Treatment of Pernicious Anemia from Autoimmune Gastric Atrophy
Lifelong intramuscular vitamin B12 (cyanocobalamin) at 100 mcg monthly is the definitive treatment for pernicious anemia caused by autoimmune gastric atrophy, following an initial loading regimen of 100 mcg daily for 6-7 days, then alternate days for seven doses, then every 3-4 days for 2-3 weeks. 1
Initial Loading Phase
- Administer 100 mcg vitamin B12 intramuscularly or deep subcutaneously daily for 6-7 days to rapidly replenish depleted stores 1
- Continue with 100 mcg on alternate days for seven doses if clinical improvement and reticulocyte response are observed 1
- Then give 100 mcg every 3-4 days for another 2-3 weeks until hematologic values normalize 1
- Avoid intravenous administration as almost all vitamin will be lost in urine 1
Maintenance Therapy
- Continue 100 mcg intramuscular B12 monthly for life after the loading phase 1
- Oral B12 is not dependable in pernicious anemia due to lack of intrinsic factor required for absorption 1
- Add iron supplementation (ferrous sulfate 200 mg twice daily or one tablet daily if better tolerated) as iron deficiency commonly coexists with B12 deficiency in autoimmune gastritis 2
- Continue iron therapy for 3 months after normalization to replenish stores 2
Essential Diagnostic Confirmation
Before initiating treatment, confirm the diagnosis through:
- Endoscopy with topographical biopsies from body and antrum to document corpus-predominant atrophic gastritis and rule out gastric neoplasia including neuroendocrine tumors 2
- Check antiparietal cell antibodies and anti-intrinsic factor antibodies to confirm autoimmune etiology 2
- Measure fasting gastrin levels (typically markedly elevated >1,500 pg/mL in pernicious anemia) 3
- Assess for H. pylori infection and eradicate if present, as this can coexist and contribute to iron malabsorption 2
Surveillance and Monitoring
- Screen for type 1 gastric neuroendocrine tumors with upper endoscopy, as these develop in the hypergastrinemic environment 2
- Perform surveillance endoscopy every 1-2 years if neuroendocrine tumors are found and removed 2
- Consider endoscopic surveillance every 3 years for advanced atrophic gastritis to detect dysplasia or adenocarcinoma 2
- Screen for autoimmune thyroid disease, as it is present in up to one-third of patients with autoimmune gastritis 2
- Monitor hemoglobin response within 4 weeks of initiating therapy 2
Critical Pitfalls to Avoid
- Do not delay B12 replacement while awaiting endoscopy, as neurologic damage can become irreversible 4, 5
- Do not rely on oral B12 preparations for treatment of pernicious anemia, as intrinsic factor deficiency prevents absorption 1, 6
- Do not assume normalization of B12 levels means cure—lifelong monthly injections are mandatory 1
- Do not overlook iron deficiency, which occurs in up to 25% of autoimmune gastritis cases and requires separate treatment 2
- Do not miss concomitant folic acid deficiency, which should be administered if present 1
Special Considerations
Neurologic manifestations (paresthesias, ataxia, subacute combined degeneration of spinal cord) may occur without anemia or macrocytosis and require urgent B12 replacement to prevent permanent damage 4, 5, 3. The diverse symptomatology includes fatigue (55%), loss of sensation in limbs (32%), excessive weight loss (27%), and glossitis (23%) 7. Pernicious anemia represents a late-stage manifestation of autoimmune gastritis, indicating years of corpus-predominant atrophy 2.