Sickle Cell Disease

Question 1

The distribution of which deadly disease matches the distribution of sickle cell disease?

Answer 1

Plasmodium falciparum (malaria)

up to 25% Africans- sub-saharan and 10% Caribbeans carry sickle gene

Question 2

What mutation is responsible for sickle haemoglobin?

Answer 2

Missense mutation of codon 6 of the beta globin gene
The mutation changed glutamic acid to valine.
In sickle haemoglobin there are TWO normal alpha chains and TWO variant beta chains

Question 3

Describe the difference in character between the amino acids thatchange in this mutation.

Answer 3

Glutamic acid = polar + soluble (negatively charged)

Valine = non-polar + insoluble

Question 4

Under which conditions do the cells sickle and why do they sickle?

Answer 4

Conditions:
The effects of valine substitution are only seen in the deoxyhaemoglobin S conformation, which is insoluble

Why do RBCs sickle:
HbS polymerises to form fibres called tactoids, by forming intertetrameric contacts with other HbS (Hb is a tetramer, so intertetrameric means between Hb) thus stabilising structire

This polymerization causes the distortion of the cells

Question 5

What are the 3 stages in the sickling of red blood cells?

Answer 5

1. Distortion
   
   • Polymerisation initially reversible with formation of oxyHbS because initially, small polymers are formed and the Hb in these polymers are reoxygenated before polymers get too big so the polymers get broken down
   • Subsequently irreversible, because deoxygenated HbS is adherent and will therefore increase the transit time. Eventually, a large enough polymer (that cannot be broken down) can be formed before being reoxygenated
2. Dehydration
3. Increased adherence to vascular endothelium (membrane expresses different adhesion molecules)

Question 6

Use three words to describe sickled red blood cells.

Answer 6

Rigid
Dehydrated
Adherent

Question 7

Originally, it was not understood why sickling causes such profound clinical problems because it appeared that the normal transit time of red blood cells is sufficient for the red cells tobecome reoxygenated and for the polymers to be broken downbefore much sickling takes place. What key feature of sickle cells explains their ability to cause such problems?

Answer 7

The sickle cells are more adherent to the vascular endothelium so they stick to the vessel walls and increase their transit time
This allows more time for the polymerisation to occur. When a big enough polymer is formed before reoxygenation, then the polymer cannot be broken down and Hb cannot be reoxygenated

Question 8

What is the difference between sickle cell disease and sickle cell anaemia?

Answer 8

Sickle cell disease = generic term that encompasses all disease syndromes due to sickling
Sickle cell anaemia = homozygous (SS)

Question 9

What effect does sickling have on the lifespan of red blood cells?

Answer 9

Shortened red cell lifespan-20 days- haemolysis

Question 10

What are the complications of increased haemolysis? What can result after a parvovirus B19 infection in haemolytic anaemia patients?

Answer 10

Anaemia
Gallstones
Aplastic Crisis (Parvovirus B19)

Question 11

Other than the increased break down of red blood cells, what else is partly responsible for anaemia in sickle cell patients?

Answer 11

REDUCED EPO DRIVE

HbS has a low affinity for oxygen so it delivers the oxygen more effectively to tissues. HbS shifts curve to the right so basically Hb can deliver more O2. There will also be less EPO released to stimulate new RBC production.

SO overall 2 reasons:

1. sickling causes RBC to be removed more quickly from spleen (which leads to eg gallstones)
2. However, the rate of RBC prodction is reduced due to less EPO

Question 12

Why does increased haemolysis cause gallstones?

Answer 12

Increased haemolysis means increased release of bilirubin and other red cell breakdown products
These get excreted through the biliary tract and carry a risk of causing gallstones

Question 13

How can sickle cell disease lead to aplastic crisis?

Answer 13

Aplastic crisis is caused by Parvovirus B19 infection (a common respiratory virus)
The virus infects developing red cells in the bone marrow and blocks their production
This doesn’t have much effect on normal people with a 120-day red cell lifespan
But because the lifespan of red cells in sickle cell disease is so low, a parvovirus infection could cause a steep drop in haemoglobin (anaemia)

Question 14

Blockage of the microvascular circulation (vaso-occlusion) is a major problem in sickle cell disease. State three effects

3 tissues that are affected

Answer 14

Tissue damage and necrosis (infarction)
Pain
Dysfunction

Spleen – leads to hyposplenism (increased risk of life-threatening infection by capsulated bacteria (mainly pneumococcal))
Bones and joints – dactylitis (inflam of fingers/toes), avascular necrosis (necrosis as a result of no blood supply), osteomyelitis
Skin – ulceration (ischaemia causes skin cells to die. This promotes ischaemic ulcers)

Question 15

How can sickle cell disease affect the lungs?

Answer 15

Acute chest syndrome (vaso-occlusive crisis of the pulmonary vasculature)
Chronic damage – pulmonary hypertension due to free Hb binding to NO

Question 16

How can sickle cell disease affect the urinary tract?

Answer 16

Haematuria (due to papillary necrosis)

Hyposthenuria (abnormally low concentration of urine leading to urine having low specific gravity)

Renal failure

Priapism (painful erections)

Question 17

How can sickle cell disease affect the brain?

Answer 17

Stroke

Cognitive impairment

Question 18

How can sickle cell disease affect the eyes?

Answer 18

Proliferative retinopathy

Question 19

Pulmonary hypertension correlates with the severity of the haemolysis. What is the likely mechanism explains this effect?

Answer 19

The free plasma Hb resulting from the haemolysis will scavenge nitric oxide and, hence, deplete the nitric oxide
This takes away the vasodilation effects of nitric oxide, causing vasoconstriction of certain vascular beds (including pulmonary circulation)

Question 20

What are the early presentations of sickle cell disease?

Answer 20

Dactylitis
Splenic sequestration (accumulation of red cells in the spleen leading to a drop in blood count)
Infection (pneumococcal (capsulated))

Question 21

What are painful crises and what could they be triggered by?

Answer 21

Painful crises in sickle cell disease are episodes of extreme pain caused by blood vessels becoming occluded. This is one of the biggest problems that doctors face in these patients
They can be triggered by: 
 Dehydration  
 Exertion 
 Hypoxia
 Infection  
 Psychological stress

Question 22

State some general measures taken for patients with sickle celldisease.

Answer 22

Folic acid supplementation 
Vaccines (due to hyposplenism)
Penicillin (hyposplenism)
Monitor spleen size  
Blood transfusions for acute anaemia events, acute chest syndrome and stoke

Question 23

Describe how you would manage a painful sickle cell crisis.

Answer 23

Pain management (opioids)  
Hydration  
Keep warm 
Oxygen if hypoxic  
Exclude infection (blood and urine cultures; CXR)

Question 24

What complications are transfusions used to prevent?

Answer 24

Stroke

Acute chest syndrome

Question 25

In which group of patients is haematopoietic stem cell transplantation effective and just how effective is it?

Answer 25

Young patients (< 16 yrs with severe disease) 
Curative in 85-90%
But relatively few children will find perfectly matched donors

Question 26

What drugs would you give in the management of patients with SCD and how do they work?
for MOA, include:
effect on current sickled RBCs 
Effect on WBCs
What vasodilator do these drugs promote?

Answer 26

Hydroxyurea(aka Hydroxycarbamide) and Butyrate

Main action: Induce of HbF (reverse adult Hb back to HbF), HbF reduces polymerisation from HbS and so less symptoms

Other actions:
effect on RBCs- reduce stickiness of sickled cells, improves their hydration

These drugs also help to generate NO which is a vasodilator

Question 27

State some laboratory features of sickle cell disease.

Answer 27

Hb LOW (typically 6-8 g/dL) 
Reticulocytes HIGH (except in aplastic crisis) 
Blood film: 
 Sickle cells 
 Boat cells 
 Target cells 
 Howell-Jolly bodies

Question 28

State two methods of diagnosing sickle cell disease.

Answer 28

1. Solubility Test:
   a. Blood sample is mixed with a reducing agent, Oxyhaemoglobin is converted to deoxyhaemoglobin, Solubility decreases. In the presence of deoxyhaemoglobin S – the solution becomes TURBID (cloudy)

HOWEVER, this does NOT distinguish between HbS and HbAS

2. DEFINITIVE DIAGNOSIS: Electrophoresis – takes advantage of the difference in charge between HbS, HbAS and HbA

Question 29

Describe the features of a patient with sickle cell trait.

Answer 29

Genotype: HbAS 
Normal life expectancy 
Normal blood count  
Usually asymptomatic 
Rarely painless haematuria (due to papillary necrosis) Under conditions of hypoxia they may have some complications due to sickling of red cells (e.g. anaesthesia, high altitude, extreme exertion)

Question 30

Deoxyhaemoglobin S is soluble or insoluble

Answer 30

Insoluble