Video summary
Polycythemia is a condition characterized by an abnormal increase in hemoglobin concentration and red blood cell mass, standing in direct contrast to anemia where these levels are low. This elevation typically occurs when hemoglobin exceeds 17 g/dL in men or 15 g/dL in women, or when hematocrit levels rise above 50% in men and 45% in women. The underlying mechanism involves the expansion of the erythron, which encompasses both circulating red blood cells and their precursors stored in the bone marrow. While anemia often triggers a compensatory increase in precursor proliferation without necessarily increasing total red cell mass, polycythemia represents a genuine surplus where the entire erythron volume is significantly increased.
The causes of polycythemia are broadly categorized into relative, primary, and secondary types. Relative or spurious polycythemia arises not from an actual increase in red cells but from conditions like severe dehydration that reduce plasma volume, thereby artificially concentrating the blood components. A rare form known as stress polycythemia or Gaisböck syndrome also falls under this category. In contrast, real polycythemia involves a true increase in red cell mass and is further divided into primary and secondary causes. Primary polycythemia, specifically polycythemia vera, occurs when bone marrow precursors proliferate autonomously due to a malignant condition. Secondary polycythemia, however, results from an external stimulus that drives the production of erythropoietin (EPO), the hormone responsible for stimulating red blood cell formation in the bone marrow.
Secondary polycythemia is most commonly triggered by hypoxia, which prompts the kidneys to release excess EPO to compensate for low oxygen levels. Various conditions can induce this state of tissue hypoxia, including chronic lung diseases like COPD and uncontrolled asthma, congenital heart defects involving right-to-left shunts that bypass pulmonary oxygenation, and living at high altitudes. Additionally, elevated carbon monoxide levels from smoking or occupational exposure can cause polycythemia because carbon monoxide binds to hemoglobin with 200 times greater affinity than oxygen, preventing oxygen release to tissues. Other factors include abnormal hemoglobin variants that hold onto oxygen too tightly and tumors such as renal carcinoma, uterine cancers, hepatomas, or cerebellar hemangiomas that autonomously produce erythropoietin without the need for hypoxic stimulation.
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Hello everyone. On one hand, there we
talk about anemia where there is
decrease in hemoglobin concentration.
Polycythemia is a totally opposite
concentration where there is increase in
hemoglobin concentration above normal.
And the cause of this increase in
hemoglobin concentration above normal is
increase in red cell mass. So yes, in
polycythemia there is increase in
hemoglobin concentration above normal.
And when it increases above 17 g per
deciliter in men and above 15 g per
deciliter in women. Or there is
increased hematocrit. Both is same only.
Increased hematocrit, there will be
increased hematocrit more than 50% in
men and more than 45% in women. So that
is known as polycythemia. Now the causes
of polycythemia can be real when
actually there is increase in red cell
mass or it can be relative or spurious.
So that is when suppose there is
dehydration. So that is going to
increase the hemoglobin because this
denominator actually is decreasing,
okay? So that's why hematocrit is
increasing. So that is known as relative
or spurious causes. So before going into
the actual cause, let us see a little
bit underlying physiology so that we are
able to understand better.
So this term erythron, if you understand
what is this, that is the the number of
RBCs which are circulating plus the
total pool of its precursors in the bone
marrow, okay? So bone marrow
erythroblast plus the circulating
erythrocytes, it composes of erythron.
And if these increase, any of these
increase, it can lead to
polycythemia, okay?
So yes, erythron total may decrease in
anemia. It will decrease in anemia and
there may be compensatory increase in
the
proliferation of the precursors in bone
marrow, right? But, in case of
polycythemia, this erythron totally
increases and it can be due to both of
these, right? So, what is happening if
RBCs are less in circulation, this leads
to hypoxia to the tissues. If they are
less in circulation or the oxygen which
is bound to hemoglobin is not released
properly. If oxygen is not being
released, then that is also leading to
hypoxia. Okay. So, any cause hypoxia
will lead to release from the kidney of
a hormone that is erythropoietin and
this erythropoietin goes and acts on the
precursors, erythroid precursors in the
bone marrow to produce more RBCs. Okay.
So, with this fundamental, let us see
what will be the causes of polycythemia.
So, first moving to just we'll finish
off the relative part. Relative
polycythemia will occur when there is
severe dehydration and there is
something known as stress polycythemia
which rarely occurs, that is Gaisböck
syndrome.
But, what are the real causes of
polycythemia where actually the red cell
mass increases? So, again, real causes
can be either primary or secondary. So,
primary is basically when the erythroid
precursors in the bone marrow
proliferate. So, that can occur due to a
malignant condition known as
polycythemia vera. Okay, so that cause
we'll see later what happens in
polycythemia vera. So, that is the
primary cause. Secondary cause is where
there is no disturbance in the
precursors as such, but there is some
cause which is leading to increase in
production of erythropoietin. So, we
have seen here that erythropoietin is
the stimulator. Basically, it acts on
the precursors and causes more
proliferation. So, anything which
increases EPO will lead to more
proliferation of the precursors and more
RBCs will be released into the
circulation.
So, when will EPO release? I told you
what is the stimulus? That is hypoxia.
So, if there is compensatory
increase in
erythropoietin production due to
hypoxia. So, any diseases which are
causing hypoxia. So, it can be the lung
diseases, that is COPD, uncontrolled
asthma, or there can be congenital heart
diseases where right to left left shunt
that means the blood is passing directly
from the right to left side of the heart
without going into the lungs. So, they
the blood is not getting oxygenated,
right?
So, that is right to left shunt which
will lead to hypoxia. High altitude,
common cause of hypoxia. Then elevated
carbon monoxide. Elevated carbon
monoxide which occurs in a smokers or
due to occupational exposure. This
carbon monoxide has high affinity with
hemoglobin. So, hemoglobin, instead of
binding to oxygen, it will bind to
carbon monoxide. So, simply said, oxygen
is going to displace whatever bound is
there, it is going to displace.
Second thing is that carbon monoxide
also has another effect, that whatever
oxygen is left binding to hemoglobin,
that is not even released, okay? So,
there is a decrease in the release of
oxygen as well. But this is the primary
cause where carbon monoxide binds to
hemoglobin because carbon monoxide has
200 times more affinity to hemoglobin
compared to oxygen, right?
So, that can also be the cause. Again,
it will lead to hypoxia. Then elevated
oxygen hemoglobin affinity, which I told
you one that carbon monoxide itself will
prevent the release of oxygen from
hemoglobin, or there can be some
abnormal hemoglobin variants which have
more affinity to oxygen so that it is
not being released to the tissues.
So, all these are going to lead to
hypoxia, and hence that is going to lead
to increase in erythropoietin
production.
Or there can be autonomous
erythropoietin production, and that
happens in certain tumors as a source of
erythropoietin. Like renal carcinoma or
cyst, uterine cancer, uterine leiomyoma,
hepatoma, cerebellar hemangioma.
Actually, this cerebellar hemangioma
will present first with the neurological
localized signs and symptoms, but yes,
it causes erythropoietin production as
well. What I mean to say that generally
the patient doesn't present with
polycythemia, the signs and symptoms of
polycythemia.
So, those were the causes of
polycythemia. I hope you understood the
concept behind that. Thanks for watching
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