Book of Medical Disorders in Pregnancy - Tintash

or because the hemoglobin A is replaced
by an abnormal hemoglobin, as in sickle
cell disease. The abnormalities in
hemoglobin formation are genetically
determined.
Sickle cell Anaemia - The hemoglobin
molecule consists of two pairs of
polypeptide chains which are attached to
the haem group. Abnormal hemoglobins
are formed when there is substitution of
specific amino acids in either α or the β
chain. In all cases the haem portion of
the molecule remains unaltered and the
oxygen-binding power of the
hemoglobin is therefore unaffected. The
most common abnormal hemoglobin is
'S' which is produced by substitution of
valine for glutamic acid radical in β
chain of the normal hemoglobin. In
hemoglobin C the same glutamic acid
radical in the β chain is replaced by
lysine. Sickle cell disease - This disorder
is inherited through an autosomal
dominant gene transmitted equally by
males and females. The reduced form of
Hb. S has only about one hundred the
solubility of Hb-S. It crystallizes out of
solution, when the oxygen tension in the
tissues is lowered to 35 to 40 mm Hg.
When crystallization of hemoglobin
occurs the red cells become distorted and
assume the typical sickle shape. When
sickling takes place the blood becomes
increasingly viscid and multiple
intracapillary thrombi are produced. This
causes sludging of the affected cells,
particularly in the bones, spleen,
alimentary tract and the kidneys where
the effected Islands of cells may die, due
to break of blood supply and
oxygenation. The survival time of red
blood cells is about one fifteenth that of
normal erythrocytes. These cells are
destroyed at a great speed by the
phagocytes of the reticuloendothelial
13
system. The end result of these changes
is chronic hemo-lytic anemia. Incidence
the incidence of sickle cell anaemia (5-5)
among the Negroes of the United States
is about one in 600, the sickle cell trait
(A-S), is present in about 8 per cent of
the Negro population.
In heterozygous patients where genetic
pattern is (A-S) on1y 20 to 30 per cent
of the hemoglobin is in the S form, while
the remaining 70 per cent hemoglobin is
adult type. In these patients sickling does
not occur at the normal oxygen tension
in the body, therefore there is no anemia.
When these patients are subjected to
abnormal anoxia which may occur during
anesthesia or in the non pressurized
air craft, sickling can easily occur and
result in severe crisis.
Clinical features
Sickle cell Anaemia (Homozygous 5-5)
Variant. The patients with this disease
are relatively infertile. They also die in
very early life from recurrent infection.
Heterozygous A-S disease - This is
usually asymptomatic but hematuria and
pyelonephritis may occur in this
condition. These changes are produced
by infarction of the medullary pyramids.
Hyperosmolarity of the papillary tips
predisposes to sickling and sludging of
the red cells. Sickle cell hemoglobin C
disease. This is quite mild and is first
suspected during a hemolytic crisis in
the course of pregnancy. The patients are
often chronically ill. Mild attacks of
hemolytic anaemia occur every now and
then. There are often hemolytic or
painful crisis which may be precipitated
by deoxygenation or an acid PH. These
changes are often associated with
infection or stasis of the blood flow.