Pernicious anemia

Article written and reviewed by Dr Luca Moretti

Pernicious anemia is a specific form of megaloblastic anemia due to vitamin B12 deficiency caused by selective impairment of intestinal cobalamin absorption resulting from loss of gastric intrinsic factor, usually through an autoimmune mechanism. It is characterized by autoimmune atrophic gastritis, which causes progressive destruction of gastric parietal cells with consequent reduction or absence of intrinsic factor secretion, an element essential for vitamin B12 transport and absorption in the distal ileum.

This condition, typical of adults and older people, is an important cause of vitamin B12 deficiency in Western countries. It differs from other forms of malabsorption because of its demonstrable immune basis and important associations with other systemic autoimmune diseases. Pernicious anemia progresses slowly, often insidiously, and may present not only with the classic picture of megaloblastic anemia but also with neurologic and gastrointestinal signs and symptoms of variable severity, which not infrequently precede the onset of anemia itself.

Diagnosis integrates clinical, laboratory and immunologic criteria: vitamin B12 deficiency, anti-intrinsic factor antibodies and endoscopic-histologic documentation of autoimmune atrophic gastritis provide complementary diagnostic evidence, without all of them necessarily being present; anti-gastric parietal cell antibodies are less specific and mainly have a supportive role. Prompt vitamin B12 replacement resolves the hematologic abnormalities and prevents neurologic complications, but loss of intrinsic factor makes lifelong maintenance therapy necessary. Particular attention should be paid to the increased risk of gastric neoplasms and management of associated autoimmune comorbidities.

Etiology and risk factors

Pernicious anemia is the late hematologic manifestation of autoimmune atrophic gastritis when loss of parietal cells and gastric intrinsic factor causes malabsorption and vitamin B12 deficiency; autoimmune gastritis and pernicious anemia are therefore not synonymous, because gastritis may precede cobalamin deficiency and anemia by years. This process almost always results from autoimmune atrophic gastritis, a condition in which the immune system produces autoantibodies against parietal-cell antigens, particularly the H+/K+-ATPase proton pump, and/or against intrinsic factor itself. The result is progressive atrophy of the gastric mucosa, loss of acid secretory capacity and disappearance of intrinsic factor.

Pernicious anemia differs from other causes of vitamin B12 deficiency, which may include surgical, inflammatory, nutritional, pharmacologic, toxic or congenital conditions already discussed on the general vitamin B12 deficiency page, because of its immune-mediated pathogenesis related to autoimmune atrophic gastritis and loss of intrinsic factor.

Risk factors for pernicious anemia reflect the autoimmune nature of the disease:

• Advanced age: prevalence increases after age 60, with a peak in the seventh decade of life.
• Female sex: the disease is more frequent in women, as is the case with many other autoimmune diseases.
• Family history of autoimmune diseases: the presence of other autoimmune diseases in the family (thyroiditis, type 1 diabetes, vitiligo, Addison disease) increases the risk of pernicious anemia.
• Associations with other autoimmune diseases: the disease is frequently associated with autoimmune thyroiditis, type 1 diabetes, Addison disease, vitiligo, celiac disease and autoimmune polyglandular syndromes.
• Geographic origin and genetic factors: a higher incidence has been documented in populations of Northern European origin and in some ethnic groups, for example African American and Latino populations in the United States, suggesting a role of genetic predisposition; reported HLA associations vary among populations and should be interpreted cautiously.

Risk is also increased in people with autoimmune polyglandular syndromes and in those with several combined predisposing factors. Pernicious anemia is rare before age 30, while in children and adolescents it is exceptionally uncommon and is often related to genetic conditions or rare autoimmune syndromes.

It is important to remember that autoimmune pathogenesis distinguishes pernicious anemia from other forms of vitamin B12 deficiency: specific antibodies and autoimmune atrophic gastritis are strong supportive diagnostic findings and represent the principal distinguishing features compared with other causes of cobalamin deficiency.

Pathogenesis and pathophysiology

The pathogenesis of pernicious anemia is based on an organ-specific autoimmune process selectively affecting the gastric mucosa and causing progressive autoimmune atrophic gastritis of the fundus and body of the stomach. This process destroys parietal cells, which are responsible for secretion of both hydrochloric acid and gastric intrinsic factor (IF), the glycoprotein essential for vitamin B12 absorption in the terminal ileum.

The main immune mechanism involves production of autoantibodies directed against two principal targets:

• Anti-gastric parietal cell antibodies (PCA): directed mainly against the H+/K+-ATPase proton pump and serving as a marker of autoimmune gastritis. Parietal-cell injury is mediated predominantly by T-cell immunity; these autoantibodies are not considered the principal direct effector of mucosal destruction.
• Anti-intrinsic factor antibodies (IFA): divided into “blocking” antibodies (type I), which inhibit binding between IF and vitamin B12, and “precipitating” antibodies (type II), which interfere with the interaction between IF and ileal receptors. These autoantibodies reduce residual IF efficacy and worsen vitamin B12 malabsorption even when a small amount of IF is still produced.

Over time, immune-mediated loss of parietal cells causes progressive atrophy of the gastric mucosa, with disappearance of normal gastric glands and replacement by a lymphoplasmacytic infiltrate and, in advanced stages, intestinal metaplasia. Chronic injury causes not only absence of intrinsic factor but also loss of acid secretion (achlorhydria) and reduced pepsin secretion, profoundly altering digestive physiology and mucosal protection.

From an immunopathogenetic perspective, pernicious anemia shows marked activation of CD4+ T lymphocytes specific for gastric parietal-cell antigens, which orchestrate a Th1 response with production of pro-inflammatory cytokines (IFN-γ, TNF-α) and activation of local macrophages and dendritic cells. The inflammatory infiltrate, composed of T lymphocytes, plasma cells and natural killer cells, contributes to tissue injury through direct cytotoxicity and release of enzymes and free radicals.

Progressive loss of parietal cells therefore causes:

• Absolute intrinsic factor deficiency: prevents formation of the IF-vitamin B12 complex and consequently selective ileal cobalamin absorption, causing vitamin deficiency even with an adequate diet.
• Achlorhydria: absence of acid secretion impairs release of vitamin B12 from food proteins and alters the gastric environment, favoring bacterial overgrowth and modifying iron and other micronutrient homeostasis.
• Hypergastrinemia: loss of acid function causes a compensatory increase in gastrin secretion by antral G cells, often detectable as elevated blood gastrin.
• Intestinal metaplasia and neoplastic risk: chronic atrophy and persistent inflammation predispose to dysplastic lesions and gastric neoplasms (adenocarcinoma and carcinoid tumors).

Systemically, loss of intrinsic factor causes selective vitamin B12 malabsorption, which over time leads to the classic abnormalities of erythropoiesis (megaloblastosis, macrocytic anemia), neurologic complications and extrahematologic manifestations typical of cobalamin deficiency, as already discussed on the general page.

In summary, the pathophysiology of pernicious anemia is a paradigm of organ-specific autoimmune disease: a chronic immune response against gastric antigens causes destruction of parietal cells, absence of intrinsic factor and vitamin B12 deficiency. Intrinsic-factor-mediated absorption is impaired, but a small amount of passive absorption can make high-dose oral therapy effective in selected patients; in autoimmune gastritis, permanent replacement is generally indicated, often intramuscularly.

Clinical manifestations

Pernicious anemia often presents insidiously, with a combination of symptoms attributable both to vitamin B12 deficiency and to the specific abnormalities related to autoimmune atrophic gastritis. Presentation is usually subtle and progression slow, so diagnosis may be delayed for several years.

Hematologically, the picture is dominated by macrocytic megaloblastic anemia, frequently accompanied by pancytopenia and, in more severe forms, subclinical jaundice due to increased indirect bilirubin from ineffective erythropoiesis. In older people or those with cardiovascular comorbidities, anemia may precipitate heart failure or angina, whereas younger patients often have progressive asthenia, exertional dyspnea, tachycardia and palpitations.

Neurologic manifestations are a distinctive feature of pernicious anemia and may precede diagnosis of anemia by months or years. Symmetric paresthesias of the hands and feet, hypoesthesia, loss of vibration and proprioceptive sensation, gait instability and sensory ataxia are common, as are spasticity, hyperreflexia, lower-limb weakness and, in advanced cases, sphincter disturbances. Central nervous system involvement may cause cognitive changes, psychomotor slowing, emotional lability and, in the most severe cases, true subacute dementia. These symptoms result from subacute combined degeneration of the spinal cord and require prompt recognition to prevent permanent sequelae.

Gastrointestinal manifestations frequently include atrophic glossitis, with a smooth, red and painful tongue, taste changes, angular cheilitis and swallowing disturbances. Gastric mucosal atrophy is associated with achlorhydria and impaired digestion, with a tendency toward dyspeptic symptoms, anorexia, weight loss, diarrhea or constipation. Autoimmune atrophic gastritis also confers an increased risk of gastric neoplasms, both carcinoma and carcinoid tumors, related to mucosal atrophy and metaplasia in the case of carcinoma and chronic hypergastrinemia in the case of carcinoid tumors.

An important element in the differential diagnosis from other forms of cobalamin deficiency is the frequent coexistence of other autoimmune diseases, including thyroiditis, type 1 diabetes, Addison disease and vitiligo, which may contribute to more complex and varied clinical presentations. In older patients, onset may present as worsening general condition, loss of autonomy, recurrent falls or cognitive decline, whereas the disease is extremely rare in children and young people.

Occasionally, skin hyperpigmentation, brittle nails, thin sparse hair, mucosal ulceration and general susceptibility to infection related to pancytopenia may occur. The variability and subtlety of symptoms make a high index of clinical suspicion essential, particularly in the presence of atypical neurologic signs or association with other autoimmune diseases.

Diagnosis and investigations

Diagnosis of pernicious anemia requires an integrated pathway combining clinical, laboratory and immunologic data, with particular attention to the features that distinguish it from other forms of vitamin B12 deficiency. Clinical suspicion typically arises in the presence of macrocytic anemia, pancytopenia and associated neurologic symptoms, especially in older patients or people with a personal or family history of autoimmune disease.

The complete blood count shows macrocytic megaloblastic anemia, often with reduced white blood cells and platelets. Peripheral smear shows macro-ovalocytes, poikilocytosis, anisocytosis and hypersegmented granulocytes, findings that are typical but not exclusive to pernicious anemia. Increased indirect bilirubin and LDH due to ineffective erythropoiesis are common.

Serum vitamin B12 measurement is the first step: low values (<200 pg/mL) are strongly suggestive of deficiency. With borderline values, increased homocysteine and methylmalonic acid help identify functional deficiency even at subclinical stages, confirming true vitamin B12 deficiency and distinguishing it from folate deficiency.

The distinctive diagnostic element of pernicious anemia is documentation of the autoimmune process by testing for specific autoantibodies. Anti-intrinsic factor antibodies have limited sensitivity, approximately 40-60% with variation among methods and populations, but are highly specific; their presence together with vitamin B12 deficiency is strongly indicative of pernicious anemia, while a negative result does not exclude it. Anti-gastric parietal cell antibodies are more sensitive but less specific and may also be found in other autoimmune conditions or in older people without anemia. The presence of both further strengthens diagnostic suspicion.

In patients with newly diagnosed pernicious anemia and no recent gastroscopy, gastroscopy with topographic gastric biopsies is indicated to document and stage autoimmune atrophic gastritis: histologic examination may show atrophy of the fundic and body mucosa, lymphoplasmacytic infiltration, loss of oxyntic glands and, in advanced stages, intestinal metaplasia. Biopsy also allows identification of associated dysplastic or neoplastic lesions, given the increased risk of gastric adenocarcinoma and neuroendocrine tumors in these patients.

Assessment of gastric function may be completed by measurement of serum gastrin, which is frequently elevated because of loss of acid secretion and compensatory stimulation of antral G cells. In some cases, measurement of pepsinogen I may also be useful because it is reduced in fundic atrophic gastritis.

Assessment for associated autoimmune diseases (thyroiditis, type 1 diabetes, Addison disease) should be guided by symptoms, history and clinical context rather than performed as indiscriminate universal screening.

Bone marrow examination remains reserved for selected cases, particularly atypical cytopenic presentations or suspected myelodysplastic syndromes. Instrumental neurologic assessment by electromyography or nerve conduction studies may be useful for subclinical signs or to monitor the evolution of peripheral neuropathy.

In summary, diagnosis of pernicious anemia integrates demonstration of vitamin B12 deficiency with the presence of anti-intrinsic factor antibodies and/or endoscopic-histologic evidence of autoimmune atrophic gastritis; no single element should be considered obligatorily positive in every patient. Clinical, laboratory, immunologic and histologic data allow complete characterization and prompt identification of possible complications.

Treatment and prognosis

Management of pernicious anemia is based on several specific principles that distinguish it from other forms of vitamin B12 deficiency. The cornerstone is prompt correction of vitamin deficiency, but management of complications, prevention of recurrence and monitoring of associated autoimmune conditions and gastric complications are equally important.

Replacement treatment requires lifelong vitamin B12. In autoimmune gastritis, intramuscular cyanocobalamin or hydroxocobalamin is generally recommended according to local protocols; high oral doses may be effective in selected patients through passive absorption independent of intrinsic factor. Regimen and intervals depend on formulation, neurologic severity and clinical response.

Response to treatment typically includes a reticulocyte response, with increased circulating reticulocytes, within 5-7 days after therapy begins, followed by progressive normalization of hematologic values and resolution of anemic symptoms. Neurologic manifestations improve more slowly: complete recovery is possible only when treatment is started early, whereas injury caused by prolonged deficiency may be partially or completely irreversible.

A fundamental component of management is prevention of recurrence and long-term follow-up. Because loss of intrinsic factor is irreversible, vitamin B12 replacement must be continued for life. Interrupting treatment exposes patients to recurrence of symptoms and complications, particularly neurologic ones. Follow-up is based on hematologic parameters and monitoring of neurologic symptoms; during intramuscular treatment, routine repeat measurement of serum vitamin B12 is not indicated. These assessments allow therapy to be adjusted and make it possible to intervene promptly when necessary.

Patients with pernicious anemia have an increased risk of gastric neoplasms because of autoimmune atrophic gastritis. Endoscopic assessment is indicated at diagnosis if not recently performed; subsequent surveillance should be determined according to the extent and grade of atrophy, metaplasia, dysplasia and additional risk factors.

Another aspect is management of associated autoimmune diseases. Thyroid, adrenal, pancreatic or other autoimmune evaluation should be guided by history, symptoms, clinical signs and individual risk factors.

The prognosis of patients treated early is generally favorable, with complete hematologic recovery and prevention of neurologic sequelae in most cases. However, delays in diagnosis or treatment may lead to permanent neurologic injury, impaired quality of life and increased mortality, especially in older people or those with comorbidities.

Patient education regarding the chronic nature of the disease, the need for lifelong therapy, the importance of periodic follow-up and surveillance of neoplastic risk is essential to ensure adherence and optimal outcomes.

Complications

Complications of pernicious anemia reflect both chronic vitamin B12 deficiency and the consequences of autoimmune atrophic gastritis. The main risk is neurologic injury, which may become irreversible if deficiency is not corrected early. Subacute combined degeneration of the spinal cord may progress to permanent motor and sensory deficits, ataxia, sphincter disturbances and cognitive changes up to subacute dementia. In the most severe cases, loss of autonomy and persistent disability impair quality of life.

Hematologically, prolonged deficiency may cause severe pancytopenia, with increased risk of recurrent or severe infections, mucocutaneous bleeding and worsening of pre-existing cardiovascular disease. In older patients, the combination of anemia, muscle weakness and neurologic abnormalities increases the risk of falls, fractures and loss of self-sufficiency.

A distinctive aspect of pernicious anemia is gastric neoplastic risk: chronic autoimmune atrophic gastritis promotes development of gastric carcinoma and carcinoid tumors. Indications for and intervals of endoscopic surveillance should be individualized according to histologic findings and risk profile.

Possible autoimmune complications should also be considered: patients with pernicious anemia have a greater likelihood of developing other autoimmune diseases during life, such as chronic thyroiditis, Addison disease, type 1 diabetes, vitiligo and celiac disease, which may require dedicated monitoring and treatment.

Finally, the chronic nature of the disease and the need for lifelong replacement therapy have a non-negligible psychological and social impact related both to treatment adherence and management of long-term complications.

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