How is Sickle Cell Diagnosed and Treated?

Diagnosis

A blood test can check for haemoglobin S — the defective form of haemoglobin that underlies sickle cell anaemia.  In the Ontario, this blood test is part of routine newborn screening done at the hospital.  But older children and adults can be tested, too.

In adults, a blood sample is drawn from a vein in the arm.  In young children and babies, the blood sample is usually collected from a finger or heel. The sample is then sent to a laboratory, where it’s screened for haemoglobin S.

If the screening test is negative, there is no sickle cell gene present. If the screening test is positive, further tests will be done to determine whether one or two sickle cell genes are present.

Additional tests

If you or your child has sickle cell anaemia, a blood test to check for a low red blood cell count (anaemia) will be done.  Your doctor might suggest additional tests to check for possible complications of the disease.

If you or your child carries the sickle cell gene, you should seek the assistance of a genetic counsellor and contact the Sickle Cell Association of Ontario for further support.

Tests to detect sickle cell genes before birth

Sickle Cell Disease can be diagnosed in an unborn baby by sampling some of the fluid surrounding the baby in the mother’s womb (amniotic fluid) to look for the sickle cell gene. If you or your partner has been diagnosed with sickle cell anaemia or sickle cell trait, ask your doctor about whether you should consider this screening.  Ask for a referral to a genetic counsellor who can help you understand the risk to your baby.

Complications of Sickle Cell Disease (SCD)

Sickle cell patients often suffer from other conditions or complications of SCD, called “comorbidities”.  The following are the percentages of SCD patients that have the specific comorbidities mentioned:

  • 58% acute chest syndrome (ACS) or pneumonia
  • up to 50% avascular necrosis (AVN)
  • 26% systemic hypertension (high blood pressure)
  • 26% congestive heart failure
  • 21% myocardial infarction (heart attack)
  • 14% arrhythmia (abnormal heartbeat)

Below is more detailed information on the various complications of Sickle Cell Disease.

Acute Chest Syndrome (ACS)

The acute chest syndrome (ACS) is an acute pulmonary illness that occurs in patients with sickle cell disease. ACS is currently defined as a new infiltrate on chest radiograph in conjunction with one other new symptom or sign: chest pain, cough, wheezing, tachypnea, and/or fever (Platt, 2000). The term acute chest syndrome was first suggested in 1979 by Charache et al (1979) and was developed to reflect the unique nature of acute pulmonary illness in patients with sickle cell disease. ACS can be caused by a variety of mechanisms, both infectious and noninfectious. Diagnostic considerations and treatment modalities are not typical of any other specific pulmonary illness experienced by the general population. Additionally, the typical course, possible complications, and outcomes are unique. For these reasons, the terminology persists and remains useful for both research purposes and effective clinical communication. As a leading cause of hospitalization and death in adults with sickle cell disease, the importance of ACS in sickle cell patients cannot be overstated (Hassel et al., 1994). Emergency physicians, family practitioners, pediatricians, internists, and hematologists who encounter these patients on a regular basis have the potential to prevent significant morbidity and mortality through early recognition and aggressive treatment of ACS.

Epidemiology

The incidence of ACS in patients with homozygous sickle cell disease is 12.8 episodes per 100 patient years (Castro et al., 1994). Incidence is inversely related to age, with children aged 2 through 4 years having the highest incidence (25.3 episodes/100 patient-years). ACS is associated with all genotypes but occurs most frequently in patients homozygous for the disease. Hematologic risk factors for the development of ACS include a high steadystate leukocyte count, low steady-state hemoglobin F concentration, and a high steady-state hemoglobin level (Castro et al., 1994). The risk for developing an ACS episode appears to be increased following surgery, with an average time for development of ACS of 3 days postsurgery (Vichinsky et al., 1995). Children often have a febrile episode preceding the event and are more likely to have an episode in the winter. Finally, both children and adults frequently have a painful event preceding the development of ACS (Vichinsky et al., 1997).

Infection

Infection has long been recognized as a cause of ACS. Initially Streptococcus pneumoniae was considered the most common infectious agent (Barrett-Connor 1971, 1973) but subsequent research has consistently reported lower rates of pneumococcal disease (Hassel et al., 1994; Sprinkle et al., 1986;  Kirkpatrick et al., 1991).  Chlamydia pneumoniae and Mycoplasma pneumoniae are now the most common documented infectious causes of ACS (Vichinsky et al, 2000). Other viral and bacterial organisms that have been linked to ACS include Haemophilus influenzae, Staphylococcus aureus, Klebsiella pneumoniae, adenovirus, influenza viruses, parainfluenza viruses, respiratory syncytial virus, parvovirus B19, and cytomegalovirus (Vichinsky et al., 2000).

Fat Emboli

Pulmonary vascular occlusion has long been suspected as a cause of ACS, but its exact role remains unclear. It was first considered as a cause of ACS after alveolar wall necrosis, pulmonary arterial thrombosis, and pulmonary infarction were found on postmortem evaluations (Haupt et al., 1982; Thomas et al., 1982). Further supporting evidence was provided by perfusion defects identified during ACS by computed tomography, angiography, and nuclear ventilation and perfusion scans (Bhalla et al., 1993; Lisbona et al., 1997). Although pulmonary vascular occlusion by in situ thrombosis and thromboembolism might account for some of these perfusion defects, current evidence suggests that many of these defects are due to fat emboli (Hassell et al., 1994; Vichinsky et al., 2000; Naprawa et al., 2005) that originate from bone marrow that becomes infarcted during vaso-occlusive crises. Reduced blood flow to bone marrow during these crises can cause painful ischemia and necrosis of the marrow, and necrotic pieces of marrow that break loose can become emboli. Postmortem studies have found fatty necrotic bone marrow in the pulmonary vasculature (Haupt et al, 1982). Additionally, some studies have noted clinical similarities between the fat emboli syndrome of trauma patients and ACS and have used this as supportive evidence (Levy, 1990). More recent investigation has identified fat droplets within cells recovered by bronchoscopy with bronchoalveolar lavage, confirming an association between fat embolism and ACS (Vichinsky et al., 1994, 2000). Finally, increased serum levels of free fatty acids and the enzyme secretory phospholipase A2 that occur during the syndrome are similar to the levels seen in the fat emboli syndrome (Naprawa et al., 2005).

Rib Infarction

Another likely cause of ACS is infarction of the ribs and other bones of the thorax. Infarction has been documented during sickle cell vaso-occlusive crisis and ACS by nuclear medicine techniques, and these infarctions are often in proximity to infiltrates on chest radiograph (Bellet et al, 1995). It is proposed that bone infarction during vaso-occlusive crisis leads to localized splinting, atelectasis, radiographic infiltrates, and ACS.

Pathogenesis

These varied causes of ACS have in common the ability to create regional hypoxia and lung injury, which are followed by a cascade of events made possible by the inherent pathophysiology of sickle cell disease. Regional hypoxia prevents reoxygenation of red blood cells returning to the lung and leaves them in their sickled form. Sickled red blood cells, with polymerized hemoglobin, are presumed to have difficulty passing through small vascular beds both because of their deformed shape and inflexibility and because of expression of adhesion molecules on the cell wall (Aldrich et al., 1996; Platt, 2000). The injured lung and hypoxia promote upregulation of adhesion molecules on the vasculature endothelium, causing sickled red blood cells to adhere to the endothelium (Lubin, 1997; Hammerman et al., 1999). Inflammatory mediators (Platt, 2000; Naprawa et al., 2005), free radical species (Klings et al., 2001), and interactions between red blood cells and white blood cells (Hofstra et al., 1996) are also induced by hypoxia and lung injury. Failure to reoxygenate, vascular stasis, and inflammation are suspected to create further red blood cell sickling, microvascular occlusion (Bhalla et al., 1993) and pulmonary infarction (Aldrich et al., 1996). A cycle of injury is also promoted by the development of shunt physiology, which creates more hypoxia (Gladwin and Schechter 1999). This cascade of events is believed to cause ACS from a precipitant as simple as atelectasis.

Clinical Symptoms

The clinical presentation of ACS varies, with the most common symptoms in all age groups being fever, cough, and chest pain (Vichinsky et al., 2000). Other less common presenting symptoms have been documented, including shortness of breath, productive cough, wheezing, and hemoptysis. The symptoms appear to be age-related, with fever and cough occurring most commonly in children and becoming less common with increasing age. Chest pain, shortness of breath, and chills, on the other hand, are less common in childhood and become more common with increasing age. Tachycardia, tachypnea, and hypoxia are variably seen on presentation. The most common physical examination finding is rales, but notably, the second most common finding is a normal physical examination. Furthermore, no clinical finding is predictive of the degree of hypoxia. The presenting symptoms in a patient’s first event are predictive of symptoms during subsequent events. Notably, up to 50% of patients diagnosed with ACS are initially admitted to the hospital for other reasons and subsequently develop the disease. The reason for admission in these cases is most often vaso-occlusive crisis. The average time to development of ACS after hospitalization is 2.5 days (Vichinsky et al., 2000).

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Avascular Necrosis

Other terms for Avascular Necrosis (AVN): osteonecrosis, bone infarction, aseptic necrosis, ischemic bone necrosis

Definition

A disease where there is a lack of blood supplied to bone tissue, resulting in cell death and bone collapse. It can also affect joints, with destruction of the articular cartilage surfaces causing arthritis. AVN primarily affects the shoulder, knee and hip joints.

Who is affected?

As many as 50% of people with SCD may be chronically affected by AVN, especially in the hip or shoulder. Eight out of ten people with SCD and AVN have it in both the hip and shoulder. The jaw and spine can also be affected in people with SCD. AVN in the hip is more common in SCD patients from Kuwait, Africa, the Mediterranean and the USA (Marti-Carvajal et al., 2010).

Diagnosis

Your physician may ask questions like:

  • When did the pain start?
  • Does the pain spread (radiate) anywhere?
  • Is the pain constant, or does it get better at night or at rest?
  • Have you noticed any difference in how much or how far you can move (your mobility)?
  • Do pain relievers help?

In the early stages of the disease, AVN can be detected by:

  • bone scintigraphy (bone scan)
  • MRI

In the later stages of the disease, AVN can be detected by X-ray or CT scans because nearby live bone appears more opaque due to increased bone resorption.

Symptoms

  • None in early stages
  • Later stages:
    • Pain in the affected joint that may increase over time
      • Pain will be severe if bone collapses
      • Pain can occur even at rest
    • Limited range of motion
    • Groin pain, if the hip joint is affected
    • Limping, if the affected site is below the hips

Prognosis

Prognosis depends on:

  • The cause of AVN
  • Disease stage when diagnosed
  • Amount of bone affected
  • Age
  • Overall health

Prevention

  • Avoid alcohol
  • Avoid corticosteroids
  • Avoid diving

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Pulmonary Hypertension

Pulmonary hypertension (PH) is abnormally high blood pressure in the arteries of the lungs, which forces the right side of the heart to work harder than usual (Blaivas, 2011). Blood becomes oxygenated by being pumped to the lung by the right side of the heart. When the small arteries in the lung become narrow and pressure builds up in the lung, the heart needs to work to pump the blood through the lung. If the heart is constantly working harder to pump blood, it can become enlarged and lead to heart failure. When there is not enough blood flowing through the lung and becoming oxygenated, symptoms will start (Blaivas, 2011). The true prevalence of PH within the SCD community cannot be accurately estimated due to overdiagnosis (Hebbel, 2011), but most recent reports suggest that the true prevalence is ~9% (Gladwin et al., 2004; Simmoneau et al., 2009).

Causes

Pulmonary hypertension can be caused by:

  • any condition that causes chronic low oxygen levels in the blood, including sickle cell disease
  • autoimmune diseases that damage the lungs
  • some birth defects of the heart
  • some diet medications
  • congestive heart failure
  • history of blood clot in the lung
  • HIV infection
  • lung or heart valve disease
  • obstructive sleep apnea
  • unknown idiopathic pulmonary arterial hypertension (IPAH)

Symptoms

You should contact your health care provider if you develop any of the following symptoms.  Common first symptom: shortness of breath or lightheadedness during activity.  Patients may feel ppalpitationsdue to their fast heart rate. Over time, symptoms will begin to occur during light activity or even while at rest. Patients with pulmonary hypertension have good days and bad days.

Other symptoms:

  • ankle and leg swelling
  • cyanosis (bluish colour of lips or skin)
  • chest pain or pressure
  • dizziness or fainting
  • fatigue
  • weakness

Diagnosis

Your doctor can do a physical examination, however the exam result may be normal in the early stages of pulmonary hypertension. In addition, the symptoms are very similar to the symptoms of asthma, which needs to be ruled out. After several months with the condition, the exam may show:

  • abnormal heart sounds
  • enlargement of neck veins
  • ability to feel a pulse over the breastbone
  • heart murmur
  • leg swelling
  • liver and spleen swelling
  • normal breathing sounds

Further testing is required to confirm pulmonary hypertension. Some tests include:

  • cardiac catheterization
  • chest x-ray
  • CT scan of the chest
  • echocardiogram
  • ECG
  • nuclear lung scan
  • pulmonary arteriogram
  • pulmonary function tests
  • sleep study

Many studies indicate that cardiac catheterization is an “indispensable part of the examination and should never be omitted” (Hebbel, 2011). Tricuspid valve jet velocity (TJV) should not be used as the sole diagnostic measure for PH as it can lead to false positives (diagnosis of PH when the patient does not actually have PH) (Hebbel, 2011). The six-minute walk test should not be used for sickle cell disease patients because the results could be due to anemia plus comorbidities, but falsely attributed to PH (Hebbel, 2011).

Before subjecting a sickle cell patient to catheterization for diagnosis, the doctor should identify any comorbidities that may be causing increased pulmonary pressure, such as chronic lung disease, thrombosis or thromboembolism, left-sided heart disease, recurrent significant nocturnal desaturation from disordered sleep. The doctor should then treat these comorbidities directly before making a diagnosis of PH (Hebbel, 2011).

Prognosis

The prognosis is currently poor, but new therapies may improve it. See the Current Research Section on Nitric Oxide for more details. Some people with pulmonary hypertension develop progressive heart failure, which can lead to death. In hospitalized SCD patients, there is a high prevalence of sudden death with unknown cause (Powars, 1991; Platt et al., 1994). One study suggested that PH may be responsible for up to 33% of sudden deaths in hospitalized SCD patients (Graham et al., 2007).

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Gall Stones

The formation of gall stones is one complication of sickle cell disease. Historically, the standard treatment was to remove the gall stones to avoid bile duct occlusion. In Jamaica, Dr. Graham Serjeant conducted a pioneering study to determine whether leaving otherwise asymptomatic gall stones would yield better clinical outcomes. The study showed that patients who had not undergone invasive surgery to remove the gall stones had less mortality and morbidity than their counterparts who had their gall stones removed. Currently, the treatment for gall stones in Jamaica and across the world is to leave gall stones unless they become symptomatic.

 

Hemolytic Anemia and Iron Overload

What is  Anemia?

Anemia comes from an Ancient Greek word meaning lack of blood. Anemia is characterized by either a lower number of red blood cells (RBCs) or a lower quantity of hemoglobin in the RBCs than normal.  In the sickle cell disease case, there is a mutation in the hemoglobin gene (HbS), which leads to their polymerization, causing the RBCs to sickle.  These RBCs have a shorter lifespan than normal RBCs.  This is known as hemolytic anemia, which leads to vasculopathy (problems with the circulatory system), including:

  • systemic and pulmonary hypertension (high blood pressure)
  • problems with the function of the cells that line blood vessels (endothelial cells) and the smooth muscle surrounding the vessels, such as changes in their proliferation (division and replication) (Rees et al., 2010).

How does iron overload occur?

Normally the body prevents iron overload by only absorbing the iron it needs from the food you eat, so it does not have a way to excrete excess iron from the blood stream (Inati et al., 2011). This is why iron overload is an inevitable consequence of chronic blood transfusions. In patients with thalassemia, iron overload occurs after 10-20 transfusions (Porter, 2001). In conditions where there are problems with red blood cell formation, such as thalassemia, the body actually increases dietary iron absorption to help with red blood cell formation. This bodily response can occur even if the patient is receiving excess iron by transfusion, and it contributes even more to the patient’s iron overload. In patients with sickle cell disease, red blood cell formation is normal, so increased dietary absorption does not occur and does not affect iron overload (Inati et al., 2011).

How does iron overload affect the patient?

There is a protein in the blood, called transferrin, which binds and carries iron to sites in the body where it is needed (Papanikolaou and Pantopolous, 2005). In the case of iron overload, there is not enough transferrin to bind all of the iron, leaving non-transferrin-bound iron (NTBI) in the blood. Some of the NTBI is pathological and is called labile plasma iron (LPI). The LPI can form reactive oxygen species (ROS) (Cabantchik et al., 2005). Cells normally produce ROS in small amounts, which are then cleared away by antioxidants. In the case of iron overload, there is a lot of LPI forming a lot of ROS, which can then enter organs and damage lipids, proteins and DNA, eventually leading to organ damage and death, if not treated (Britton et al., 2002). Iron overload decreases the success rate of hematopoietic stem cell transplantation by increasing the rate of graft vs host disease, infections and death (Pullarkat et al., 2008; Platzbecker  et al., 2008). In patients with sickle cell disease, iron overload also increases the frequency of acute events, hospitalizations and death (Fung et al., 2007; Kushner et al., 2001; Ballas, 2001; Aduloju et al., 2008). In patients with B-thalassemia, iron overload causes serious heart complications, which is the primary cause of death for these patients (Olivieri et al., 1994; Zurlo et al., 1989; Modell et al., 2000; Borgna-Pignatti et al., 1998; Cohen et al., 2004). In contrast, heart complications due to iron overload are a very rare occurence in patients with SCD (Batra et al., 2002). Similarly, iron overload-induced endocrine system problems, such as diabetes, hypogonadism, hypothyroidism and growth failure, are frequent in patients with thalassemia, but rare in patients with SCD (Fung et al., 2006). Since excess iron tends to accumulate in the liver, ROS can also lead to liver fibrosis (scarring), which is also seen more frequently in patients with thalassemia than in patients with SCD (Inati et al., 2011).

How is iron overload assessed?

Patients with thalassemia are more likely to be routinely assessed for iron overload than patients with SCD because they require more frequent and regular transfusions. However, there is evidence to suggest that the iron burden on both patient groups is similar (Fung et al., 2008), so patients with SCD should be assessed routinely as well. Since SCD patients have increased life expectancy than in the past, they may require more transfusions, resulting in higher iron overload, which further supports the need for routine assessment.

Ferritin is a protein that stores iron and releases it when it’s needed. “Serum ferritin” or the level of ferritin in the blood is typically used in assessment for iron overload because it is easy and quick to test. Its validity and reliability as a marker of iron load has been questioned though for a few reasons. Firstly, serum ferritin levels can fluctuate due to inflammation and vaso-occlusive crises, which are both characteristic of SCD. Several serum ferritin readings at different times under steady states are therefore required to get a more accurate estimate of true body iron levels (Karam et al., 2007; Porter and Huehns, 1987). Secondly, it is unclear whether serum ferritin truly correlates with liver iron concentration (LIC) as some studies have indicated a positive correlation, while others have failed to find a correlation. Also, within studies that have found a positive correlation, some found the correlation to be linear, while others found a nonlinear correlation (Inati et al., 2011). One study tested the correlation over a large range of iron overload and found that serum ferritin cannot accurately estimate LIC at intermediate ranges of iron overload (Adamkiewicz et al., 2009). Current guidelines state that serum ferritin assessments may be used when a patient is in a steady state, but that it should not be used as the sole method of assessment (Sickle Cell Society, 2008).

Since most of the excess iron ends up in the liver, the best way to assess iron overload is to assess the liver iron concentration (LIC) directly. This is usually done by a liver biopsy, where a small piece of the patient’s liver is removed surgically and tested for iron levels. Although this method is the most accurate and reliable, it is invasive, has a hospitalization rate of 2% and may be inaccurate if the patient has liver fibrosis (scarring) (Adamkiewicz et al., 2009).

Other methods of detemining iron overload have been investigated, but were unsuccessful. Methods to measure levels of non-transferrin-bound iron (NTBI) were developed (Cabantchik et al., 2005) and correlated well to transfusion burden, but had even worse predictability of liver iron concentration (LIC) than serum ferritin (Inati et al., 2010). Transfusion rate was suggested as an indicator of LIC because some studies found that transfusion number and duration correlated to LIC, but other studies found no correlation (Inati et al., 2011).

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Immune Responses and Malaria

Malaria

The plasmodium parasite that causes malaria is transmitted from mosquitoes to men. The parasites spend part of their life cycle in the mosquito and part of it in the human host. The infective plasmodial sporozoites enter the bloodstream from the saliva of the feeding female anopheles mosquito. The Kupfer cells of the liver clear the sporozoites from the blood stream and kill many of the organisms. A fraction of the sporozoites escape destruction however, and penetrate the hepatocytes where they take up residence.


The parasites within the hepatocytes transform into a new entities called “schizonts”. The nuclear genetic material in the schizonts replicates to the point that the hepatocytes are totally filled with new forms called “merozoites”. A single schizont can produce thousands of merozoites. Erumpent hepatocytes release the merozoites into the bloodstream where they invade circulating red cells. After penetrating the red cells, the merozoites assume a ring form called trophozoites. These organisms consume the hemoglobin in the red cells and enlarge until they fill the cell completely. During their growth, the trophozoites metamorph into schizonts and produce new merozoites inside the red cells. The red cells subsequently lyse and release merozoites that can penetrate new red cells and restart the pernicious process.


Some of the trophozoites in the red cells take a different developmental pathway and form gametocytes. Gametocytes are the sexual form of the parasite and do no lyse the red cells. A mosquito taking a blood meal from a person whose red cells contain gametocytes acquires the malarial parasite. The sexual reproduction cycle then begins in the mosquito. The mosquito subsequently transmits the parasite when it attacks another human host.

Malaria Defenses

The complex nature of the malaria parasite life cycle in the human host presents several points at which the organism could be targeted for destruction. The sporozoites injected into the blood stream with the initial mosquito bite are attacked there by components of the immune system. These include antibodies, lymphocytes called “natural killer cells” as well as lymphocytes that attack the malarial parasites because of prior exposure to the organisms (conditioned lymphocytes).


Host immunity is crucial to survival of people infected with the malaria parasite. This is particularly true with respect to the nocuous falciparum parasite. The immune system works best when it has been primed against the invader. Children who suffer their first or second bout of malaria have not developed the immune response needed to provide adequate defense against the parasite. This explains in part the high mortality seen in children infected with P. falciparum. Vaccines are a common way of achieving host immunity prior to pathogen exposure. Polio immunization is a well-known example. Unfortunately, the malarial parasite constantly changes its immune makeup, making it difficult to produce an effective vaccine.

Malaria and Sickle Cell Anemia

Sickle hemoglobin provides the best example of a change in the hemoglobin molecule that impairs malaria growth and development. The initial hints of a relationship between the two came with the realization that the geographical distribution of the gene for hemoglobin S and the distribution of malaria in Africa virtually overlap. A further hint came with the observation that peoples indigenous to the highland regions of the continent did not display the high expression of the sickle hemoglobin gene like their lowland neighbours in the malaria belts. Malaria does not occur in the cooler, drier climates of the highlands in the tropical and subtropical regions of the world. Neither does the gene for sickle hemoglobin.


The sickle trait (heterozygous) provides a survival advantage over people with normal hemoglobin in regions where malaria is endemic, however, the trait provides neither absolute protection nor invulnerability to the disease. Rather, people (and particularly children) infected with P. falciparum are more likely to survive the acute illness if they have sickle cell trait. When people with sickle cell trait procreate, both the gene for normal hemoglobin and that for sickle hemoglobin are transmitted to the next generation.


The genetic selective scenario in which a heterozygote for two alleles of a gene has an advantage over either of the homozyous states is called “balanced polymorphism”. A key concept to keep in mind is that the selection is for sickle cell trait. A common misstatement is that malaria selects for sickle cell disease. This is not true. A person with sickle cell disease is at an extreme survival disadvantage because of the ravages of the disease process. This means that a negative selection exists for sickle cell disease. Having the sickle cell trait is the genetic condition selected for in regions of endemic malaria. Sickle cell disease is a necessary consequence of the existence of the trait condition because of the genetics of reproduction. The precise mechanism by which the sickle cell trait imparts resistance to malaria is unknown. A number of factors likely are involved and contribute in varying degrees to the defense against malaria.


Red cells from people with the sickle trait do not sickle to any significant degree at normal venous oxygen tension. Very low oxygen tensions will cause the cells to sickle, however. For example, extreme exercise at high altitude increases the number of sickled erythrocytes in venous blood samples from people with the sickle cell trait (Martin, et al., 1989). Sickle trait red cells infected with the P. falciparum parasite deform, presumably because the parasite reduces the oxygen tension within the erythrocytes to very low levels as it carries out its metabolism. Deformation of sickle trait erythrocytes would mark these cells as abnormal and target them for destruction by phagocytes (Luzzatto, et al., 1970).


Experiments carried out in vitro with sickle trait red cells showed that under low oxygen tension, cells infected with P. falciparum parasites sickle much more readily than do uninfected cells (Roth Jr., et al., 1978). Since sickle cells are removed from the circulation and destroyed in the reticuloendothelial system, selective sickling of infected sickle trait red cells would reduce the parasite burden in people with the sickle trait. These people would be more likely to survive acute malarial infections.


Other investigations suggest that malaria parasites could be damaged or killed directly in sickle trait red cells. P. falciparum parasites cultured in sickle trait red cells died when the cells were incubated at low oxygen tension (Friedman, 1978). In contrast, parasite health and growth were unimpeded in cells maintained at normal atmospheric oxygen tensions. The sickling process that occurs at low oxygen tensions was presumed to harm the parasite in some fashion. Studies showed extensive vacuole formation in P. falciparum parasites inhabiting sickle trait red cells that were incubated at low oxygen tension, suggesting metabolic damage to the parasites (Friedman, 1979). Prolonged states of hypoxia are not physiological, raising questions about degree to which these data can be extrapolated to human beings. However, they do suggest mechanisms by which sickle hemoglobin at the concentrations seen with sickle cell trait red cells could impair parasite proliferation.


The thalassemias also reached levels of expression in human populations by protecting against malaria. The imbalance in hemoglobin chain production characteristic of thalassemia produces membrane oxidation by hemichromes and other molecules that generate reactive oxygen species (Grinberg, et al., 1995; Sorensen, et al., 1990). Reactive oxygen species also injure and kill malaria parasites (Clark, et al., 1989). In vitro malaria toxicity of thalassemic red cells is most easily seen in cells containing hemoglobin H (ß-globin tetramers) (Ifediba, et al., 1985; Yathavong, et al., 1988). Hemoglobin H occurs most often in people with three-gene deletion alpha-thalassemia (Zhu, et al., 1993). The compound heterozygous condition of two-gene deletion alpha thalassemia and hemoglobin Constant Spring also produces erythrocytes that contain hemoglobin H (Derry, et al., 1988). Alpha thalassemia may protect against malaria in part by altering the immune response to parasitized red cells (Luzzi, et al., 1991) In any event, epidemiological studies show clear evidence of protection provided by two-gene deletion alpha thalassemia (Flint, et al., 1986; Modiano, et al., 1991).

 

Future Direction

An important future goal for healthcare providers and policymakers is to be able to more effectively monitor patients for early diagnosis and intervention of complications. England is a pioneer in this regard. Their rationale has been that early diagnosis and intervention will result in less pain for patients as well as less cost for taxpayers.  Canada has followed suit with the implementation of newborn screening for Sickle Cell Disease in most provinces.

References

A good reference for hydroxyurea: http://www.ahrq.gov/downloads/pub/evidence/pdf/hydroxyurea/hydroxscd.pdf

  1. Charache S, Terrin ML, Moore RD, Dover GJ, Barton FB, Eckert SV, McMahon RP, Bonds DR. Effect of hydroxyurea on the frequency of painful crises in sickle cell anemia. Investigators of the Multicenter Study of Hydroxyurea in Sickle Cell Anemia. N Engl J Med. 1995 May 18;332(20):1317-22.
  2. Rees, DC, Williams, TN, and Gladwin, MT. Sickle-cell disease. The Lancet. 376: 2018-31, 2010.
  3. Inati, A, Khoriaty, E, and Musallam, KM. Iron in sickle-cell disease: what have we learned over the years? Pediatr Blood Cancer. 56: 182-190, 2011.
  4. Porter, JB. Practical management of iron overload. Br J Haematol. 115: 239-252, 2001.
  5. Papanikolaou, G, and Pantopolous, K. Iron metabolism and toxicity. Toxicol Appl Pharmacol. 202: 199-211, 2005.
  6. Cabantchik, ZI, Breuer, W, Zanninelli, G, et al. LPI-labile plasma iron in iron overload. Best Pract Res Clin Haematol. 18: 277-287, 2005.
  7. Britton, RS, Leicester, KL, and Bacon, BR. Iron toxicity and chelation therapy. Int J Hematol. 76: 219-228, 2002.
  8. Pullarkat V, Blanchard S, Tegtmeier B, et al. Iron overload adversely affects outcome of allogeneic hematopoieticcell transplantation. Bone Marrow Transplant 2008;42:799–805.
  9. Platzbecker U, Bornhauser M, Germing U, et al. Red blood cell transfusion dependence and outcome after allogeneic peripheral blood stem cell transplantation in patients with de novo myelodysplastic syndrome (MDS). Biol Blood Marrow Transplant 2008;14:1217–1225.
  10. Fung EB, Harmatz P, Milet M, et al. Morbidity and mortality in chronically transfused subjects with thalassemia and sickle cell disease: A report from the multi-center study of iron overload. Am J Hematol 2007;82:255–265.
  11. Kushner JP, Porter JP, Olivieri NF. Secondary iron overload. Hematol Am Soc Hematol Educ Program 2001;47–61.
  12. Ballas SK. Iron overload is a determinant of morbidity and mortality in adult patients with sickle cell disease. Semin Hematol 2001;38:30–36.
  13. Aduloju SO, Palmer S, Eckman JR. Mortality in sickle cell patient transitioning from pediatric to adult program: 10 years Grady comprehensive sickle cell center experience. Blood 2008;112: Abst 1426.
  14. Olivieri NF, Nathan DG, MacMillan JH, et al. Survival in medically treated patients with homozygous b-thalassemia. N Engl J Med 1994;331:574–578.
  15. Zurlo MG, De Stefano P, Borgna-Pignatti C, et al. Survival and causes of death in thalassaemia major. Lancet 1989;2:27–30.
  16. Modell B, Khan M, Darlison M. Survival in b-thalassaemia major in the UK: Data from the UK Thalassaemia Register. Lancet 2000;355:2051–2052.
  17. Borgna-Pignatti C, Rugolotto S, De Stefano P, et al. Survival and disease complications in thalassemia major. Ann NY Acad Sci 1998;850:227–231.
  18. Cohen AR, Galanello R, Pennell DJ, et al. Thalassemia. Hematol Am Soc Hematol Educ Program 2004;14–34.
  19. Batra AS, Acherman RJ, Wong WY, et al. Cardiac abnormalities in children with sickle cell anemia. Am J Hematol 2002;70:306–312.
  20. Fung EB, Harmatz PR, Lee PD, et al. Increased prevalence of iron-overload associated endocrinopathy in thalassaemia versus sickle-cell disease. Br J Haematol 2006;135:574–582.
  21. Fung EB, Harmatz PR, Milet M, et al. Disparity in the management of iron overload between patients with sickle cell disease and thalassemia who received transfusions. Transfusion 2008;48:1971–1980.
  22. Karam LB, Disco D, Jackson SM, et al. Liver biopsy results in patients with sickle cell disease on chronic transfusions: Poor correlation with ferritin levels. Pediatr Blood Cancer 2007;50:62–65.
  23. Porter JB, Huehns ER. Transfusion and exchange transfusion in sickle cell anaemias, with particular reference to iron metabolism. Acta Haematol 1987;78:198–205.
  24. Adamkiewicz TV, Abboud MR, Paley C, et al. Serum ferritin level changes in children with sickle cell disease on chronic blood transfusion are non-linear, and are associated with iron load and liver injury. Blood 2009;114:4632–4638.
  25. Sickle Cell Society. Standards for the clinical care of adults with sickle cell disease in the UK. 2008. Available at: www.sicklecellsociety.org/CareBook.pdf.
  26. Inati A, Musallam KM, Cappellini MD, et al. Non-transferrinbound iron in transfused patients with sickle cell disease. Int J Lab Hematol 2010; DOI: 10.1111/j.1751-553X.2010.01224.x. Epup ahead of print.
  27. Abetz LN, Baladi JF, Jones P. Transfusion and iron overload (IO) in MDS: Impact of infusion chelation therapy (ICT) on quality of life (Qol) and adherence. Leuk Res 2005;29:S54.
  28. Treadwell MJ, Law AW, Sung J, et al. Barriers to adherence of deferoxamine usage in sickle cell disease. Pediatr Blood Cancer 2005;44:500–507.
  29. Schnog JB, Duits AJ, Muskiet FA, et al. Sickle cell disease: A general overview. Neth J Med 2004;62:364–374.
  30. Steinberg MH, Barton F, Castro O, et al. Effect of hydroxyurea on mortality and morbidity in adult sickle cell anemia: Risks and benefits up to 9 years of treatment. JAMA 2003;289:1645–1651.
  31. Verduzco LA, Nathan DG. Sickle cell disease and stroke. Blood 2009;114:5117–5125.
  32. Hebbel RP. Reconstructing sickle cell disease: A data-based analysis of the “hyperhemolysis paradigm” for pulmonary hypertension from the perspective of evidence-based medicine. Am J Hematol 2011; 86:123-154.
  33. Blaivas, Allen J. “Pulmonary hypertension.” NCBI PubMed Health. 7 May 2011. <http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001171/>
  34. Hebbel, RP. Reconstructing sickle cell disease: A data-based analysis of the “hyperhemolysis paradigm” for pulmonary hypertension from the perspective of evidence-based medicine. American Journal of Hematology. 86:123-154, 2011.
  35. Gladwin MT, Sachdev V, Jison ML, et al. Pulmonary hypertension as a risk factor for death in patients with sickle cell disease. N Engl J Med 2004;350: 886–895.
  36. Simonneau G, Robbins IM, Beghetti M, et al. Updated clinical classification of pulmonary hypertension. J Am Coll Cardiol 2009;54:S43–S54.
  37. Powars DR. Sickle cell anemia: Beta s-gene-cluster haplotypes as prognostic indicators of vital organ failure. Semin Hematol 1991;28:202–208.
  38. Platt OS, Brambilla DJ, Rosse WF, et al. Mortality in sickle cell disease. Life expectancy and risk factors for early death. N Engl J Med 1994;330:1639–1644.
  39. Graham JK, Mosunjac M, Hanzlick RL, et al. Sickle cell lung disease and sudden death: A retrospective/prospective study of 21 autopsy cases and literature review. Am J Forensic Med Pathol 2007;28:168–172.
  40. Marti-Carvajal AJ, Sola I, Agreda-Perez LH. Treatment for avascular necrosis of bone in people with sickle cell disease (Review). The Cochrane Collaboration 2010, Issue 11

Doctors treat most complications of sickle cell anemia as they occur.  Treatment might include antibiotics, vitamins, blood transfusions, pain-relieving medicines, other medications and possibly surgery, such as to correct vision problems or to remove a damaged spleen.  Treatments for specific complications can be explored below.

Acute Chest Syndrome (ACS)

Antibiotics

All patients with ACS should receive antibiotics at presentation, which should include a third-generation cephalosporin to cover S. pneumoniae, H. influenzae, and K. pneumoniae, and a macrolide to cover M. pneumoniae and C. pneumoniae (Vichinsky et al., 2000).  Risk factors for more virulent organisms and culture results can guide further therapy. Because of the inherent limitations of blood cultures and the established difficulty of clinically excluding an infectious etiology, a full course of antibiotics is recommended regardless of culture results (Kennedy et al, 2005).

Bronchodilators

It appears that ACS includes a reactive airway component that often responds to treatment, which may be partly attributed to the high prevalence of asthma in the sickle cell population. The mean forced expiratory volume for patients with ACS has been documented as 53% of predicted normal. In the NACSSG, 20% of patients demonstrated clinical improvement with administration of bronchodilators (Vichinsky et al., 2000).

Transfusion Therapy

Transfusion therapy is often used in the treatment of ACS, with 72% of patients in the NACSSG receiving some form of transfusion (Vichinsky et al., 2000). Reports of dramatic improvement in clinical condition after initiation of transfusion are well documented in the literature (Hassell et al., 1994). Both the partial pressure of arterial oxygen and oxygen saturation has also been shown to significantly improve with transfusion (Emre et al., 1995). The mechanism by which this therapy works is not entirely known, but it is likely related to improved oxygenation and lowering the hemoglobin S concentration. The latter mechanism may promote blood flow through the pulmonary vasculature. Transfusion also may have an effect on the inflammatory mediators of disease.

Prevention

Prevention of ACS is possible and is essential to the long-term health of patients with sickle cell disease. Each episode of ACS places the patient at risk for event-related mortality and long-term lung injury. Recurrent episodes are thought to contribute to chronic lung disease, pulmonary hypertension, and cor pulmonale (Powars et al., 1988). Hydroxyurea is a relatively new treatment option available for patients with sickle cell disease. Hydroxyurea is a ribonuclease reductase inhibitor with many physiologic effects, including increasing fetal hemoglobin production, decreasing white blood cell counts, and altering adhesion molecules on reticulocytes. Use of this medication has been associated with fewer episodes of pain crises and ACS, decreased need for transfusion, and lower mortality (Zimmerman et al., 2004; Hankins et al., 2005).

Avascular Necrosis

If the disease is diagnosed early, treatment may consist of:

  • taking pain relievers
  • limiting use of the affected area
  • physical therapy (exercises to increase range-of-motion)
  • bisphosphonate medication to reduce bone resorption and prevent bone collapse

The above treatments can slow the progression of AVN, but most patients will require surgery. Surgical options may include:

  • Total joint replacement
    • This requires long recovery, and the replacements have short lifespans.
  • Hip resurfacing (more details on site)
  • Bone graft
  • Vascularised bone graft (bone grafted along with its blood supply)
    • Example: Free vascular fibrular graft (FVFG)
  • Osteotomy (cutting the bone and re-aligning it to relieve stress on the bone or joint)
  • Core decompression (removing some of the inside of the bone to relieve pressure and allow new blood vessel formation)

One study showed that outcomes for people with SCD and AVN undergoing physical therapy were not significantly improved by surgery, specifically hip core decompression (Marti-Carvajal et al., 2010), suggesting that physical therapy alone may be a sufficient treatment option.

 

Pulmonary Hypertension

Once a positive diagnosis has been made, a patient with sickle cell disease and Pulmonary Hypertension (PH) should consult a pulmonary hypertension expert in order to choose and monitor appropriate therapy (Hebbel, 2011).

Since there is no known cure for PH, treatment consists of managing the symptoms. If the PH is caused by a medical disorder, such as sickle cell disease, it is important to treat the disorder.

People with PH are strongly advised to:

  • avoid pregnancy
  • avoid strenuous activity and heavy lifting
  • avoid travelling to high altitudes
  • get your yearly flu vaccine
  • get your pneumococcal pneumonia vaccine
  • stop smoking
  • make necessary changes in the home environment and get help around the home as the condition progresses.

Some medicines prescribed for treatment include:

  • Ambrisentan (Letairis)
  • Bosentan (Tracleer)
  • calcium channel blockers
  • diuretics
  • Prostacyclin or similar medications
  • Sildenafil

Some patients are prescribed blood thinners to reduce the risk of blood clots in the legs and lungs.  If the condition is advanced, patients may need oxygen.
If the patient does not respond to medications, some may require a lung and/or heart transplant.

 

Hemolytic Anemia and Iron Overload

How is hemolytic anemia treated?

Usually hemolytic anemia is treated with Hydroxyurea, but in cases where a patient is unresponsive to hydroxyurea (Schnog et al., 2004; Steinberg et al., 2003) or has over 3-4 crises per year, transfusions may be recommended. Transfusions remain the primary treatment for some complications, particularly stroke (Verduzco et al., 2009).  In most cases, however, transfusions are avoided due to the possibility of alloimmunization (Inati et al., 2011), which is when the patient’s immune system attacks the donor red blood cells, and the development of a dependence on transfusions.

Blood transfusions are used to treat anemia by decreasing the percentage and synthesis of HbS and reducing hemolysis (RBC breaking, bursting, dying) (Rees et al., 2010).  Blood transfusions may be used acutely or chronically to treat patients with SCD for a variety of reasons (Inati et al., 2011), as shown in table below.

 
Acute blood transfusion Chronic blood transfusions
worsening anemia* (due to parvovirus infection, vasoocclusive episode or malaria-induced severe hemolytic anemia) prevention of recurrent stroke
acute splenic sequestration (red blood cells get stuck in the spleen, causing it to enlarge) prevention of first stroke in some cases
severe or long-lasting aplastic crises (no red blood cells produced due to parvovirus infection) chronic pulmonary hypertension (high blood pressure in the lungs) when other treatments have failed
acute chest syndrome (severe pneumonia) congestive heart failure (when other treatments have failed)
multiple organ-failure syndrome previous acute splenic sequestration in a child under 3 years (before splenectomy)
in some pre-operative cases chronic pain
Table adapted from Inati et al., 2011.

There are different types of blood transfusions; each with its own uses, advantages and disadvantages as outlined in table below.

 

Type Uses Advantages Disadvantages
Simple/top-up: patient given more blood
  • severe anemia
  • transient red cell aplasia (problems with development of red blood cells)
  • acute splenic sequestration (red blood cells get stuck in the spleen, causing it to enlarge)
  • acute hepatic sequestration (red blood cells get stuck in the liver, causing it to enlarge)
  • simple and effective
  • widely available
  • reduced exposure to donor blood
  • iron loading
  • cannot achieve <30% HbS without risk of blood thickening
Exchange/red cell pheresis: sickle cells removed and replaced with normal red cells
  • used when rapid alteration of Hb is required without blood thickening
  • acute stroke
  • acute chest syndrome (severe pneumonia)
  • severe sepsis (infection)
  • acute multi-organ failure
  • progressive intrahepatic cholestasis (bile stuck in the liver)
  • fast
  • little iron loading
  • reduction of iron overload in some patients
  • better control of HbS without changing other aspects of blood
  • increased use of red cells
  • increased rate of donor exposure, risk of infection and alloimmunization (immune system attacking red cells)
  • access to veins can limit use
  • requires trained staff and special equipment
  • extra care required in children
Rapid partial exchange: blood removed from one arm, while donor cells transfused into other arm
  • many of the above uses
  • little iron loading
  • slow
  • careful control of removal vs. transfusion required
Table adapted from Inati et al., 2011.

However, chronic transfusions can lead to iron overload, with iron usually accumulating in the liver (Rees et al., 2010).

How is iron overload treated?

Iron chelators can be used to treat iron overload. Iron chelators are molecules that bind iron and remove it from the body and can be delivered orally or parenterally (Rees et al., 2010). Desferoxamine (DFO, Desferal(R), Novartis Pharma AG, Basel, Switzerland) was first introduced in the 1960s and is currently the treatment standard reference to which other treatments are compared (Modell et al., 2000). The efficacy of the drug in improving moribidity and mortality in patients with chronic transfusions has been proven through decades of clinical use, although little formal research has been done in this area (Inati et al., 2011). The fact that subcutaneous infusion of the drug takes 8-12 hours and needs to be done 5-7 nights per week can significantly decrease the patient’s quality of life and many patients tend to have difficulty adhering to the treatment regimen (Abetz et al., 2005; Treadwell et al., 2005). Additionally, possible side effects of the drug include local reactions, growth retardation, eye, ear, lung and allergic reactions (Treadwell et al., 2005).

 

The oral iron chelator deferasirox (Exjade(R), Novartis Pharma AG, Basel, Switzerland) is the first oral chelator to be licensed for use in patients with SCD. Studies indicate that it is effective for most patients, however, some patients require a high dose to see good effects, while other patients see no effect even at high doses. This drug has similar side effects to those of DFO, as well as possible kidney, liver and gastrointestinal problems. Serious and fatal side effects are usually seen in older patients or patients with underlying kidney, liver or blood problems. A significant advantage to deferasirox is that it taken orally so most patients prefer it over desferoxamine and are more likely to stick to their treatment regimen (Inati et al., 2011).

The oral iron chelator deferiprone (Ferriprox(R), Apotex Inc., Toronto, ON, Canada) is only available in some countries outside North America. It is taken three times daily and is only used in patients with thalassemia major when parental therapy with desferoxamine (DFO) cannot be used. Only a small number of short-term studies have been done, and until larger-scale longer-term studies are completed to determine efficacy and safety, it will not be licensed for use in patients with SCD or in North America (Inati et al., 2011).

 

Experimental treatments

Scientists are studying new treatments for sickle cell anemia, including:

  • Gene therapy.  Researchers are exploring whether inserting a normal gene into the bone marrow of people with sickle cell anemia will result in normal hemoglobin.  Scientists are also exploring the possibility of turning off the defective gene while reactivating another gene responsible for the production of fetal hemoglobin — a type of hemoglobin found in newborns that prevents sickle cells from forming.  Potential treatments using gene therapy are a long way off, however.
  • Nitric oxide.  People with sickle cell anemia have low levels of nitric oxide in their blood. Nitric oxide is a gas that helps keep blood vessels open and reduces the stickiness of red blood cells. Treatment with inhaled nitric oxide might prevent sickle cells from clumping together. Studies on nitric oxide have shown little benefit so far.
  • Drugs to boost fetal hemoglobin production.  Researchers are studying various drugs to devise a way to boost the production of fetal hemoglobin.  This is a type of hemoglobin that stops sickle cells from forming.

References

A good reference for hydroxyurea: http://www.ahrq.gov/downloads/pub/evidence/pdf/hydroxyurea/hydroxscd.pdf

  1. Charache S, Terrin ML, Moore RD, Dover GJ, Barton FB, Eckert SV, McMahon RP, Bonds DR. Effect of hydroxyurea on the frequency of painful crises in sickle cell anemia. Investigators of the Multicenter Study of Hydroxyurea in Sickle Cell Anemia. N Engl J Med. 1995 May 18;332(20):1317-22.
  2. Rees, DC, Williams, TN, and Gladwin, MT. Sickle-cell disease. The Lancet. 376: 2018-31, 2010.
  3. Inati, A, Khoriaty, E, and Musallam, KM. Iron in sickle-cell disease: what have we learned over the years? Pediatr Blood Cancer. 56: 182-190, 2011.
  4. Porter, JB. Practical management of iron overload. Br J Haematol. 115: 239-252, 2001.
  5. Papanikolaou, G, and Pantopolous, K. Iron metabolism and toxicity. Toxicol Appl Pharmacol. 202: 199-211, 2005.
  6. Cabantchik, ZI, Breuer, W, Zanninelli, G, et al. LPI-labile plasma iron in iron overload. Best Pract Res Clin Haematol. 18: 277-287, 2005.
  7. Britton, RS, Leicester, KL, and Bacon, BR. Iron toxicity and chelation therapy. Int J Hematol. 76: 219-228, 2002.
  8. Pullarkat V, Blanchard S, Tegtmeier B, et al. Iron overload adversely affects outcome of allogeneic hematopoieticcell transplantation. Bone Marrow Transplant 2008;42:799–805.
  9. Platzbecker U, Bornhauser M, Germing U, et al. Red blood cell transfusion dependence and outcome after allogeneic peripheral blood stem cell transplantation in patients with de novo myelodysplastic syndrome (MDS). Biol Blood Marrow Transplant 2008;14:1217–1225.
  10. Fung EB, Harmatz P, Milet M, et al. Morbidity and mortality in chronically transfused subjects with thalassemia and sickle cell disease: A report from the multi-center study of iron overload. Am J Hematol 2007;82:255–265.
  11. Kushner JP, Porter JP, Olivieri NF. Secondary iron overload. Hematol Am Soc Hematol Educ Program 2001;47–61.
  12. Ballas SK. Iron overload is a determinant of morbidity and mortality in adult patients with sickle cell disease. Semin Hematol 2001;38:30–36.
  13. Aduloju SO, Palmer S, Eckman JR. Mortality in sickle cell patient transitioning from pediatric to adult program: 10 years Grady comprehensive sickle cell center experience. Blood 2008;112: Abst 1426.
  14. Olivieri NF, Nathan DG, MacMillan JH, et al. Survival in medically treated patients with homozygous b-thalassemia. N Engl J Med 1994;331:574–578.
  15. Zurlo MG, De Stefano P, Borgna-Pignatti C, et al. Survival and causes of death in thalassaemia major. Lancet 1989;2:27–30.
  16. Modell B, Khan M, Darlison M. Survival in b-thalassaemia major in the UK: Data from the UK Thalassaemia Register. Lancet 2000;355:2051–2052.
  17. Borgna-Pignatti C, Rugolotto S, De Stefano P, et al. Survival and disease complications in thalassemia major. Ann NY Acad Sci 1998;850:227–231.
  18. Cohen AR, Galanello R, Pennell DJ, et al. Thalassemia. Hematol Am Soc Hematol Educ Program 2004;14–34.
  19. Batra AS, Acherman RJ, Wong WY, et al. Cardiac abnormalities in children with sickle cell anemia. Am J Hematol 2002;70:306–312.
  20. Fung EB, Harmatz PR, Lee PD, et al. Increased prevalence of iron-overload associated endocrinopathy in thalassaemia versus sickle-cell disease. Br J Haematol 2006;135:574–582.
  21. Fung EB, Harmatz PR, Milet M, et al. Disparity in the management of iron overload between patients with sickle cell disease and thalassemia who received transfusions. Transfusion 2008;48:1971–1980.
  22. Karam LB, Disco D, Jackson SM, et al. Liver biopsy results in patients with sickle cell disease on chronic transfusions: Poor correlation with ferritin levels. Pediatr Blood Cancer 2007;50:62–65.
  23. Porter JB, Huehns ER. Transfusion and exchange transfusion in sickle cell anaemias, with particular reference to iron metabolism. Acta Haematol 1987;78:198–205.
  24. Adamkiewicz TV, Abboud MR, Paley C, et al. Serum ferritin level changes in children with sickle cell disease on chronic blood transfusion are non-linear, and are associated with iron load and liver injury. Blood 2009;114:4632–4638.
  25. Sickle Cell Society. Standards for the clinical care of adults with sickle cell disease in the UK. 2008. Available at: www.sicklecellsociety.org/CareBook.pdf.
  26. Inati A, Musallam KM, Cappellini MD, et al. Non-transferrinbound iron in transfused patients with sickle cell disease. Int J Lab Hematol 2010; DOI: 10.1111/j.1751-553X.2010.01224.x. Epup ahead of print.
  27. Abetz LN, Baladi JF, Jones P. Transfusion and iron overload (IO) in MDS: Impact of infusion chelation therapy (ICT) on quality of life (Qol) and adherence. Leuk Res 2005;29:S54.
  28. Treadwell MJ, Law AW, Sung J, et al. Barriers to adherence of deferoxamine usage in sickle cell disease. Pediatr Blood Cancer 2005;44:500–507.
  29. Schnog JB, Duits AJ, Muskiet FA, et al. Sickle cell disease: A general overview. Neth J Med 2004;62:364–374.
  30. Steinberg MH, Barton F, Castro O, et al. Effect of hydroxyurea on mortality and morbidity in adult sickle cell anemia: Risks and benefits up to 9 years of treatment. JAMA 2003;289:1645–1651.
  31. Verduzco LA, Nathan DG. Sickle cell disease and stroke. Blood 2009;114:5117–5125.
  32. Hebbel RP. Reconstructing sickle cell disease: A data-based analysis of the “hyperhemolysis paradigm” for pulmonary hypertension from the perspective of evidence-based medicine. Am J Hematol 2011; 86:123-154.
  33. Blaivas, Allen J. “Pulmonary hypertension.” NCBI PubMed Health. 7 May 2011. <http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001171/>
  34. Hebbel, RP. Reconstructing sickle cell disease: A data-based analysis of the “hyperhemolysis paradigm” for pulmonary hypertension from the perspective of evidence-based medicine. American Journal of Hematology. 86:123-154, 2011.
  35. Gladwin MT, Sachdev V, Jison ML, et al. Pulmonary hypertension as a risk factor for death in patients with sickle cell disease. N Engl J Med 2004;350: 886–895.
  36. Simonneau G, Robbins IM, Beghetti M, et al. Updated clinical classification of pulmonary hypertension. J Am Coll Cardiol 2009;54:S43–S54.
  37. Powars DR. Sickle cell anemia: Beta s-gene-cluster haplotypes as prognostic indicators of vital organ failure. Semin Hematol 1991;28:202–208.
  38. Platt OS, Brambilla DJ, Rosse WF, et al. Mortality in sickle cell disease. Life expectancy and risk factors for early death. N Engl J Med 1994;330:1639–1644.
  39. Graham JK, Mosunjac M, Hanzlick RL, et al. Sickle cell lung disease and sudden death: A retrospective/prospective study of 21 autopsy cases and literature review. Am J Forensic Med Pathol 2007;28:168–172.
  40. Marti-Carvajal AJ, Sola I, Agreda-Perez LH. Treatment for avascular necrosis of bone in people with sickle cell disease (Review). The Cochrane Collaboration 2010, Issue 11