A 58-year-old male from rural Maharashtra presents with progressive weakness, paresthesias in both feet, and difficulty walking for 3 months. On examination, he has diminished vibration and proprioception sense in lower limbs, hyperreflexia, and a positive Romberg sign. His diet consists mainly of polished rice and minimal protein intake. Blood investigations show macrocytic anemia (Hb 9.2 g/dL, MCV 104 fL) and serum cobalamin level of 145 pg/mL (normal >200). What is the most appropriate initial pharmacological intervention?

Explanation

## Clinical Diagnosis This patient presents with **pernicious anemia** secondary to vitamin B₁₂ (cobalamin) deficiency, evidenced by: - Macrocytic anemia with elevated MCV - Subacute combined degeneration (SACD) of spinal cord: paresthesias, diminished proprioception/vibration, hyperreflexia - Low serum cobalamin (145 pg/mL; normal >200) - Risk factor: diet poor in B₁₂ (polished rice, minimal protein) ## Why Intramuscular Cyanocobalamin Is Correct **Key Point:** In established neurological manifestations of B₁₂ deficiency (SACD), intramuscular (IM) administration is mandatory because: 1. **Bypasses intestinal absorption:** Oral forms rely on intrinsic factor and ileal absorption, which are often impaired in B₁₂ deficiency states. 2. **Achieves rapid, high tissue levels:** IM injection ensures predictable pharmacokinetics and CNS penetration. 3. **Prevents neurological progression:** Neurological damage is irreversible if left untreated; IM therapy halts further deterioration. 4. **Standard dosing:** 1000 mcg IM weekly × 6 weeks (induction), then monthly maintenance is the evidence-based regimen [cite:KD Tripathi 8e Ch 45]. **High-Yield:** Neurological symptoms in B₁₂ deficiency demand **parenteral replacement**. Oral therapy alone is inadequate and risks permanent spinal cord damage. ## Cyanocobalamin vs. Methylcobalamin | Feature | Cyanocobalamin | Methylcobalamin | |---------|---|---| | **Route** | IM preferred for SACD | IV/IM, but not standard | | **Tissue penetration** | Excellent, crosses BBB | Theoretical advantage, no proven superiority | | **Cost** | Cheaper | Expensive | | **Evidence base** | Gold standard for neurological B₁₂ deficiency | Limited evidence in SACD | **Clinical Pearl:** Cyanocobalamin is converted to methylcobalamin and adenosylcobalamin in tissues; both are the active forms. There is no clinical advantage to giving methylcobalamin directly in SACD. ## Why Oral Therapy Fails Here **Warning:** Oral cyanocobalamin (option A) is contraindicated in neurological B₁₂ deficiency because: - Absorption is unreliable in deficiency states - Cannot achieve therapeutic CNS levels quickly enough - Risk of irreversible neurological damage during the lag period - Oral therapy is reserved for **asymptomatic** deficiency or maintenance after IM induction.