Sickle cell anaemia Flashcards
Describe the first description of sickle cell anaemia
‘Peculiar elongated forms of the red corpuscles’
‘Some change in the composition of the corpuscle itself may be the determining factor’
What mutation is responsible for sickle cell anaemia
Sickle haemoglobin (HbS) differs from HbA by a single amino acid. The defect is in the β globin chain and results in replacement of glutamic acid at position 6 of the β chain by valine. 3-D models of the deoxyhaemoglobin indicate that the residue at position 6 sits on the surface of the protein. Although glutamate is a highly polar amino acid, the side chain of valine is distinctly nonpolar and this alteration markedly reduces the solubility of deoxyhaemoglobin.
MISSENSE mutation
What is important to remember about HbS
In sickle haemoglobin we have two NORMAL alpha chains and two variant beta chains. These are almost identical to normal beta globin but
the AMINO ACID valine rep laces glutamic acid at position 6 of the beta chain.
While Glutamic acid is a polar aminoacid, valine is not and this will inevitably influence the solubility of the molecule and the potential bonds that the globin chains can form.
Compare glutamine (HbA) to valine (HbS)
Glutamine is polar and soluble
Whereas valine is non-polar and insoluble
Describe the consequences of this mutation for HbS
Deoxyhaemoglobin S is insoluble
HbS polymerises to form fibres - “tactoids”
Intertetrameric contacts stabilise structure
Outline the stages in the sickling of red cells
Distortion
· Polymerisation initially reversible with formation of oxyhaemoglobin S
Dehydration (irreversible from now on)
Increased adherence to the vascular endothelium
Use 3 words to describe sickled red cells
Rigid - normal red cells should have a biconcave shape - sickle shape and polymerisation makes them less deformable- harder to pass through micro-vasculature
Dehydrated - further concentrates HbS and promotes polymerisation
Adherent - less oxygen delivered to tissues- precipitates to ischaemia and tissue damage
Describe the changes that may make the red blood cells more adherent
There are also changes in the red cell membrane, which are not fully understood. The membrane expresses a different profile of adhesion molecules which make the red cells sticky to vascular endothelium.
Summarise the population genetics associated with sickle cell anaemia
Distribution matches that of endemic Plasmodium falciparum malaria
Up to 25% Africans (sub-Saharan) and 10% Caribbeans carry sickle gene
Around 300,000 affected births annually worldwide
Why has the sickle gene also arisen in different populations
Sickle gene arisen independently in several occasions- based on different haplotypes for beta globin gene- multi-centric origin- selected for by evolution due to protection against malaria
Describe the global migration and distribution of the HbS gene
Births/yr
Affected 300,000
HbAS 5.5 million
Patients
US 100,000
Europe 60,000
UK 12-15,000
As migration increase- the number of migrants with HbS has increased- leading to a greater number of origin countries of HbS
Summarise the epidemiology of sickle cell disease in the U.K
Prevalence 12000-15000
70% reside in Greater London
350 new births per annum. Most common monogenic disorder
National Haemoglobinopathy Registry (NHR) established 2013. Currently 11,000 SCD patients registered
~ 600 patients at ICHT
Mostly picked up by new born screening
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 to become reoxygenated and for the polymers to be broken down before much sickling takes place. What key feature of sickle cells explains their ability to cause such problems?
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
What are the sickle cell disorders
Sickle cell anaemia (SS) and compound
heterozygous states e.g. SC, Sb thalassaemia
Genetically simple – Autosomal recessive
Clinically heterogeneous
What is the difference between sickle cell disease and sickle cell anaemia
Sickle cell disease = generic term that encompasses all disease syndromes due to sickling
Sickle cell anaemia = homozygous (SS)
What is important to remember about SCD
Although sickle cell disease is genetically simple, clinically it is complex with a wide range of symptoms affecting the whole body
What are the different sickle cell conditions
Sickle cell disease: sickle cell anaemia βS.βS S
Sickle cell disease: sickle cell / haemoglobin C disease βS.βC S and C
Sickle cell trait βS.βA A and S
Normal βA.βA A
Which type of Hb is affected in SCD and why
Only adult Hb is affected because HbF does not have any beta chains. The problems therefore start at 4-6 months or older, after the HbF level decreases and the adult Hb level increases.
Therefore HbF and HbA2 will be normal.
What effect does sickling have on the lifespan of RBCs
As the cells are distorted, the body more avidly removes them
They have a lifespan of around 20 days
What are the consequences of increased haemolysis
Anaemia
Gallstones
Aplastic Crisis
What else is responsible for the anaemia in SCD besided increased haemolysis
There is reduced erythropoietic drive as HbS has a low affinity for oxygen so it delivers the oxygen more effectively to tissues
So hypoxia doesn’t stimulate EPO release from the kidneys as much
Why does haemolysis caused increased gallstones
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
How can SCD lead to aplastic crisis
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)
Describe the consequences of blockages to the micro-vascular circulation
Blockage to microvascular circulation (vaso-occlusion)
Tissue damage and necrosis (Infarction)
Pain
Dysfunction
Why is circulation impaired in sickle cell anaemia
As sickled red cells become trapped in the small blood vessels, circulation is impaired and there is damage to multiple organs. In children, infarcts of the small bones of the hands of feet may occur and lead to a painful dactylitis called the “hand-foot“ syndrome and, as a later result, shortening of the digits. In adults, generalised pains are more typical and result from
oxygen deprivation of tissues and avascular necrosis of the bone marrow
What is the most common cause of hospital admissions in sickle cell anaemia
Distorted sticky red cells can clog up blood vessels and lead to tissue death. This is called infarction.
It is associateed with severe pain and loss of function.
Pain crisis- due to bone marrow ischaemia – need strong opiods- most common cause of admission to hospital- acute pain crisis
Repeat ischaemic damage= chronic organ damage
Describe the consequences of infarction of the spleen
Splenic sequestration/hyposplenism
auto-infarcation- repeptitive infarction damage
The spleen can become engorged and act as a reservoir for the bodies blood supply…the so called sequestration crisis which also requires urgent blood transfusion. As the patients gets older the spleen shrinks and the patients become at risk of death from infections which are normally held in check by splenic activity such as malaria or pneumococcal sepsis.
Suceptible to infections from encapsulated bacteria (which the spleen normally clears)
Describe the consequences of infarction on the bones and joints
dactylitis /osteomyelitis/avascular necrosis of the hip
Particularly at risk are the weight bearing femoral head and the fingers.
Osteomyelitis: infection of bone (dead tissue is susceptible to bacterial infection)
Avascular necrosis will result in death of the tissue
Describe the consequences of infarction on the skin
chronic/recurrent leg ulcers
Describe the pathogensis of vaso-occlusion in sickle cell disease
Polymerisation of HbS (HbS polymers can be seen on X-Ray diffraction)
Trapping in micorvasculature (rigid)
Adherence to endothelium (attraction of other cells- neutrophils)
Vaso-occlusion
Describe the relationship between sickle cell vasculopathy and nitric oxide
Cell-free haemoglobin limits nitric oxide bioavailability in sickle cell disease
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)
Leading to pulmonary hypertension
Describe pulmonary hypertension in sickle cell disease
Pulmonary hypertension correlates with the severity of haemolysis
The likely mechanism is that the free plasma haemoglobin resulting from intravascular haemolysis scavenges NO and causes vasoconstriction
Associated with increased mortality
Describe the impact of pulmonary hypertension on survival rates in patients with SCD
Pulmonary blood pressure can be measured using the Measurement of tricuspid regurgitant
jet velocity by echocardiography (essentially, the speed of blood moving back over the tricuspid valve)
TRV >2.5m/sec associated with lower survival rates- due to pulmonary hypertension.
Outline the effects of the pathogenesis of SCD on the lungs
Lungs
Acute chest syndrome (most common cause of death in adults with SCD)- VASO-OOCLUSIVE CRISIS OF PULMONARY VASCULATURE
Chronic damage
Pulmonary hypertension
Outline the effects of the pathogenesis of SCD on the urinary tract
Haematuria (due to papillary necrosis)
Hyposthenuria (inability to control urine concentration – can lead to dehydration)
Renal failure
Priapism (painful erections)
Outline the effects of the pathogenesis of SCD on the brain and eyes
Brain:
Stroke and cognitive impairment – affects 8% of SS, most common 2-9yrs.
Macrovasculature- middle cerebral artery- can monitor stroke risk using Doppler studies.
Eyes:
Proliferative retinopathy- similar to that seen in diabetic patients.
What is important to remember about the clinical course of SCD
Clinical course variable and unpredictable even within same family
Explain the clinical onset of SCD
Symptoms rare before 3-6 months
Onset coincides with switch from fetal to adult Hb
At birth- 80% of Hb is HbF
What are the earliest clinical manifestations of SCD
Dactylitis
Splenic sequestration (accumulation of red cells in the spleen leading to a drop in blood count)
Infection (pneumococcal (capsulated))
How can we distinguish between sequestration and aplastic crises
Sequestration and aplastic crises can be distinguished by the presence/absence of reticulocytes, but both require urgent transfusion.
Low reitulocytes in aplastic anaemia
The reticulocyte count is raised in a sequestration crisis but in an aplastic crisis it is much lower than normal.
Describe the importance of early diagnosis of SCD
Significant reduction
in deaths from
pneumococcal
infection and acute
splenic sequestration
With prophylactic penicillin and education on how to palpate and examine spleen to monitor sequestration of spleen- prevent it getting to advanced stages.
What did the PROPS trail show (prophylactic penicillin)
PROPS trial <3 years 84% reduction in pneumococcal Infection
Describe the factors leading to the improvement in the life expectancy in patients with SCD
National Sickle Cell Act
Preventative Penicillin (and new born screening programme)
Hydroxycarbamide treatment
Transfusion for stroke prevention
Describe the mortality associated with SCD
In one large survey the median life expectancy for men and women with homozygous sickle cell anaemia was 42 and 48 years respectively and the causes of death were: 21% associated with a painful crisis 14% associated with a chest syndrome 9% associated with renal failure 7% associated with infection 6% perioperative
List the sickle emergencies
Septic shock (BP <90/60)
Neurological signs or symptoms (ischaemic in children- mixture of ischaemic and haemorrhagic in adults)
SpO2 <92% on air (hypoxia)
Symptoms/signs of anaemia with Hb <5 or fall >3g/dl from baseline (acute anaemic event)
Priapism >4 hours (medical emergency- if not managed- will lead to permanent erectile dysfunction)
Why is hypoxia important to spot in patients with SCD
Hypoxia important to detect- may set up further cycle of further sickling- most commonly due to acute chest syndrome
Describe the radiological features of acute chest syndrome
Typically seen as segmental or subsegmental atelectasis/consolidation with a lower lobe predilection, and/or pleural effusion.
A chest radiograph may also show other sequelae from sickle cell disease such as bone infarcts, rib enlargement and cardiomegaly (from anaemia).
Bi-basal changes
Shadowing
Consolidation
What is important to remember about acute chest syndrome
Responds well to early intervention- non-invasive respiratory support and exchange transfusion
Describe the key features of acute chest syndrome
New pulmonary infiltrate on chest X-ray
with Fever
Cough
Chest pain
Tachypnoea
Develops in context of vaso-occlusive crisis
surgery pregnancy
Describe the relative incidences of acute chest syndrome in the different sickle cell diseases
Incidence SS>SC>SB+ Thal
Describe the statistics associated with acute chest syndrome
Diagnosis often delayed
Mechanical ventilation 15%
Mortality > 18 yr 9%
Describe avascular necrosis of the femoral head
Significant disability
Loss of ball and socket shape to joint
Due to ischaemic damage of head of femur
May require hip replacement
What are the most common infectious causes of osteomyelitis
- Salmonella
2. Staphylococcus bacteria
Summarise stroke in SCD
Affects 8% SS
Most common 2-9 yrs
Involves major cerebral
vessels
Also involves internal carotid artery.
Summarise the features of gallstones in patients with SCD
May present with pain, acute cholecystitis with biliary obstruction and gall stonepancreatitis
Once symptomatic- treatment with laproscopic cholecystectomy is recommended
By 25 years prevalence
of gallstones
50% in SS
What genetic modifier can increase the risk of getting gallstones in patients with hereditary haemolytic anaemia (like sickle cell disease)?
Co-inheritance of Gilbert’s syndrome
Describe Gilbert’s syndrome
Coinheritance of Gilbert
syndrome(UGT 1A1
TA7/TA7 genotype)
further increases risk
Gilbert- benign variant of BR conjugation- urodine glucornoniside transferase gene (UGT1A1)- in promoter- element with TATA box- required for normal expression- normal 6 TA repeats- extra in gilbert on each allele- TO GIVE 7 TA REPEATS - Caused by an extra TA dinucleotide in the promoter on each chromosome (there are normally 6 TA repeats)
REDUCING EFFICIENCY OF TRANSCRIPTION- REDUCED BR CONJUGATION- REDUCED ENZYME ACTIVITY- INCREASED RISK OF G.S 3-5 FOLD
Summarise the laboratory features of SCD
Hb low (typically 6-8 g/dl) Reticulocytes high (except in aplastic crisis)
Film Sickled cells Boat cells Target cells Howell Jolly bodies
Ultimately, what does the area of central pallor reflect
The shape of the RBC
What type of inclusion is a Howell-Jolly body
Basophilic
Outline the solubility test to check for presence of HbS
In presence of a reducing agent oxyHb converted to deoxy Hb ( reducing agent- sodium diathianite or sodium metabisulfate)
Solubility decreases
Solution becomes turbid
Does not differentiate AS from SS (limitation- so only checks to see if you have one or more sickle traits.
Place against two black lines- won’t be able to see black lines if HbS present (makes the solution more opaque)
What is needed for a definitive diagnose of SCD and why
Electrophoresis or high performance liquid
Chromatography (HPLC) separates proteins according
to charge
Electrophoresis – takes advantage of the difference in charge between HbS, HbAS and HbA
Describe cellulose acetate membrane Hb electrophoresis
Separates proteins according to charge.
§ You can see homozygous sickle patients have NO HbA and only HbS and a little HbA2.
§ Heterozygous sickle patients have both HbS and HbA.
S travels further to the right than A
In heterozygous state- A band will be thicker (60% of Hb is HbA)
Summarise the general measures in the management of SCD
General measures
Folic acid (5mg/day)
Penicillin (by 3 months)
Vaccination (pneumococcoas, meningococcous and haemophilus, also influenzae- consequences more serious- bacterial pneumonia risk
Monitor spleen size (pre-empt the development of acute splenic sequestration)
Blood transfusion for acute anaemic events, chest syndrome and stroke
Pregnancy care (greater risk of maternal obstetric complications and fetal loss)
Why do we need to monitor folic acid in SCD
Increased production of RBCs- need adequate folic acid- DNA synthesis
Why do we need to monitor spleen size in SCD
Acute splenic sequestration can mean you get an enlarged spleen and often requires prophylactic therapy (against pneumococcal infection), transfusions to correct anaemia and maybe splenectomy.
Summarise the management of a painful crisis
PAINFUL CRISIS
Pain relief (opioids)- parenteral- short periods of time to prevent addiction-
Hydration- in pain- may find it hard to take in oral take or i.v fluids
Keep warm
Oxygen if hypoxic (establish cause of hypoxia)
Exclude infection:
Blood and urine cultures
CXR
What could be part of the differential for hypoxia in SCD
Acute chest syndrome
SCD patients also at risk of thrombo-embolic episodes so consider pulmonary embolism in differential.
Describe the importance of excluding infection in a painful crisis
IF FEBRILE OR INCREASE IN CRP OR OTHER INFLAMMATORY MARKERS- INFECTION SCREEN
If infection is present, antibiotics are needed.
List the factors that can trigger a painful crisis
Painful crises triggered by: Infection Exertion Dehydration Hypoxia Psychological stress
Why do we need to give penicillin prophylactically in SCD
Prophylactic penicillin to prevent some of the infections caused by hyposplenism.
Summarise the pain management of a painful crisis
Opioids
- Marked individual variation in response
- Diamorphine most widely used
- Most children receive oral opioid
Individual analgesia protocols
Patient controlled analgesia
Adjuvants – paracetamol, NSAIDs, Pregabalin/Gabapentin
Explain the pain management of a painful crisis
Opiate naïve- small doses
;larger doses for more tolerant opiods
Many children managed without opiods- maintain this in adult care
Individual analgesia protocol- doses and nature that are most effective
Patient controlled analgesia- bolus that patient can control themselves- lock out of 20 mins- patient can control dose- allows for lower doses
Pregabalin/Gabapentin - neuropathic pain- feature of prolonged pain crisis
Describe the different types of blood transfusion used in the management of SCD
Blood transfusion:
a) top up blood transfusion e.g. if aplastic or sequestration crises.
b) exchange blood transfusion if life threatening/severe disease such as a stroke, or chest crisis. An exchange transfusion aims to reduce the HbS to less than 20%.
NOTE: top-up blood transfusion is NOT a treatment for painful crisis; it will increase blood viscosity and may make the painful crisis worse.
What are the current disease modifying therapies for SCD
Transfusion
Hydroxycarbamide (Hydroxyurea)
Haemopoietic stem cell transplantation
Describe exchange transfusion
Stroke
Acute chest syndrome
Exchange diffusion
Machine seperates patients blood components by centrifugation- retunrs WBC, platelets, byt changes RBCs- aim to reduce % of HbS below 30
Blood will come from matched donor with normal Hb
Describe haemopoietic stem cell transplantation
<16 yr with severe disease
Survival 90-95% Cure 85-90%
Results optimal in cases where there is a matched HLA compatible related donor
Small mortality and risk of loss of fertility with stem cell- myeloablative conditioning
Potential pubertal failure
What are the two pharmacological agents available for the induction of HbF
Hydroxyurea
Butyrate
Name a drug that is used to induce HbF in sickle cell patients and explain the principle behind this treatment
Hydroxyurea/Hydroxycarbamide
· It is a ribonucleotide reductase inhibitor (cytotoxic)
· It induces the production of red cell in the bone marrow that mainly contain HbF
· So, over time, there will be an increase in the number of red cells that are unable to sickle
· It significantly reduces the frequency of crises
Butyrate also has a similar effect
Summarise hydroxyurea
HbF inhibits polymerisation of HbS
Infants with SCD do not usually develop symptoms until > 3 months
Patients with higher HbF levels have fewer complications and improved survival
What is the rationale for hydroxyurea in the treatment of SCD
Increases production of baby (fetal) haemoglobin (HbF)
Decreases ‘stickiness’ of sickle red blood cells
Reduces white blood cell production by the bone marrow (higher WBC count associated with worser outcomes)
Improves hydration of red blood cells (reduce conc of HbS-less polymerisation)
Generates nitric oxide which improves blood flow (theoretically beneficial)
Cytotoxic and so need to manage patient closely,
Describe the multicentre study of hydroxyurea in SCA
Double blind randomised trial 1992
299 adult patients (HU152 placebo 147)
> 3 painful crises/yr
HU at maximum tolerated dose
Concluded early after 30 months
Crisis rate significantly reduced (44%) HU 2.5/yr Placebo 4.5/yr
Reduction in
Hospitalisation 58%
Acute chest syndrome 51%
Transfusion 34%
Describe the use of hydroxyurea to treat SCA
Commonly used worldwide for both children and adults
Not universally prescribed in the U.K
Describe the long-term outlook for hydroxyurea
What is the long term outlook?
17.5 year follow up study 2010
Overall mortality 129/299 (43%)
Never received hydroxyurea 16/44 (36%)
>15 yr treatment with hydroxyurea 0/20
Describe the date supporting the use of haematopoietic stem cell transplant for SCD
33 children with HbSS
Age 3-17 years
22 CNS disease
Donors: HLA-matched sibs (n=31) or other related donor (n=2)
Overall survival (OS): 97.0%
Disease-free survival (DFS): 93.9%
aGvHD II-IV: 11.8%
cGvHD: 6.3%
What are the indications for haematopoietic stem cell transplant
Indications:
CNS disease
Recurrent severe VOC*
Recurrent ACS*
- if hydroxyurea fails
Describe CNS disease
CNS disease
Stroke
Abnormal TCD + silent infarct
Silent infarcts with cognitive deficiency
Abnormal MRA despite transfusions
Abnormal TCD + RBC alloantibodies
CNS disease requiring transfusions with iron overload despite optimal care
Describe the limitations of haematopoetic stem cell transplant
Donor availability 18% have unaffected sibling donor 1-2% of children with SCD qualify Length of Treatment: 2 months as an inpatient 4 months as outpatient Transplant Related Mortality Long Term Effects: Infertility Pubertal failure Chronic GvHD Organ toxicity Secondary malignancies
Describe the potential use of gene therapy for SCD
Lenti-viral vector
Contained modified beta globin gene
Amino acid change at 87- mimics that seen in HbF and responsible for sickling effect seen in HbF
Also provides convenient means of tracking production in RBCs of the variant- will separate from HbA and S in HLPC
Decline in HbA
Total Hb raised to higher level than that normally seen in SCD
Due to production of Hb of trans gene
Converts patient from homozygous state to state akin to sickle cell trait
Requires myeloablative conditioning to enable genetically corrected stem cells to re-populate the bone marrow
Describe the use of mAbs in SCD
Inhibits P-selectin (adhesion molecule involved in adhesion of RBC to endothelium)
P-selectin inhibitor Crizanlizumab
45.3% reduction in crisis rate (1.63 vs 2.98)
Median time to 1st crisis 4.07 vs 1.38 mo
2nd crisis 10.32 vs 5.09 mo
Works better at higher dose
Summarise the sickle cell trait
HbAS
Normal life expectancy
Normal blood count
Usually asymptomatic
Rarely painless haematuria (PAPILLARY NECLOSIS)
Caution: anaesthetic, high altitude, extreme exertion (SPLENIC INFARCTION) due to sickling of red cells
5.5 million carriers
Why does the sickle cell trait need to be identified
However it needs to be identified because certain situations can provoke sickling e.g. anaesthesia, high altitude, air travel in unpressurised planes. All patients from ethnic groups in whom βS occurs should be screened prior to surgery. Sickle trait must also be identified in pregnant women so that their partners can be tested and appropriate counselling given and action taken if necessary.
With the sickle solubility test, what must a positive result be confirmed by
A positive result must be confirmed by Hb electrophoresis.
Why does the diagnosis require Hb electrophoresis
Definitive diagnosis requires haemoglobin electrophoresis (or an equivalent test) as well as a sickle solubility test. Electrophoresis separates proteins according to their charge; this varies according to the pH at which electrophoresis is carried out. So, at alkaline pH, HbS separates readily from Hb A and F. However, there are some non-sickling haemoglobins (called HbD and HbG) that run with HbS – this is why a sickle solubility test is also needed. In sickle cell anaemia no HbA is detected and there is a variable (5-15%) amount of HbF and a small amount of HbA2. Patients with sickle trait have Hbs A and S (plus small amounts of HbA2 and HbF).
What occurs in an attempt to compensate for the shortened life span of RBCs
In an attempt to compensate for the shortened red cell life span there is an increased turnover of red cells and the body’s supply of folic acid can become low.
