Introduction

An inflammatory disease called pernicious anaemia stops the body from absorbing vitamin B12, which results in fewer red blood cells that carry oxygen throughout the body. Years may pass with no discernible changes, and if treatment is not received, it may result in major health problems, such as irreparable nervous system damage. Medical professionals prescribe vitamin B12 supplements to treat pernicious anaemia. It impacts some bodily systems, such as the nervous, heart, and digestive systems, weakness and exhaustion. With a median prevalence of 151 per 100,000 in the US, pernicious anaemia primarily affects individuals of Northern European ancestry between the ages of 60 and 80.¹

Vitamin B12 and intrinsic factor (IF) deficiencies combine to create pernicious anaemia, a complicated illness. A glycoprotein called IF, which is released by parietal cells, binds to B12 and makes it easier for the ileum to absorb it. Anti-IF antibodies stop B12 from attaching to IF, which stops intestinal absorption or the creation of the B12/IF complex. Up to 25% of patients with autoimmune gastritis (AIG), which affects the corpus and fundus but spares the antrum, have pernicious anaemia. IF and hydrochloric acid are produced when parietal cells in the oxyntic mucosa are destroyed by antiparietal cell antibodies. Because fewer parietal cells are available to create the IF required for dietary B12 absorption, achlorhydria reduces the release of cobalamin attached to dietary protein. Additionally, co-occurring autoimmune diseases such as vitiligo, autoimmune thyroid disease, and type 1 diabetes are more common in patients with pernicious anaemia.² 

The cytological and numerical characteristics of the marrow cells, the spatial interactions between cells, and the overall marrow structure can all be revealed by bone marrow aspiration and biopsy. The nucleus is big and has coarse, motley chromatin clumps, and erythroid precursors are massive and frequently oval. The nucleus's maturity and the hemoglobinization of the orthochromic megaloblastic normoblasts are not related when there is an imbalance in the nucleus's rate of maturation compared to the cytoplasm. Mature neutrophils and eosinophils are hypersegmented, and there are giant metamyelocytes and bands. 

Similar to folic acid insufficiency, cobalamin deficiency causes dramatic alterations in bone marrow histology within 12 hours of starting the proper treatment.³ The main bone marrow abnormalities in pernicious anaemia are examined in this article, along with their pathophysiology underpinnings and clinical importance for diagnosis and treatment monitoring.

Pathophysiology of Pernicious Anaemia Leading to Bone Marrow Changes

The main cause of pernicious anaemia is decreased dietary B12 absorption brought on by an IF deficit. Megaloblastic anaemia results from disturbed DNA synthesis and demyelinated nerves, which are caused by vitamin B12, which is necessary for erythropoiesis and nerve myelination. 

Erythrocytes exhibit the most notable alterations in blood components, and the degree of anaemia is directly correlated with the severity of morphologic abnormalities in red blood cells. In the acidic stomach environment, B12 is liberated from food carrier proteins by proteolysis and attaches itself to haptocorrin to prevent its breakdown. It then attaches to IF made by gastric parietal cells after being liberated from haptocorrin in the small intestine by pancreatic proteases. 

After binding to cubam receptors in the terminal ileum, the IF-B12 complex is endocytosed, releases transcobalamin into the bloodstream, and is then transported to transcobalamin cell receptors. When these coenzymes are unable to function properly due to a B12 shortage, their substrate builds up in plasma. Excess homocysteine can lead to apoptosis, cellular stress, and changes in the structure and function of proteins.²

Diagnosis

Anaemia, macrocytosis, and pancytopenia can all be detected with a complete blood count (CBC) in patients with B12 deficiency, though approximately one-third of patients may not have macrocytosis. Iron deficiency may occur concurrently with or before a diagnosis of pernicious anaemia, and 25 to 30% of patients who are eventually diagnosed with pernicious anaemia may exhibit neurologic signs and symptoms without anaemia or macrocytosis.²

Anisopoikilocytosis, hypersegmented neutrophils, and macro-ovalocytes can all be seen in a peripheral blood smear. Although they may be uncommon or nonexistent in more advanced disease, hypersegmented neutrophils usually appear before macrocytosis and anaemia. The sensitivity and specificity of an isolated serum cobalamin level are inadequate for accurately identifying a B12 deficit. Since up to 35% of patients with pernicious anaemia have falsely inflated serum cobalamin levels (≥200 ng/L), these patients may have a B12 deficiency.²

A CBC can be used to detect anaemia, macrocytosis, and pancytopenia in patients with B12 deficiency. However, about one-third of patients may not have macrocytosis. Iron deficiency may precede a pernicious anaemia diagnosis or be concomitant with it. Pernicious anaemia can present with nonanemic macrocytosis months before the diagnosis, and neurologic signs and symptoms can occur in the absence of anaemia or macrocytosis in 25 to 30% of patients eventually diagnosed with pernicious anaemia. An isolated serum cobalamin level has poor sensitivity and specificity for reliably detecting B12 deficiency. Patients with pernicious anaemia and serum cobalamin levels ≥200 ng/L may have true B12 deficiency because levels can be falsely elevated in up to 35% of these patients.²

The CobaSorb test has shown promise, but it does not discriminate between gastric and intestinal causes of B12 malabsorption and cannot be used once a patient is treated with B12. In patients with serum cobalamin levels ≥200 ng/L, diagnostic performance may be enhanced by measuring serum methylmalonic acid (MMA), fasting homocysteine, and HTC. Elevated MMA and/or fasting homocysteine are indicators of B12 deficiency in patients without evidence of impaired renal function or an inherited cobalamin processing enzyme defect. Homocysteine levels may also increase in folate deficiency, pyridoxine deficiency, and hypothyroidism. Serum HTC levels may be used for assessing B12 status, with or without MMA and/or homocysteine, and may reflect B12 absorptive capacity.²

Folate levels should be determined to exclude macrocytic anaemia secondary to folate deficiency and because treating B12-deficient patients with folate alone may worsen associated neurologic damage. Anti-IF antibodies with or without antiparietal cell antibodies are 40 to 60% sensitive in detecting pernicious anaemia, with the rate of positivity rising with disease progression. Combining anti-IF antibodies with antiparietal cell antibodies significantly increases their diagnostic performance for pernicious anaemia (73% sensitivity and 100% specificity).²

Key Bone Marrow Findings in Pernicious Anaemia

Histological examination after a bone marrow biopsy is usually unnecessary, as it will show hypercellularity with a shift toward immaturity, including abnormal maturation of myeloid and erythroid cell lines. Pernicious anaemia is found in up to 25% of patients with AIG as a late-stage manifestation. The fasting serum gastrin assay can play a role in establishing a pernicious anaemia diagnosis in difficult cases. A deficiency in gastric intrinsic factor output after pentagastrin stimulation is specific for the diagnosis of pernicious anaemia and can be used when evaluating serological markers in certain patients.²

Because folate alone may exacerbate related neurologic impairment in people with B12 insufficiency, folate levels should be measured to rule out macrocytic anaemia caused by folate deficit. With or without antiparietal cell antibodies, anti-IF antibodies have a 40–60% sensitivity in identifying pernicious anaemia, and the rate of positivity increases as the illness worsens. Their diagnostic performance for pernicious anaemia is greatly improved when anti-IF antibodies and antiparietal cell antibodies are combined (73% sensitivity and 100% specificity).²

Following a bone marrow biopsy, histological analysis is typically not required because it will reveal hypercellularity along with a change towards immaturity, including aberrant myeloid and erythroid cell line maturation. Up to 25% of people with AIG have pernicious anaemia as a late-stage symptom. In challenging situations, the fasting serum gastrin assay can help establish a diagnosis of pernicious anaemia. When assessing serological markers in some patients, a deficiency in stomach intrinsic factor output following pentagastrin stimulation can be utilised as a particular diagnostic tool for pernicious anaemia.²

Clinical Implications and Treatment Response

Monthly injections are used as part of the treatment to raise vitamin B12 levels; more injections are needed for extremely low levels. Large dosages of vitamin B12 pills or a particular kind given by the nose may be beneficial for certain people.⁴

Summary

Megaloblastic anaemia and neurological problems are caused by vitamin B12 deficiency brought on by reduced intrinsic factor production in pernicious anaemia, a crippling inflammatory disease. Serological indicators like anti-IF antibodies validate the diagnosis, whereas bone marrow abnormalities like megaloblastic erythropoiesis and hypersegmented neutrophils offer crucial diagnostic information. To avoid irreparable cognitive damage and haematological deterioration, early identification and lifelong B12 supplementation via intramuscular injections or high-dose oral/nasal therapy are crucial. Finding non-anaemic or iron-deficient instances is still difficult, despite improvements in diagnostic tests. Future studies should concentrate on enhancing screening techniques and comprehending how pernicious anaemia interacts with other autoimmune diseases. Effective care is ensured by prompt intervention, underscoring the significance of physician understanding in reducing the long-term effects of this illness.