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Clinical Applications - Sickle Cell Disease (SCD)

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Sickle cell disease is a disorder that affects the red blood cells, normally red blood cells are round and flexible so they can travel freely through the narrow blood vessels. The hemoglobin molecule in the red blood cell, which is used to transport oxygen from the lungs to the rest of the body, has two parts: an alpha and a beta. Patients with sickle cell disease have a mutation in a gene on chromosome 11 that codes for the beta subunit of the hemoglobin protein. As a result, hemoglobin molecules don't form properly, causing red blood cells to be rigid and have a concave shape (like a sickle used to cut wheat). These irregularly shaped cells get stuck in the blood vessels, especially in bends and bifurcations, and are unable to transport oxygen effectively, causing pain and damage to the organs.

Figure 1: Sickle Cell morphology

Sickle cell disease is inherited in an autosomal recessive pattern. This means that a child will not inherit the disease unless both parents pass down a defective copy of the gene. People who inherit one good copy of the gene and one mutated copy are carriers. They are clinically normal, but can still pass the defective gene to their children.

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Figure 2: Sickle Cell Anemia probability

Sickle cell disease is much more common in people of African and Mediterranean descent. It is also seen in people from South and Central America, the Caribbean, and the Middle East. The high prevalence of the defective gene in these regions may be due to the fact that carriers of a mutation in the beta-subunit of hemoglobin are more resistant to malaria.

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Figure 3: Sickle Cell Anemia World distribution

Sickle cell disease prevents oxygen from reaching the important organs, without oxygen, the cells that make up these organs will begin to die. For example, the spleen is often destroyed in these patients resulting in some loss of immune function. As a result, these patients often experience frequent infections. The red blood cells of patients with sickle cell disease don't live as long as healthy red blood cells(10-20 days in comparison to 120 days), as a result, people with this disorder often have low red blood cell counts (anemia), which is why this disease is commonly referred to as sickle cell anemia. When sickle-shaped red blood cells get stuck in blood vessels this can cause episodes of pain called crises, other symptoms include: delayed growth, strokes, and jaundice (yellowish hue to the skin and eyes because of liver damage). In the past, people with sickle cell disease often died between ages 20 and 40. Thanks to a better care people now can live to the age of 50 and beyond.

Sickle cell disease can be diagnosed by a simple blood test. Babies and young children with sickle cell disease must take a daily dose of penicillin to prevent potentially deadly infections. Patients also take folic acid, which helps build new red blood cells. It is also advised people with sickle cell disease to get plenty of rest, drink lots of water, and avoid too much physical activity. Blood transfusions that provide a patient with healthy red blood cells are a common treatment. People with more severe cases of the disease can be treated with a bone marrow transplant. This procedure provides the patient with healthy red blood cells from a donor, ideally from a sibling.

Ischemic stroke will occur in over 10% of children with sickle cell disease (SCD) by the age of 20. Transcranial Doppler (TCD) is used to evaluate cerebral arterial hemodynamics, and it plays an important role for screening and prognosis of sickle cell disease patients. The stroke prevention trials in sickle cell anemia (STOP trials I and II) used TCD to screen and identify SCD patients at greatest risk of ischemic stroke, it found that patient with increased velocity in the distal internal carotid artery (dICA) and the proximal middle cerebral artery (pMCA) have the highest risk for stroke. Once this patients identified, confirmed and treated with prophylactic blood transfusion the STOP trial shows more than 90% reduction in risk of stroke for these patients. Since then the STOP trials results were anchored in the society of vascular ultrasound (SVU) guidelines for TCD in sickle cell anemia pediatric patient from November 2012.

 

The guidelines states that a TCD examination should be done once or twice a year for SCD patient from the age of 2 years old. If the mean flow velocity at the dICA and the pMCA is more than 200cm/sec in two consecutive examinations it will indicate high risk of stroke and the patient will be treated with prophylactic blood transfusion.

 

Digi-Lite TCD is a non-invasive, easy to use, repeatable, portable, and cost-effective method for SCD patients screening, it is the common and popular method for SCD application throughout the world.