Sickle cell anemia (sickle cell disease) is a disorder of the blood caused by inherited abnormal hemoglobin (the oxygen-carrying protein within the red blood cells). Hemoglobin is a protein in red blood cells that carries oxygen throughout the body. “Inherited” means that the disease is passed by genes from parents to their children. SCD is not contagious. A person cannot catch it, like a cold or infection, from someone else.
The abnormal hemoglobin, called hemoglobin S or sickle hemoglobin, causes distorted (sickled) red blood cells. The sickled red blood cells are fragile and prone to rupture. When the number of red blood cells decreases from rupture (hemolysis), anemia is the result. This condition is referred to as sickle cell anemia. The irregular sickled cells can also block blood vessels causing tissue and organ damage and pain. Sickle cell anemia is one of the most common inherited blood anemias. The disease primarily affects Africans and African Americans.
Cells in tissues need a steady supply of oxygen to work well. Normally, hemoglobin in red blood cells takes up oxygen in the lungs and carries it to all the tissues of the body. Red blood cells that contain normal hemoglobin are disc shaped (like a doughnut without a hole). This shape allows the cells to be flexible so that they can move through large and small blood vessels to deliver oxygen. Sickle hemoglobin is not like normal hemoglobin. It can form stiff rods within the red cell, changing it into a crescent, or sickle shape. Sickle-shaped cells are not flexible and can stick to vessel walls, causing a blockage that slows or stops the flow of blood. When this happens, oxygen can’t reach nearby tissues.
Figure A shows normal red blood cells flowing freely in a blood vessel. The inset image shows a cross-section of a normal red blood cell with normal hemoglobin. Figure B shows abnormal, sickled red blood cells blocking blood flow in a blood vessel. The inset image shows a cross-section of a sickle cell with abnormal (sickle) hemoglobin forming abnormal stiff rods.
The lack of tissue oxygen can cause attacks of sudden, severe pain, called pain crises. These pain attacks can occur without warning, and a person often needs to go to the hospital for effective treatment. The red cell sickling and poor oxygen delivery can also cause organ damage. Over a lifetime, SCD can harm a person’s spleen, brain, eyes, lungs, liver, heart, kidneys, penis, joints, bones, or skin.
Sickle cells can’t change shape easily, so they tend to burst apart or hemolyze. Normal red blood cells live about 90 to 120 days, but sickle cells last only 10 to 20 days.
The body is always making new red blood cells to replace the old cells; however, in SCD the body may have trouble keeping up with how fast the cells are being destroyed. Because of this, the number of red blood cells is usually lower than normal. This condition, called anemia, can make a person have less energy.
How is sickle cell anemia inherited?
Sickle cell anemia is inherited as an autosomal (meaning that the gene is not linked to a sex chromosome) recessive condition. This means that the gene can be passed on from a parent carrying it to male and female children. In order for sickle cell anemia to occur, a sickle cell gene must be inherited from both the mother and the father, so that the child has two sickle cell genes.
The inheritance of just one sickle gene is called sickle cell trait or the “carrier” state. Sickle cell trait does not cause sickle cell anemia. Persons with sickle cell trait usually do not have many symptoms of disease and have hospitalization rates and life expectancies identical to unaffected people. When two carriers of sickle cell trait mate, their offspring have a one in four chance of having sickle cell anemia. (In some parts of Africa, one in five persons is a carrier for sickle cell trait.)
The image shows how sickle hemoglobin genes are inherited. A person inherits two hemoglobin genes—one from each parent. A normal gene will make normal hemoglobin (A). A sickle hemoglobin gene will make abnormal hemoglobin (S).
In the image above, each parent has one hemoglobin A gene and one hemoglobin S gene, and each of their children has:
A 25 percent chance of inheriting two normal genes: In this case the child does not have sickle cell trait or disease. (Case 1)
A 50 percent chance of inheriting one hemoglobin A gene and one hemoglobin S gene: This child has sickle cell trait. (Cases 2 and 3)
A 25 percent chance of inheriting two hemoglobin S genes: This child has sickle cell disease. (Case 4)
It is important to keep in mind that each time this couple has a child, the chances of that child having sickle cell disease remain the same. In other words, if the first-born child has sickle cell disease, there is still a 25 percent chance that the second child will also have the disease. Both boys and girls can inherit sickle cell trait, sickle cell disease, or normal hemoglobin.
If a person wants to know if he or she carries a sickle hemoglobin gene, a doctor can order a blood test to find out.
What are the symptoms and treatments of sickle cell anemia?
Virtually all of the major symptoms of sickle cell anemia are the direct result of the abnormally shaped, sickled red blood cells blocking the flow of blood that circulates through the tissues of the body. The tissues with impaired circulation suffer damage from lack of oxygen. Damage to tissues and organs of the body can cause severe disability in patients with sickle cell anemia. The patients endure episodes of intermittent “crises” of variable frequency and severity, depending on the degree of organ involvement.
The major features and symptoms of sickle cell anemia include:
Fatigue and Anemia
Dactylitis (swelling and inflammation of the hands and/or feet) and Arthritis
Splenic Sequestration (sudden pooling of blood in the spleen) and Liver Congestion
Lung and Heart Injury
Aseptic Necrosis and Bone Infarcts (death of portions of bone)
Some features of sickle cell anemia, such as fatigue, anemia, pain crises, and bone infarcts can occur at any age. Many features typically occur in certain age groups.
Sickle cell anemia usually first presents in the first year of life. Infants and younger children can suffer with fever, abdominal pain, pneumococcal bacterial infections, painful swellings of the hands and feet (dactylitis), and splenic sequestration. Adolescents and young adults more commonly develop leg ulcers, aseptic necrosis, and eye damage. Symptoms in adult typically are intermittent pain episodes due to injury of bone, muscle, or internal organs.
Affected infants do not develop symptoms in the first few months of life because the haemoglobin produced by the developing fetus (fetal hemoglobin) protects the red blood cells from sickling. This fetal hemoglobin is absent in the red blood cells that are produced after birth so that by 5 months of age, the sickling of the red blood cells is prominent and symptoms begin.
The treatment of sickle cell anaemia is directed to the individual features of the illness present. In general treatment is directed at the management and prevention of the acute manifestations as well as therapies directed toward blocking the red blood cells from stacking together. There is no single remedy to reverse the anaemia. It is, therefore, important that affected individuals and their family members have an optimal understanding of the illness and that communication with the doctors and medical personnel is maintained.