A 38-year-old female from rural Maharashtra presents with a 6-month history of progressive weakness, paresthesias in both feet, and gait disturbance. On examination, she has loss of vibration sense and proprioception in the lower limbs, brisk reflexes, and a positive Romberg sign. She reports a predominantly cereal-based diet with minimal animal products. Serum vitamin B12 level is 180 pg/mL (normal >200). What is the most appropriate next step in management?

Explanation

## Clinical Presentation Analysis This patient presents with **subacute combined degeneration (SCD)** of the spinal cord—a neurological emergency caused by vitamin B12 deficiency. The constellation of findings (paresthesias, loss of vibration/proprioception, hyperreflexia, positive Romberg) indicates demyelination of the dorsal and lateral columns. ## Why Immediate Parenteral B12 is Correct **Key Point:** Once neurological symptoms are present, B12 deficiency is a medical emergency. Neurological damage can become irreversible if treatment is delayed. **High-Yield:** The presence of neurological manifestations (not just macrocytic anemia) mandates **immediate intramuscular (IM) cyanocobalamin therapy**—not oral supplementation or diagnostic delays. **Clinical Pearl:** IM administration ensures rapid, high-dose delivery and bypasses any potential malabsorption (dietary deficiency vs. pernicious anemia vs. intrinsic factor deficiency). The neurological deficit is time-sensitive; waiting for Schilling test results or endoscopy risks permanent cord damage. ## Management Algorithm ```mermaid flowchart TD A[B12 deficiency + neurological signs]:::outcome --> B{Neurological symptoms present?}:::decision B -->|Yes| C[Start IM cyanocobalamin 1000 µg weekly × 6 weeks]:::action B -->|No| D[Oral supplementation or investigate cause]:::action C --> E[Monthly maintenance IM B12]:::action E --> F[Reassess neurological recovery at 3-6 months]:::outcome D --> G[Schilling test / endoscopy if needed]:::action ``` ## Dosing Regimen | Phase | Dose | Route | Duration | Rationale | |-------|------|-------|----------|----------| | **Induction** | 1000 µg | IM | Weekly × 6 weeks | Rapid repletion; neurological emergency | | **Maintenance** | 1000 µg | IM | Monthly, indefinite | Prevent relapse; oral inadequate for SCD | **Note:** Oral cyanocobalamin (option 3) is insufficient for neurological disease and delays critical therapy. ## Why Diagnostic Delays Are Harmful **Warning:** The Schilling test (option 2) and endoscopy (option 4) are useful for determining *etiology* (pernicious anemia vs. dietary) but should **never delay treatment** when neurological symptoms are present. Demyelination can become irreversible within weeks.