Pathophysiology of B12 Deficiency Neuropathy **Key Point:** Vitamin B12 (cobalamin) is essential for: - Myelin synthesis via methylmalonyl-CoA mutase and methionine synthase pathways - DNA synthesis (via methyl-transfer reactions) - Homocysteine metabolism Deficiency leads to demyelination of the **dorsal and lateral columns** of the spinal cord, producing the characteristic triad: 1. **Dorsal column involvement** → vibration/proprioception loss, ataxia 2. **Lateral corticospinal tract involvement** → hyperreflexia, Babinski sign 3. **Peripheral neuropathy** → paresthesias, weakness ## Why This Patient? **High-Yield:** Rural diet in India (polished rice, minimal animal protein/dairy) is a major risk factor for B12 deficiency. B12 is found almost exclusively in animal products (meat, eggs, milk). **Clinical Pearl:** The **normal glucose** rules out diabetic neuropathy. The **normal MRI spine** excludes structural cord lesions but does NOT exclude SCD — early SCD may show normal imaging. ## Differential Diagnosis Table | Vitamin | Neuropathy Type | Key Features | Diet Risk | | --- | --- | --- | --- | | **B12** | Dorsal column + peripheral | Vibration/proprioception loss, hyperreflexia, ataxia | Vegetarian, vegan | | **B1** | Wernicke-Korsakoff | Ophthalmoplegia, confusion, ataxia | Alcoholism, thiamine-poor diet | | **B6** | Sensory neuropathy | Dorsal root ganglion damage, pain/temperature loss | Isoniazid use, high-dose supplements | | **B3** | Pellagra (4 Ds) | Dermatitis, diarrhea, dementia, death | Corn-based diet, tryptophan deficiency | **Mnemonic for B12 deficiency signs: DAMS** — Dorsal column loss, Ataxia, Myelin loss, Subacute onset ## Diagnostic Confirmation Expect: - ↓ Serum B12 level - ↑ Methylmalonic acid and homocysteine (sensitive markers) - Macrocytic anemia (if advanced) - Positive Schilling test (if intrinsic factor deficiency) ## Treatment **Action:** Intramuscular B12 supplementation (1000 μg weekly × 8 weeks, then monthly maintenance). Early treatment prevents irreversible neuronal damage.