A 45-year-old woman with pernicious anemia is started on intramuscular vitamin B12 therapy. Which of the following is NOT a recognized hematologic or neurologic complication that may develop if B12 deficiency is left untreated?

## Neurologic and Hematologic Complications of B12 Deficiency **Key Point:** Vitamin B12 deficiency causes two distinct pathologic processes: (1) megaloblastic anemia from impaired DNA synthesis, and (2) demyelinating neuropathy from impaired myelin synthesis. Hemolytic anemia is NOT a complication of B12 deficiency. ### Recognized Neurologic Complications | Complication | Pathophysiology | Clinical Features | |--------------|-----------------|-------------------| | Subacute combined degeneration (SCD) | Demyelination of posterior columns and lateral corticospinal tracts | Paresthesias, ataxia, spasticity, positive Romberg sign | | Peripheral neuropathy | Demyelination of sensory nerves (dorsal root ganglia) | Distal paresthesias, loss of vibration/position sense, "glove-and-stocking" distribution | | Optic neuropathy | Demyelination of optic nerve fibers | Central scotoma, visual blurring, optic atrophy on fundoscopy | | Cognitive dysfunction | Impaired myelin formation in cerebral white matter | Memory loss, personality changes, psychosis ("megaloblastic madness") | | Autonomic dysfunction | Involvement of autonomic fibers | Orthostatic hypotension, impotence | ### Why Acute Hemolytic Anemia Is NOT a Complication **High-Yield:** B12 deficiency causes **megaloblastic (non-hemolytic) anemia**, not hemolytic anemia. The mechanism is impaired DNA synthesis leading to ineffective erythropoiesis, not immune-mediated RBC destruction. **Clinical Pearl:** The anemia of B12 deficiency is characterized by: - **Low reticulocyte count** (inappropriately low for degree of anemia) - **Elevated LDH and indirect bilirubin** (from intramedullary hemolysis, not peripheral hemolysis) - **Negative direct Coombs test** (no antibodies against RBC surface antigens) - **Macrocytic indices** (MCV 100–140 fL) **Mnemonic: B12-NEURO** — B12 deficiency causes: Neurologic (demyelination, SCD, optic neuropathy), Erythroid (megaloblastic, NOT hemolytic), U-turn (posterior columns), Reduced myelin, Optic nerve involvement ### Pathophysiology of B12-Mediated Neurologic Damage 1. B12 is a cofactor for **methionine synthase** (methylation reactions) 2. B12 deficiency → ↓ SAM (S-adenosylmethionine) → impaired myelin phospholipid synthesis 3. Selective vulnerability: **posterior columns** (proprioception/vibration) and **lateral corticospinal tracts** (motor) 4. Demyelination → conduction block → paresthesias, ataxia, weakness **Warning:** Do not confuse B12 deficiency with folate deficiency. Folate deficiency causes megaloblastic anemia but does NOT cause neurologic complications (no demyelination). B12 deficiency causes BOTH anemia and neuropathy.