Macrocytic Anemia

Etiology

Megaloblastic Anemia

Megaloblastic anemia occurs from deficiencies in folic acid and vitamin B12. Folate deficiency is due to diminished intake (eg, alcohol abuse or malnutrition), increased consumption (eg, hemolysis or pregnancy), and malabsorption (eg, familial, gastric bypass, or medications like cholestyramine or metformin). Vitamin B12 deficiency appears in diminished intake (eg malnutrition), malabsorptive states (eg, atrophic gastritis, either autoimmune or nonautoimmune, from Helicobacter pylori or Zollinger-Ellison syndrome, Diphyllobothrium tapeworm infection, gastric bypass, or ileal resection), or the presence of antagonists (eg, nitrous oxide).[3] Drugs that impair DNA synthesis include folic acid analogs (eg, methotrexate and trimethoprim-sulfamethoxazole), nucleic acid analogs (eg, 5-fluorouracil), and others, including hydroxyurea, pentamidine, phenytoin, pyrimethamine, sulfasalazine, and triamterene. An increase in the MCV in patients with rheumatoid or psoriatic arthritis portends a good response to methotrexate (MTX).[4] However, an increase in the MCV also warrants concern over heme toxicity. Many patients with arthritis have concurrent anemia, and the toxicity of MTX compounds this.

Nonmegaloblastic Anemia

Nonmegaloblastic anemia, which lacks hypersegmented neutrophils, occurs in various settings. Benign conditions include alcohol consumption, red blood cell (RBC) toxicity, hereditary spherocytosis (ie, impaired volume regulation increases red cell size), hypothyroidism, liver disease due to lipid deposition in the cell membrane, and marked reticulocytoses from states of excess RBC consumption, such as hemolysis or turnover in pregnancy or primary bone marrow disease (ie, reticulocytes are larger than the average RBCs).

Other Macrocytosis Etiologies

Some cases of macrocytosis are normal variants associated with a genetic predisposition or found in infants, patients with Down syndrome, and pregnant women.[5] Specific mutations associated with macrocytosis include HGprt deficiency (Lesch-Nyhan disease), ALAS2, and KLF13.[6][5][7] Other spurious findings include hyperglycemia, concentrating the blood. When diluted, the RBCs swell with volume, leukocytosis, and paraproteinemia, increasing sample turbidity for overestimates of RBC size or operator error from occlusion of microscope aperture or sample left out at room temperature too long.[8][9] 

Extreme macrocytosis (MCV >130 fL) can be seen with HIV medicines, particularly zidovudine, stavudine, and lamivudine). The resolution of the anemia from HIV is seen as a positive effect of HAART therapy.[10] However, the aforementioned agents are suspected of interfering with B12/DNA, which, in a patient who may be already malnourished, can compound the situation.  

Epidemiology

Macrocytosis affects 2% to 4% of the population, 60% of whom have anemia. Alcohol use accounts for the majority, followed by deficiencies in folate and vitamin B12, and medications. Autoimmune causes are more common in middle-aged women. Hypothyroidism and primary bone marrow disease account for more cases of macrocytic anemia in older patients. The prevalence of vitamin B12 deficiency increases in patients older than 60.

Mild macrocytic anemia (MCV 100 to 110 fL) is more likely to be caused by benign conditions compared to marked macrocytic anemia (MCV >110 fL), the latter of which is due to primary bone marrow disease or megaloblastic anemia from folate or vitamin B12 deficiencies.[11][12] 

Pathophysiology

The equation for mean corpuscular volume [MCV (fL) = Hct (%) X 10/RBC (106/µg)] explains how macrocytic anemia represents sizable RBCs in comparison to the total amount. Folate and vitamin B12 are necessary for RBC nucleic acid synthesis. Without DNA or RNA, erythropoiesis is ineffective with nuclear/cytoplasmic asynchrony, resulting in larger erythrogenic precursors with abnormal nuclei (eg, hypersegmentation) but normal cytoplasms. Anemia occurring in the presence of macrocytosis and hypersegmented neutrophils is known as megaloblastic anemia. The absence of hypersegmented neutrophils characterizes nonmegaloblastic anemia due to abnormalities involving the RBC membrane, excess erythrocytic precursors, increased cell volume, or RBC toxicity.

The average individual's daily folate requirement is 100 to 200 µg; the body can absorb 400 µg/day. Healthy individuals typically have stored sufficient folate for 4 months. Folate is primarily absorbed in the small bowel. In comparison, the average individual's daily vitamin B12 requirement is 1 µg, and the body can absorb 2 to 3 µg/day. Most individuals have several years of B12 stores. Vitamin B12 is absorbed in the ileum when bound by an intrinsic factor, a protein the gastric parietal cells produce. Abnormalities in these cascades cause deficiencies in folate or vitamin B12, respectively.[13] 

Histopathology

Megaloblastic macrocytic anemia contains hypersegmented neutrophils and macro-ovalocytes (macrocytes having an oval shape with a reduced or absent central pallor) on a peripheral blood smear (PBS) (see Image. Macrocytic Anemia). Anisocytosis and poikilocytosis are not uncommon due to ineffective erythropoiesis. Bone marrow evaluation will demonstrate hypercellularity with abnormal maturation and proliferation of myeloid cell lines, particularly the erythroid lineage. 

Nonmegaloblastic macrocytic anemia will not have hypersegmented neutrophils; instead, its PBS will show round macrocytes or macro reticulocytes, in addition to other cells representative of the underlying etiology including acanthocytes from liver disease, myeloid dysplasia, or immaturity in primary bone marrow disease, polychromatophilic RBCs with reticulocytosis, schistocytes from hemolysis, or spherocytes in inherited hemolytic anemias, eg, hereditary spherocytosis. The term "polychromasia" describes the color variation in the peripheral blood smear (PBS) from reticulocytosis. Reticulocytes have a bluish-purple-like hue, contrasting with the erythrocyte red. This contrast, this variation, is suggestive of reticulocytosis, especially when considered in a field of younger, larger red blood cells.

History and Physical

The presentations of macrocytic anemia are dependent on the underlying cause.

Clinical History

Patients with vitamin B12 deficiency complain of mood disturbances and neurologic symptoms like loss of balance, memory loss, paresthesias, and peripheral neuropathy. Other symptoms are typically more specifically associated with an underlying cause for vitamin B12 deficiency, eg, constitutional symptoms from primary bone marrow disease, gastrointestinal upset secondary to enteral malabsorption, or hypoactive metabolism from hypothyroidism.

Folate deficiency has similar clinical features to vitamin B12 deficiency; however, Patients with folate deficiency typically do not have the neuropsychiatric complaints associated with vitamin B12 deficiency. Clinicians should also obtain a clinical history that may help to identify an underlying etiology. Prior surgeries associated with macrocytic anemia, including a gastric bypass for weight loss or ileal resection, are likely causations. Family or personal medical histories may reveal autoimmune diseases, hemolytic anemias, or an inherited cause.

The Imerslund-Grasbect syndrome is an autosomal recessive disorder manifesting a decreased B12 absorption, resulting in megaloblastic anemia.[14] This syndrome is found primarily in patients of Scandinavian descent. Furthermore, certain medical conditions may require medications that increase the risk of developing macrocytic anemia. Additionally, a review of dietary intake can reveal nutritional causes of vitamin B12 or folate deficiencies, eg, excessive alcohol consumption or limited food choices (“tea and toast”). 

Physical Examination Findings

Physical exam may reveal nonspecific anemia findings (eg, conjunctival pallor) or neurologic deficits due to vitamin B12 deficiency (eg, impaired proprioception or vibration, positive Romberg sign). Stigmata of underlying diseases may also be noted, including glossitis from autoimmune atrophic gastritis, hepatosplenomegaly from familial hemolytic anemias, hypopigmentation from vitiligo, or jaundice and spider angiomata from alcohol use. 

Evaluation

Initial evaluation of macrocytic anemia includes a thorough history and physical followed by focused laboratory studies, eg PBS, reticulocyte count, and serum B12. RBC indices alone will underestimate the presence of macrocytosis by 30% because MCV represents the mean distribution curve of all RBCs and fails to identify smaller macrocytes. If the PBS has normal findings, with no megaloblastic RBCs noted, additional assessments for liver and thyroid diseases (ie, aminotransferases and thyroid stimulating hormone) should be ordered, as these are the most common causes of nonmegaloblastic anemia.

The reticulocyte count should be evaluated if megaloblastic RBCs are demonstrated on PBS. A reticulocyte count <1% indicates underproduction, while a finding of >2% is associated with hyperproliferation due to hemolysis or hemorrhage, which should prompt a hemolytic anemia work-up. Additionally, if findings consistent with hyperproliferation are reported, a vitamin B12 level should be performed.

Vitamin B12 values <100 pg/mL indicate deficiency; an RBC folate level (not serum folate due to lack of sensitivity) should be ordered for a B12 >400 pg/mL. However, low levels may also indicate folate deficiency. Normal B12 levels may indicate the need for bone marrow evaluation. Homocysteine and methylmalonic acid (MMA) should be measured in patients with a B12 level of 100 pg/mL to 400 pg/mL, as these are biochemical compounds important in cell metabolism pathways and use folate and vitamin B12 as cofactors.[15] The measurement of serum B12 alone can miss upwards of 50% of cases.

When checked alone, the shortcomings of B12 elevations cast favor towards checking both the MMA and homocysteine measurements.[15] MMA is only elevated in patients with vitamin B12 deficiency; homocysteine is elevated in patients with folate deficiency. Normal values may warrant hematology consultation for bone marrow studies. Specialist consults are recommended in patients with abnormal myeloid morphology on PBS (disordered immaturity, hypogranulated or hyposegmented neutrophils, or additional cytopenias). Clinicians should remember that the patient may have macrocytic anemia with concurrent iron-deficient anemia. Under these circumstances, the MCV may be normal (ie, automated CBC measurements may "cancel out" the high and low indices, but the RDW would be increased). 

Evaluations for specific causes of megaloblastic anemia should be based on presentation. Antibodies to intrinsic factors or parietal cells are indicative of pernicious anemia. The Schilling test, once used to measure vitamin B12 uptake before and after administration of intrinsic factors, has fallen out of favor.[8][16]   

Treatment / Management

Macrocytic Anemia Management

Management of macrocytic anemia comprises deficiency correction. Therefore, macrocytic anemia should be treated by replacing folate or vitamin B12 to resolve the underlying cause. Folic acid, 1 mg to 5 mg orally daily, is recommended, and clinicians should encourage patients to consume folate-rich diets (eg, fortified cereals and leafy vegetables). Patients who are taking folate antimetabolites (eg, methotrexate) or pregnant women (especially those with a history of neural tube defects or taking antiepileptics) should take daily supplements to prevent deficiencies. Clinicians should not overlook vitamin B12 deficiency as a cause of macrocytic anemia.

Treatment of folate deficiency without addressing vitamin B12 deficiency, if present, will result in the resolution of the anemia but not the neurologic effects. Clinicians should prescribe 1000 µg of oral vitamin B12 daily for 1 month, followed by 125 to 250 µg daily, or administer 1000 µg of intramuscular B12 every week for 4 weeks, then monthly to replace vitamin B12 stores, the latter of which is preferred for patients with pernicious anemia or altered gastrointestinal anatomy. Clinicians may prescribe empiric folate supplementation (400 µg to 1 mg/d) in patients receiving vitamin B12 replacement. Reticulocytosis will improve within 1 to 2 weeks, and anemia should resolve after 4 to 8 weeks.

Treatment Monitoring

Monitoring RBC indices or rechecking folate or vitamin B12 levels and their metabolites during active treatment is unnecessary, though some clinicians check yearly complete blood counts in patients taking long-term vitamin B12 therapy. Neurologic symptoms from vitamin B12 deficiency take longer. Alcohol-related macrocytosis resolves with abstinence. Other nonmegaloblastic anemias improve when the underlying conditions are treated. 

In other instances, macrocytosis indicates medical compliance (eg, methotrexate or zidovudine) and does not need additional management beyond supplements to prevent anemia.[17][18] Assessing MMA and serum B12 levels to judge the patient's compliance and efficacy of therapy is recommended.[19] 

Differential Diagnosis

Several differential diagnoses should also be considered when evaluating macrocytic anemia.

Folate deficiency anemia

A subset of this entity is the "functional" or "facultative" folate deficiency.[20][21][22] This occurs in high-demand states such as pregnancy or severe hemolysis, where the body attempts to augment red cell production, increasing the demand for folate and exhausting the folate reserves. Inadequate folate reserves/intake during pregnancy is a leading cause of the development of neural tube defects (NTD) in newborns. For this reason, "fortification" of cereal products with folate has been mandated.

Folate administration can help compensate for the functional deficiency caused by increased erythrocyte production in cases of significant hemolysis, such as sickle cell disease. In infants, folate deficiency decreases the prevalence of NTD, anemia, and serum homocysteine, reducing the risk of developing cardiovascular disease. 

Anemia due to liver disease

Anemia is a consistent factor in cirrhosis, with upwards of three-quarters of patients manifesting it.[23][24] Although factors exist to support macrocytosis, they are sometimes overshadowed by iron deficiency, the most common cause of anemia in these patients. For macrocytic anemia in cirrhosis, folate deficiency is prevalent in 40% of patients, while B12 deficiency is prevalent in 30% to 40%.

Alcohol use can lead to B12 and folate deficiency, as well as have a direct toxic effect on the erythroid precursors. B12 and folate coenzymes are required for Thymidylate and purine biosynthesis. A deficiency would cripple DNA synthesis leading to delayed nuclear maturation and resulting in macrocytosis.[24] In alcoholic liver disease, the resultant splenomegaly (from portal hypertension) can have a sequestration and hemolytic effect, thereby leading to macrocytosis.[23] By various pathways, alcohol can cause significant reticulocytosis, leading to an overall increase in the MCV (as reticulocytes are large cells).

With liver disease, an increased deposition of cholesterol is noted on the red cell membranes, leading to an increase in their surface area.[24] Macrocytic anemia is found to be associated with the severity of liver disease, a gauge, as it were, in following hepatic decompensation. However, cirrhosis remains the harbinger of variceal hemorrhage, spontaneous bacterial peritonitis, encephalopathy, and jaundice.[25] The 5-year survival for compensated cirrhosis is about 84%; it is <35% if decompensated. 

Hypothyroidism

Hypothyroidism anemia is most commonly normocytic, with macrocytic and microcytic findings less frequent.[26] Its presentation may be somewhat variable if other comorbidities exist, such as bone marrow depression, decreased erythropoietin production, or deficiency of iron, B12, and folate. Hypothyroidism decreases the number and proliferative potential of erythroid cells in the bone marrow.

Additionally, the bone marrow ground substance undergoes a gelatinous transformation, as shown by mucopolysaccharide accumulation. For patients with an increased RDW, in the absence of an iron deficiency, thyroid function tests are advocated, along with the testing of B12 and folate. B12 deficiency can be found with hypothyroidism in the setting of autoimmune disease.[27][26] Thyroid dysfunction should also be considered if treatment-resistant or refractory anemia should occur. 

Myelodysplastic syndrome

Macrocytosis (MCV >100 fL) is considered a hallmark of megaloblastic anemia, but other findings and incongruities can obscure the diagnosis and even point towards incorrect ones, like myelodysplasia (MDS).[28][22] Characteristics of MDS that would help avoid a misdiagnosis include the lack of clonality and cytogenetic anomalies, bilineage cytopenias, granulocytic hyposegmentation or hypogranulation, megakaryocytic hypo lobulation, hypogranular platelets, and the presence of increased blasts.

Although the presence of bilineage cytopenias does support a diagnosis of MDS, leukopenia, and thrombocytopenia occur in megaloblastic anemias. The bone marrow in this entity does show megaloblastic precursors with nuclear-cytoplasmic dysynchrony. Indirectly, evidence of hemolysis may be present, eg, an increased LDH or a decreased haptoglobin. Hypercellularity may be evident. Giant metamyelocytes and bands, hyperpigmented neutrophils (felt most specific for megaloblastic anemia), and hyperlobulated megakaryocytes may be present. The proper diagnosis can be cloaked in dysmorphisms, 

Alcohol use disorder

The etiology of macrocytosis in alcoholism can be associated with folate deficiency alone but is often multifactorial.[29] The liver stores 50% of the body's folate in deposits, but additional deposits exist in the connective tissue, erythrocytes, kidneys, and the gastrointestinal tract. About 1 in 4 patients with megaloblastic anemia show low serum folate levels and 1 in 10 show low deposits. Megaloblastic anemia with ringed sideroblasts is a well-recognized entity in alcoholism.[30] Macrocytosis in alcoholism is reversible.

Alcohol causes anemia via 2 mechanisms. The first is the development of megaloblastic hematopoiesis by inducing folate deficiency. The second is nonmegaloblastic, macrocytic anemia due to the direct toxic effect of alcohol on the erythroid precursors in the marrow, independent of folate depletion. These bone marrow anomalies are reversible. Folate deficiency in alcoholism is often attributed to dietary deficiency. However, other situations can cause it, eg, cancer (with its increased folate requirement), renal dialysis, increased renal excretion, malabsorption, drugs (eg, antiseizure medications and metformin), and impaired hepatic uptake leading to scanty deposits.[29] Abstinence from alcohol can bring some reversibility to this damage, allowing some macrocytic anemia to be treated without medication.[30]

Prognosis

The prognosis for macrocytic anemia is excellent, provided the underlying cause is identified and treated early. Specialist referral is rarely needed unless the anemia is resistant to therapy or there is evidence of underlying myelodysplasia or leukemia.

Complications

Patients with chronic megaloblastic anemia from vitamin B12 deficiency can develop permanent neurologic damage, referred to as subacute combined neurodegeneration. Patients will have gait ataxia, memory loss, peripheral neuropathy, and psychiatric disturbances. Patients with macrocytic anemia from underlying conditions will experience complications of their respective diseases.