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Sickle Cell Disease 镰状细胞贫血病

Sickle Cell disease (SCD) is an autosomal recessive disorder and is characterized by a structural abnormality in the beta-globin chain of the hemoglobin molecule within the red blood cells (RBC)   (Ndefo, U., et all., 2008).  Autosomal  refers to the fact that the beta-globin gene effects males and females equally.  Recessive  means that in order for the mutated gene to be expressed as a disease, both copies of the beta-globin gene must be mutant (NSW Health, 2015).

According to the Center for Disease Control and Prevention (CDC), when both parents have sickle cell trait (SCT), they have a 25% chance of having a child with sickle cell disease (SCD) with every pregnancy.  When both parents have SCT, they have a 50% chance of having a child with SCT with every pregnancy.  Meaning if both parents have the sickle cell trait there is a 50% chance your baby will be born with the sickle cell trait, but not have sickle cell.  During pregnancy, prenatal testing can be done to find out if a baby will have SCD, SCT, or neither one.  The prenatal tests chorionic villus sampling (CVS) and amniocentesis often are used to find out if the baby will have the disease or carry the trait. These tests usually are conducted after the second month of pregnancy.

Sickle Cell Anemia is called a recessive condition because you must have two copies of the sickle hemoglobin gene to have the disorder.  If one parent has sickle cell trait (HbAS) and the other does not carry the sickle hemoglobin at all (HbAA) then none of the children will have sickle cell anemia. There is a one in two (50%) chance that any given child will get one copy of the HbS gene and therefore have the sickle cell trait. It is equally likely that any given child will get two HbA genes and be completely unaffected (Sickle Cell Society, n.d.).

***Let’s draw a Punnett Square to determine the likelihood of both parents are carriers of sickle cell disease, having a baby with sickle cell disease: in this case, both parents are carriers of this genetic disorder, as “Ss”

S s
S SS Ss
s Ss ss

According to the Punnett Square table above, their chances of getting a child who is not a carrier and does not have the disease is 25%. Giving birth to a carrier like the parent, has the highest probability as shown on the table, 50%. And giving birth to a child that has the disease is also 25%, as the two recessive genes come together in the bottom right square of the table.

However, when one parent, who does not have sickle cell disease “SS”, grows up and marries someone who does have the disease “ss”,

S S
s Ss Ss
s Ss Ss

The chances of giving birth to a child with the disease is zero. Thus, in order for giving birth to a child with sickle cell disease, both parents must be carriers or have the disease themselves. ***

Sickle cell disorders occur most commonly in African, Mediterranean, Indian and Middle Eastern ancestry ( DeBaun, M.R., 2011).  In North America the disease is most prevalent in African Americans, Hispanic patients from the Caribbean, Central and South America. In the United States, the disease affects approximately 50,000 people.  Among newborn infants SCD occurs at the following prevalance:

  • African -Americans, 1 in 400,000 to 500,000.
  • Hispanics, 1 in 36,000.
  • Caucasions, 1 in 80,000.  (DeBaun, M.R., 2011).

“SCD is one of the most common severe, monogenic diseases worldwide.” ( Fernandes, T.A., et al., (2015).  The World Health Organization considers it as a global, public health problem.  Specific phenotype manifestation of SCD are varied, both in severity and frequency.  But it is important to understand that both genetic and acquired factors contribute to the above. Yet the most important acquired factor is the patient’s socioeconomic situation (Fernandes, .A., et al. 2015).

It is difficult enough to live with and manage SCD in a developed country with access to and the ability to pay for good health care.  The optimal setting would be a comprehensive center with a multidisciplinary team.  They would be able to diagnosis SCD at birth, ensure proper vaccinations, and education. Throughout the life span they would be able to include new treatment modalities, prevent or minimize complications and be sure patients actually received their medications (Claster, S. and Vichinsky, E.P., 2003).  But consider the health care in a low-income region of the world, a northern area of Brazil.  I would offer the suggestion that patient care in this region may be equal to or slightly worse that that of the underserved African-American population in the United States today.

These are some of the demographics seen in this region of Brazil:

  • Age:  approximately 20% of total patients with SCD are between ten and thirty years old, 20% of total in each decade.  After 35 years of age there is a significant decline.
  • 45 % of total are age 5 and under.
  • Gender: Males / Females: 53% / 47%.
  • Ethnicity: White / Mulatto, Black:  26% / 69%, other, 5% .
  • Education: over 18 years of age, 52% did not graduate high school. Illiteracy rate = 19.8%.
  • Work situation: over 18 years of age, 55% working, 44% not working.  Of those working, only 62% are making a minimum wage.  Others are making less.

Barriers to care include difficulty of transport to medical facility, inability to pay for or insufficient social services to receive medications, lack of ability to understand medical treatment, and lack of motivation and family assistance ( Fernandes, A., (2015).

The clinical course is characterized by painful crises caused by the occlusion of small blood vessels by sickled RBCs. People who with one copy for the sickling gene, heterozygous, are possess sickle cell trait, which is associated with benign clinical course.

Children with sickle cell disease are rarely symptomatic until late in first year of life, related to increased amounts of fetal hemoglobin. Child may start having signs of anemia, such as low Hemoglobin level, loss of appetite, paleness, weakness, fever, irritability, jaundice. According to Ferri (2017), with any precipitating factors, such as dehydration, infection, and trauma, may lead to sickle cell crisis. During sickle cell crisis, hemolysis and vaso-occlusive may occur, small blood vessels are occluded by the sickle-shaped cells, causing distal ischemia and infarction. According to Braun and Anderson (2017), the location of ischemic can occur in all extremities, spleen, kidneys, liver, and cerebral, in which may lead to chronic organ damage and or death (p. 155).

Pharmacological treatment for SCD remains the mainstay with emphasis on prevention of complications, reduction of disease progression and alleviation of symptoms. Thus, treatment should start with promote measurements in preventing sickling, such as immunization against childhood and community illnesses, encourage increased intake of fluids to keep body hydrated, avoid low oxygen environment such as high altitudes area, avoid strenuous physical exertion, avoid extreme heat and cold weather, and routine lab work, such as complete blood count to help monitor RBCs level and may determine infection, thus may initiate blood transfusion and antibiotic treatment as needed (Braun and Anderson, 2017, p.156).

  • In a multicenter study, the use of Hydroxyurea has been shown to decrease painful crises and mortality rates by 40-44%.  Hydroxyurea increases the amount of HbF, thus preventing the formation of HbS.  It also kills bone marrow cells which in turn will produce cells with a higher amount of HbF.  All of the above leads to fewer pain episodes, less anemia and thus fewer blood transfusions and overall improvement in the clinical manifestations of SCD (Braun and Anderson, 2017).
  • Infection is a constant concern to patients with SCD.  This is secondary to the loss of a functional spleen, causing a decrease in immune function and its ability to synthesize antibodies (Braun and Anderson, 2017).  It is therefore imperative for patients to keep abreast of all childhood immunizations and vaccines for community diseases.  Penicillin prophylaxis in children between three months and three years of age has been known to decrease infection rates by 83% (Ndefo, U., et al, 2008).
  • The hallmark, overt manifestation of SCD and primary cause of hospitalization is pain.  Proper management, identification of location and cause of pain and avoidance of precipitating factors is essential.  Although mild pain can be controlled with NSAIDS such as, ibuprofen and ketorolac, and analgesics such as tramadol, frequently the patient must be treated with opiates (Braun and Anderson, 2017).  In today’s opiod addiction climate, one should search for alternative methods of pain relief, as pain will be part of the continuum of this difficult disease.

There is only one real cure for SCD.  That is a bone marrow transplant.  Human lymphocyte antigen must be transplanted from an identical family member.  This potential cure, has a rate of disease-free survival of a healthy 83%, a transplant-related mortality rate of 7% and a graft rejection rate of 9%. That said, barriers exist.  Being an inherited disease, often in an underserved population, there is a lack of suitable bone marrow donors and also a need to identify patients with acceptable risk-to-benefit ratio (Ndefo, U., et al., 2008).

 

References:

Braun, C.A., and Andreson, C.M., (2017) Applied pathophysiology: a conceptual approach to the Mechanisms of Disease. (3rd edition) Baltimore: Wolters Kluwer.

Claster, S. and Vichinsky, E.P., (2003). Managing Sickle Cell Disease. British Medical Journal  327(7427): 151-155.

DeBaun, M.R., (2011). About Sickle Cell Disease-Demographics. Department of Pediatrics, Vanderbilt University, Nashville.

Fact Sheet44 / Sickle Cell Disease. (2015) NSW Health    www.genetics.edu.au 1-4.

Fernandes, T.A., de Medieros, T.M., Alves, J.J., Bezerra, C.M., Fernandez, J.,Sonati, M., (2015). Socioeconomic and demographic characteristics of sickle cell disease patients from a low-income region of northeastern Brazil.  Rev Bras Hematol Hemotes 37(3): 172-177.

Ferri, F. F. (2017) Ferri’s clinical advisor, 2017, 5 books in 1

Ndefo, U.A., Maxwell, A.E., Nguye, H., Chiobi, T.T., (2008) Pharmacological Management of Sickle Cel Disease.  Pharmacy and Therapeutics  33(4): 238-243.

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