This lesson will go over the different types and causes of macrocytic anemia. This will include a discussion of something known as megaloblastic anemia as well.
Really Big Things
Things that are really big grab people's attention. Big signs with big writing, big cars and big diamond rings are just a few of them.
Big red blood cells are also something that grab a clinician's and a pathologist's attention. They are an indicator of something going awry in the body, and this lesson will seek to explain why red blood cells may be overly large.
MCV and RDW
On a CBC, a complete blood count, your doctor has several values with which to help classify the cause of anemia. Anemia is a decreased function of or number of red blood cells.
One of the values is mean corpuscular (cell) volume, or MCV. In essence, this value tells you how big or small a cell is. It can be macrocytic, an abnormally large red blood cell; microcytic, an abnormally small red blood cell; or normocytic, a normally sized red blood cell.
The other value on your CBC that can help classify anemia is red cell distribution width, or RDW. This value measures the variation in red blood cell volume.
Macrocytic Anemia with Normal RDW
One type of anemia is called macrocytic anemia. I think you won't be surprised when I tell you this is when red blood cells are abnormally large. A subtype of macrocytic anemia occurs with a normal RDW.
One of the reasons for this is something known as aplastic anemia. Aplastic anemia refers to a disease where the bone marrow doesn't make enough red blood cells. The prefix 'a-' refers to being without something and 'plasia' refers to growth. So we are without the growth of red blood cells in the bone marrow.
There are a ton of reasons for aplastic anemia, including:
- Infections with HIV
- Toxins, such as benzene
- Radiation exposure
- Genetic disorders
Macrocytic Anemia with Increased RDW
The other subtype of macrocytic anemia occurs with an increased RDW. When there is a high RDW, we expect anisocytosis. This is a fancy word for red blood cells that have unequal sizes.
Macrocytosis with increased RDW can occur as a result of autoimmune hemolytic anemia. This is when your own body's (auto) antibodies attack your red blood cells and destroy them. The antibodies are proteins created by white blood cells. These proteins are actually supposed to attack foreign invaders, like bacteria, viruses and fungi. However, sometimes, due to improper genetics, a cross reaction between an invader, or unknown reasons, the antibodies target your red blood cells instead.
I sometimes think these autoantibodies are like boomerangs. They're created by you, they're thrown by you for your own needs, but if you're not careful they can comically fly back and hit you in the head, hurting you in the process. Of course, autoimmune hemolytic anemia isn't comical as it can result in a person's death.
Macrocytosis with increased RDW can also occur due to chronic liver disease. Many times this is as a result of alcohol abuse over a long period of time.
Finally, another more famous reason for macrocytosis and increased RDW is a deficiency in folate, vitamin B9, or cobalamin, vitamin B12. Both of these vitamins are important in DNA synthesis. If there's a deficiency of these vitamins, then DNA synthesis is impaired. This means a cell cannot divide, but it can still continue to grow in size, leading to the formation of an abnormally large mega cell called a megaloblast. The megaloblasts are just immature, gigantic, nincompoop red blood cell precursors. I like to think of them as the orcs from the Hobbit movies. They're big, they're ugly and their presence is not a good sign for any elf, dwarf or human.
By the way, one famous cause of vitamin B12 deficiency is known as pernicious anemia, which is when you do not properly absorb vitamin B12 despite consuming it in normal quantities. Even though you aren't deficient in vitamin B12 in a dietary sense, you are still technically deficient because your body isn't absorbing it, leading to megaloblastic anemia. It's sort of like if you were given a computer for Christmas but weren't allowed to use it. It's there in front of you, but it's of no use to you if you can't use it. With cobalamin, if you can't absorb it, you can't use it, and thus, it's useless.
Hopefully, the presents you get during the holidays are more useful than the sweaters I constantly get. And hopefully this lesson was not like one of my holiday sweaters for you. Let's take a look at the important stuff real quick.
Elevated mean corpuscular (cell) volume, or MCV, on a CBC refers to a cell that is macrocytic, or an abnormally large red blood cell.
MCV is used with RDW, red cell distribution width, to help figure out the cause for anemia.
If MCV is elevated but RDW is normal, then a reason for this type of macrocytic anemia may be aplastic anemia. Aplastic anemia refers to a disease where the bone marrow doesn't make enough red blood cells.
If both RDW and MCV are elevated, the cause for the anemia may be autoimmune hemolytic anemia, liver disease, or a deficiency in folate, vitamin B9, or cobalamin, vitamin B12.
By the end of this lesson, you should be able to:
- Recall the two values of a CBC count used in diagnosing anemia
- Discuss the causes of macrocytic anemia with normal RDW and with elevated RDW
- List some of the causes for aplastic anemia