9.2 Disease of Red Cell Morphology

Question 1

What is a sickledex test?

Answer 1

A qualitative solubility test in which a sample of the patient’s blood is mixed
with a

deoxygenating agent and
a solubility buffer to determine the presence
of > 10% of haemoglobin S (HbS) in the sample.

It determines the presence of HbS but does not diagnose sickle cell disease.

Question 2

What test is used to diagnose sickle disease? What genetic variants may be diagnosed using this test?

Answer 2

Haemoglobin electrophoresis

• Quantifies the amount of HbS to determine
  hetero/homozygous genotyping (sickle trait versus sickle disease)
• Also measures other abnormal haemoglobins including HbC and thalassaemia

Question 3

What is the underlying defect in Hbs?

Answer 3

• Inherited haemoglobinopathy resulting from

mutation on chromosome 11,
causing substitution of
Glutamine by Valine in the 6th amino acid of
β chains resulting in the formation of haemoglobin S.

• Haemoglobin S is biochemically unstable and
  can precipitate out of solution when in the deoxygenated state,
  forming the pointed, slightly curved ‘sickle cells’.

Question 4

What are the different types of sickling crises?

Answer 4

1. • Vaso-occlusive crisis
2. • Aplastic crisis
3. • Splenic sequestration crisis
4. • Haemolytic crisis

Question 5

• Vaso-occlusive crisis

Answer 5

Sickle-shaped red blood cells obstruct capillaries
and restrict blood flow to an organ,
resulting in ischaemia, pain, necrosis, and often organ damage.

Question 6

• Aplastic crisis

Answer 6

Acute worsening of the patient’s baseline anaemia.

This crisis is normally triggered by parvovirus B19,
which directly affects erythropoiesis
by invading the red cell precursors and
multiplying in them and destroying them.

Question 7

• Splenic sequestration crisis

Answer 7

Spleen is affected in the process of clearing defective red blood cells.

It is usually infarcted before the end of childhood
in patients with sickle-cell anaemia.

Question 8

• Haemolytic crisis

Answer 8

Accelerated drops in haemoglobin.

Common with co-existent G6PD deficiency.

Question 9

What other system manifestations or comorbidities might the patient present with, related to sickle cell disease?

Answer 9

Respiratory
* Acute chest syndrome
* Pulmonary hypertension

Neurological
* Acute brain syndrome
* Recurrent infarcts

Haematology
* Anaemia

Musculoskeletal
* Bone marrow dysplasia

Renal
* Renal failure

Gastro-intestinal
* Asplenism, may require antibiotic prophylaxis
* Gallstones
* Jaundice

Genitourinary
* Priapism

Question 10

What is acute chest syndrome?

Answer 10

• Dyspnoea, cough, haemoptysis, and
  pleuritic chest pain caused by
  recurrent pulmonary infarctions
• Radiological finding of a new infiltrate on X-ray
• Management—
  broad-spectrum antibiotics,
  fluid management,
  oxygenation,
  chest physiotherapy,
  bronchodilators,
  intermittent incentive spirometry

Question 11

What are your clinical priorities in managing patients with sickle cell disease undergoing surgery?

Answer 11

Careful, well-balanced anaesthesia

• Avoid hypoxia.
• Ensure adequate hydration.
• Maintain normothermia.
• Optimal analgesia.
• Avoid oxidant drugs such as
  prilocaine,
  nitroprusside,
  vitamin K,
  aspirin,
  and penicillin,

as they may precipitate haemolysis.

Consider regional anaesthetic techniques where likely to be beneficial.

Avoid venostasis—caution with tourniquets.

Question 12

At what Pao2 does sickling occur? What is the mechanism behind the process?

Answer 12

• Homozygous (SS) cells begin to sickle at much higher oxygen saturation,
  typically 85% (Pao2 5–5.5 kPa).
• Heterozygous (AS) cells may deform below the saturation 40%
  (PaO2 2.5–4.0 kPa). .
  (Sickling with sickle cell trait is therefore rarely a problem
  without concomitant stasis.)
• Desaturation of Hb S results in the polymerisation of haemoglobin,
  forming large aggregates called tactoids,
  which deform the red cells into the typical sickle shape.

Question 13

When is exchange transfusion used?

Answer 13

• Traditionally, an aggressive transfusion policy targeting an HbS
  concentration of < 30% was suggested.

Now a simple transfusion policy to a haemoglobin of 10 g d/L is recommended.

• In high-risk cases,
  a more aggressive approach may be required.

This must be discussed in advance with a haematologist,
as the risk of transfusion-related complications such as iron overload,
alloimmunisation,
transfusion-related acute lung injury,
allergic reactions are all increased

Question 14

What happens to the oxygen dissociation curve in sickle cell disease?

Answer 14

Rightward shift due to increased 2, 3–DPG in
response to chronic anaemia

Question 15

Describe the structure of RBC

How is it different from other body cells?

Answer 15

Red blood cells are biconcave discs (7 microns in diameter);

This structure gives it a higher surface area
to volume ratio for diffusion of oxygen and
allows it to negotiate tight passages in the vasculature.

RBC contain large amounts of protein but
do not have a nucleus, mitochondria, or ribosomes.

Question 16

What are the diseases of the red cell membrane?

Answer 16

Red cell membrane disorders are

inherited diseases due to mutations in
membrane or skeletal proteins,
resulting in decreased red cell deformability,
life span, and premature death of the erythrocytes.

The red cell membrane disorders include:
hereditary spherocytosis,
elliptocytosis,
ovalocytosis,
stomatocytosis.

Question 17

What is hereditary spherocytosis?

Describe the pathogenesis and issues.

Answer 17

Hereditary spherocytosis is the most common congenital haemolytic
anaemia, with an autosomal dominant inheritance.

_______________________________________

Pathogenesis

The molecular defect involves the genes encoding for proteins
(spectrin, ankyrin, band 3, and protein 4.2)

that are involved in the attachment of the
cytoskeleton to the red cell membrane.

This results in loss of surface area and leads to spherical,
osmotically fragile cells that get trapped in the spleen.

Question 18

Hereditary spherocytosis

Diagnostic test

clinical features

Treatment

Answer 18

DX
* Blood film
* Osmotic fragility test
* Electrophoresis

Clinical features
* Haemolytic anaemia sometimes requiring exchange transfusion
* Jaundice, splenomegaly, and cholelithiasis.

Treatment
* Folate therapy in mild forms; red cell transfusions in severe forms.

• Splenectomy, with appropriate counseling
  about the risk of infections and
  post-splenectomy antibiotic and vaccinations.

Question 19

What are the other disorders of red cells?

Answer 19

Red cell destruction
* Haemoglobinopathy
* Enzymopathy (G6PD deficiency)
* Autoimmune
* Membrane disorder (already discussed)

Disorders in red cell production
* Thalassemias
* Myelodysplasia
* Aplastic anaemia

Question 20

Explain thalassemia

Answer 20

Thalassemia is the genetic defect in
haemoglobin synthesis resulting in
decreased or absent synthesis
of one of the two globin chains (α or β).

• Imbalance of globin chain synthesis leads to decreased haemoglobin
  production and precipitation of excess globin (toxic).
• Found in people of African, Asian, and Mediterranean heritage.

Question 21

Explain thalassemia

Answer 21

Thalassemia is the genetic defect in
haemoglobin synthesis resulting in
decreased or absent synthesis
of one of the two globin chains (α or β).

• Imbalance of globin chain synthesis leads to decreased haemoglobin
  production and precipitation of excess globin (toxic).
• Found in people of African, Asian, and Mediterranean heritage.

Question 22

What is 𝛃 thalassemia

Answer 22

𝛃 thalassemia
It is an autosomal recessive disorder.

In the homozygous state (i.e. thalassemia major)
it causes severe, transfusion-dependent anaemia.

In the heterozygous state or β thalassemia trait
(i.e. thalassemia minor), it causes
mild to moderate microcytic anaemia.

Question 23

Thalassemia Clin feats

Dx

Answer 23

clinical features
* Anaemia
* Extramedullary haematopoiesis
* Complications of long-term transfusion
* Increased risk for infections
* Cholelithiasis

Diagnostic test
* Blood smear: microcytic/hypochromic anaemia
* Hb electrophoresis (HbA2, HbF)
* Iron stores usually elevated

Question 24

Thalassemia treatment

Answer 24

• Definitive treatments are stem cell transplant and simple transfusion.
• Chelation therapy to avoid iron overload has to be started early.

The major causes of morbidity and mortality in β thalassemia are anaemia
and iron overload.

Question 25

Discuss G6PD deficiency.

Answer 25

Glucose-6-phosphate dehydrogenase (G6PD) deficiency

is inherited as an X-linked disorder (G6PD locus at Xq28),
which primarily affects men.

The G6PD enzyme catalyses the
oxidation of
glucose-6-phosphate to
6-phosphogluconate

while concomitantly reducing the oxidized form
of nicotinamide adenine dinucleotide phosphate (NADP+)

to reduced nicotinamide adenine dinucleotide phosphate (NADPH).

NADPH, a required cofactor in many biosynthetic reactions,
maintains glutathione in its reduced form.

See Figure 9.4.

GSH and GSSG – reduced and oxidised form of glutathione

Question 26

What are the sequeale

Answer 26

Reduced glutathione acts as a scavenger

for dangerous oxidative metabolites in the cells.

Red blood cells rely heavily upon glucose-6- phosphate dehydrogenase (G6PD) activity

because it is the only source of NADPH that protects the cells against oxidative stresses.

Question 27

What are the sequelae G6PD

Answer 27

Reduced glutathione acts as a scavenger

for dangerous oxidative metabolites in the cells.

Red blood cells rely heavily upon glucose-6- phosphate dehydrogenase (G6PD) activity

because it is the only source of NADPH that protects the cells against oxidative stresses.

Haemolysis is triggered by:
* Infections

• Ingestion of fava beans
• Oxidant drugs such as sulfa, dapsone, antimalarial drugs, and chloramphenicol
• Surgery

Question 28

G6PD
Clin features

Treatment

Answer 28

Clinical features
* Asymptomatic
* History of neonatal jaundice, requiring exchange transfusion
* Haemolysis
* Gallstones and splenomegaly

Treatment
* Avoid oxidant drugs
* During acute crisis, stop the offending factor
* Folic acid supplementation
* Blood transfusion is rarely necessary