Macrocytic anemia: Pathology review

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Case Study0:00–0:34

In the hematology ward. Two people came in with the same symptoms, easy fatiguability, exertional dyspnea and weight loss.
One of them is a 65 year old Caucasian individual named Bobby and the other one is a 50 year old Hispanic individual named Sarah Bobby complains of frequent falls.
While Sarah admits she is a chronic user of alcohol. Their lab tests show decreased hemoglobin levels.
Both Bobby and Sarah are suffering from anemia which is defined as lower than average levels of hemoglobin typically below 13.5 g per deciliter in adult men and below 12.0 g per deciliter in adult women.

Pathology0:34–1:48

Now, anemias can be broadly grouped into three categories based on mean corpuscular volume or MCV, which reflects the volume of a red blood cell or RBC.
So, microcytic anemia is where the M CV is lower than 80 femtoliter. Normocytic with an M CV between 8100 femtolitre and macrocytic with an M CV, larger than 100 femtolitre.
Now, let's focus on the macrocytic anemias. The two most common causes are vitamin b12 deficiency and folate deficiency orotic aciduria.
Fanconi anemia. Diamond Blackfan anemia are also macrocytic.
Finally, liver diseases. And hypothyroidism can also cause this type of anemia but their mechanisms are not well understood.
Ok. Macrocytic anemias can be classified based on the presence of megaloblasts.

Megaloblastic macrocytic1:48–3:23

These are large immature red blood cells produced when the cytoplasm develops normally, but the DNA synthesis is impaired and cell division is delayed.
So when there's defective DNA synthesis or defective DNA repair, like in folate deficiency, vitamin b12 deficiency orotic aciduria and fanconi anemia.
there's megaloblastic macrocytic anemia. Megaloblastic anemia can also affect white blood cell production.
So the bone marrow starts releasing large immature neutrophils with hyper segmented nuclei, meaning their nucleus has more than five lobes.
These are called hypersegmented neutrophils and are a key finding on the peripheral blood smear of individuals suffering from megaloblastic anemia.
So, for your exams, remember that the peripheral blood smear and megaloblastic anemia shows RBC macrocytosis, megaloblasts in hyper segmented neutrophils.
All right. Now, in diamond Blackfan anemia and liver disease DNA synthesis is not impaired and there's nonmegaloblastic macrocytic anemia.
The peripheral blood smear shows RBC macrocytosis without megaloblasts or hypersegmented neutrophils. Ok.
So let's take a closer look at these different anemias starting with vitamin b12 or cobalamin deficiency. One cause is decreased dietary intake because vitamin B12 is found in animal and dairy products such as eggs, meat or milk.

Vitamin B12 Deficiency 3:23–6:51

So, it's often seen in vegans who don't take vitamin supplements. An important fact is that large quantities of B12 are stored in the liver.
So it could take years for decreased dietary intake symptoms to develop for your exams. It's important to know that the most common cause of vitamin B12 deficiency is defective absorption and you need to know the different ways this could happen.
One cause is gastric atrophy. Now, normally dairy and animal products are broken down in the stomach by Pepsin which releases B12.
For this to happen. The stomach must be very acidic in order to convert pepsinogen into pepsin.
Now, in gastric atrophy, there's a decrease of hydrochloric acid production in the stomach which leads to decreased acidity and prevents the release of vitamin B12.
Gastric bypass is another potential cause since food passes through the stomach too quickly and not enough B12 is released.
Another cause of decreased absorption is pernicious anemia. Ok.
So the stomach parietal cells make a protein called intrinsic factor, which binds to vitamin b12. And this complex passes into the intestines.
When the complex reaches the terminal ileum. The enterocytes recognize the intrinsic factor and absorb the whole complex in pernicious anemia.
The body produces antibodies against intrinsic factor or parietal cells leading to decreased absorption. Also any terminal ileal diseases like inflammatory bowel disease or ileal resection can damage the enterocytes and prevent absorption.
Finally, infections like Diphyllobothrium latum or fish tapeworm and bacterial overgrowth in the ileum also interfere with absorption leading to vitamin b12 deficiency.
Now, vitamin B12 is used to synthesize DNA precursors which is essential for cell division when B12 levels are low or absent cell division is impaired, so rapidly dividing cells, like those in the bone marrow are the most affected.
This includes precursors for platelets and both red and white blood cells. Leading to thrombocytopenia anemia and possibly to leukopenia.
When all three blood cell lines are affected, it's called pancytopenia. Another high yield fact is that b12 deficiency increases homocysteine and methylmalonic acid in the body.
This is because vitamin b12 is an important cofactor that aids the conversion of homocysteine to methionine and the conversion of methyl malonyl coa to succinyl coa.
Ok. Moving on to folate or Vitamin B9 deficiency, which can be caused by decreased dietary intake, increased demand or impaired absorption.

Folate Deficiency6:51–10:01

Main sources of Vitamin B9 include leafy greens and citrus fruits like oranges and lemons. Now, folic acid present in these foods are generally in the polyglutamate form which is almost nonabsorbable.
So, special enzymes in the jejunal mucosa like the intestinal conjugase, cut down the polyglutamate residues into monoglutamate, which can be absorbed once inside the enterocytes of the jejunum.
An enzyme called dihydrofolate reductase converted into tetrahydrofolic acid or th F which then goes into the portal circulation.
Now, the liver also stores some Vitamin B9 along with vitamin B12, but only enough Vitamin B9 for a few months while there's enough B12 stored for years.
So for your exams, remember that an individual with a folate deficient diet will experience symptoms more rapidly than B12 deficiency folate deficiency due to increased demand occurs in conditions like pregnancy and hemolytic anemias.
Because more red blood cells need to be produced for impaired absorption. Medications like phenytoin trimethoprim, methotrexate sulfaSALAzine, as well as alcohol can interfere with folic acid absorption in the jejunum.
More specifically phenytoin inhibits the intestinal conjugase, which normally converts the polyglutamate form of folate into monoglutamate trimethoprim.
Inhibits dihydrofolate reductase which converts dihydrofolate into tetrahydrofolate. An alcohol like sulfaSALAzine decreases the expression of a transporter protein that moves folic acid from the enterocytes to the portal vein.
Now, folic acid is used to synthesize purines and pyrimidines which are essential for DNA replication and cell division.
So, in the absence of folic acid cell division is impaired and therefore rapidly dividing cells in the bone marrow are affected like red and white blood cells as well as platelet precursors leading to anemia, leukopenia and thrombocytopenia for the test.
Remember, homocysteine levels are elevated like vitamin b12 deficiency but methylmalonic acid levels are normal. This is because folate is a cofactor that aids the conversion of homocysteine to methionine but does not affect the conversion of methylmalonyl coa to succinyl coa.
Next up is orotic aciduria where there's excessive excretion of arotic acid in urine. The main concept you'll need to know is that erotic aciduria is an autosomal recessive disorder characterized by a deficiency of the enzyme.

Orotic Aciduria 10:01–10:39

Uridine monophosphate synthase or up synthase which normally converts uro acid to um p um P is an important precursor of pyrimidines and thus orotic aciduria leads to defective DNA synthesis and megaloblastic anemia.
Now, there's fanconi anemia which is a genetic form of aplastic anemia caused by mutations in DNA repair enzymes. So, in fanconi anemia, many precursor blood cells in the bone marrow are affected and accumulate DNA damage causing them to undergo cell death.

Fanconi Anemia 10:39–11:22

If enough cells are affected, it can lead to bone marrow failure and pancytopenia. Affected individuals are also at risk of developing cancers like acute leukemia, as well as tumors of the head, neck, skin gastrointestinal system or genital tract.
Ok. Moving on to nonmegaloblastic anemia, which includes diamond blackfan anemia and liver disease.

Diamond-Blackfan Anemia11:22–12:00

Diamond Blackfan anemia is a genetic disorder caused by mutations in the genes for ribosomal proteins. The ribosomal proteins are mainly involved in protein synthesis but also regulate cell division and cell death.
Impaired function of ribosomal proteins affects the erythroid progenitor cells resulting in anemia. But remember for your test that white blood cells and platelets are usually not affected.
Now, all anemias can cause fatigue, pallor, shortness of breath and glossitis. But other symptoms can help you identify the specific disease for vitamin b12 deficiency anemia.

Symptoms12:00–14:16

There is neurological dysfunction because when there's too much methylmalonic acid in the body, it builds up in the neurons specifically in their myelin sheath, which degenerates the main sites of neurological involvement include the peripheral nerves and the spinal cord causing subacute combined degeneration or SCD which due to affected dorsal columns cause decreased vibratory and position sense and affected lateral corticospinal tracts cause spastic paresis.
It affects the cerebrum causing dementia or psychiatric symptoms. As early signs of B12 deficiency.
It also affects the cerebellum. And as a result, patients experience ataxia and recurrent falls in folate deficiency, methyl malonic acid levels are normal.
So, no neurologic symptoms occur and that is something you absolutely have to remember for orotic aciduria. It usually manifests in childhood and presents as failure to thrive and developmental delay.
For fanconi anemia. There are physical abnormalities like short stature, thumb and arm defects and skin markings called cafe ole spots, which are flat patches that are the color of milky coffee.
Now diamond blackfan anemia usually manifests during infancy and the individuals affected also have physical abnormalities like short stature, craniofacial abnormalities such as small head size or microcephaly, cleft, palate, cleft lip and small lower jaw called micrognathia as well as upper extremity malformations.
Most commonly malformed or absent thumbs. Let's switch gears and look at some important lab tests.

Diagnosis14:16–15:46

The diagnosis of vitamin B12 deficiency relies on a peripheral blood smear which shows large red blood cells and hypersegmented neutrophils.
A bone marrow study can also be done to look for the megaloblastic changes in RBC precursors at various stages of differentiation.
Homocysteine and methylmalonic acid levels are elevated. Next diagnosis of folate deficiency also relies on a peripheral blood smear and bone marrow study both of which shows the same finding as B12 deficiency.
However, keep in mind that the homocysteine level is elevated. But methylmalonic acid levels are normal orotic aciduria is diagnosed via the very high orotic acid excretion in urine.
Genetic testing can be done to confirm the diagnosis. The definitive test for fanconi anemia is a chromosome breakage test where the ability of the patient's blood cells to repair DNA damage is tested.
Diamond Blackfan anemia, diagnosis is mainly made through bone marrow biopsy which shows diminished erythroid precursors.
The important fact to remember for exams in diamond Blackfan anemia is that fetal hemoglobin or HB F is elevated to compensate for the decreased production of red blood cells.
Ok. To review macrocytic anemias can occur when there is defective DNA synthesis like in vitamin b12 deficiency, folate deficiency and orotic aciduria, defective DNA repair like in fanconi anemia and in other conditions such as diamond Blackfan anemia and liver disease.

Review15:46–16:32

Vitamin b12 deficiency and folate deficiency. However, are by far the two most common causes of macrocytic anemia.
A diagnosis can be made based on peripheral blood smear and bone marrow study, but also consider the other symptoms, clinical history, lab findings and genetic tests.

Summary16:32–17:21

Now, back to the patients due to his age and frequent falls indicating neurological dysfunction. Bobby most likely has vitamin b12 deficiency, which can be confirmed with the peripheral blood smear.
First, low dietary intake should be ruled out then to pinpoint the cause of decreased absorption. One can look for anti intrinsic factor antibodies for pernicious anemia or carry out endoscopic or imaging studies in younger patients that might have Crohn's disease.
Meanwhile, Sarah probably has folate deficiency anemia based on her history of chronic alcohol use. Confirmation of the diagnosis can be done with peripheral blood smear findings along with homocysteine and methylmalonic acid levels.
Macrocytic anemia: Video, Causes, and Symptoms | Osmosis