What is sickle cell trait?
People can dream, but their road to success can be derailed in the most unusual ways, perhaps none as strange as that which affected Ryan Clark of the Pittsburgh Steelers. When his team plays in Denver, the Mile High City, so named because it sits at an altitude of 5,280 feet, Clark is sidelined. This is because of an incident in 2007 in which Clark ended up critically ill requiring emergency surgery because of his genetics; he suffers from sickle trait, a condition that affects the shape and function of red blood cells. So when his teammates compete in the Mile High City, Clark is relegated to the sidelines.
Red blood cells are manufactured in the bone marrow. Their unique biconcave shape (think of squeezing a marshmallow between your fingers) increases their storage capacity for hemoglobin molecules that carry oxygen. They also make the cells pliable and soft so they can squeeze through the tiniest blood vessels in the body. In sickle disease, the red blood cells form an abnormal crescent shape that is rigid, causing the red blood cells to be damaged. The cells aren't malleable enough to get through tight spaces, and this can increase the risk of forming blood clots in the small capillaries of different organs causing the potential for organ damage.
What is sickle cell disease?
Sickle cell disease is a genetic disease that most commonly affects people whose heritage is usually African or Caribbean. Patients with full blown sickle cell anemia have inherited a sickle gene from each parent and are usually diagnosed in childhood as having this autosomal recessive disease. Autosomal means that the gene is not located on either the X or Y sex chromosome. Recessive means that you need to have two copies of the involved gene to have the disease. If only one parent passes on that "bad gene," the disease may not be present, or in the case of sickle disease, the patient is said to have sickle trait. People with sickle trait usually live normal lives and have very few, if any, symptoms. Unfortunately, that is not always the case.
In the U.S., full-blown sickle cell anemia affects about 100,000 people, but almost two million more have the sickle trait. In sickle trait, symptoms rarely occur but are usually are brought on by exercise, presumably because of lactic acid production as the body moves from aerobic to anaerobic production. This may be why Mr. Clark became ill playing football in Denver. There is less oxygen available in the air at high altitudes and the intense exercise and dehydration caused his normally-shaped red blood cells to morph into the sickle shape.
How does sickle cell affect the spleen?
Perhaps one of the most affected organs of the body in sickle disease is the spleen, whose job it is to filter the blood, get rid of debris, and to remove damaged red blood cells. Sickled red blood cells clog up the filter system and can cause the spleen to become enlarged, swell with blood, and cause shock. And a variety of other organs can also be involved, most often causing a pain crisis because the blood supply is interrupted when the sickle cells obstruct the small capillaries. Catastrophes like stroke, loss of vision, kidney failure, and other complications may occur. There are too many complications to discuss here, but commonly in sickle disease, spleen function is gradually lost as blood vessels become blocked and the spleen dies or infarcts. The role of the spleen in the immune system is also lost, and immunizations may be needed to fight off common causes of infection, especially the Pneumococcus bacteria. This same immunization is also given to any patient who has needed a splenectomy (an operation to remove the spleen).
How does sickle cell trait affect athletes?
A person who has sickle trait does not have any symptoms unless he or she is unlucky enough, because of unusual circumstances, to have normal red blood cells turn into sickled ones. The risk of sickle trait in athletes is well recognized. After the death of a college athlete in 2006, the NCAA began recommending testing of all athletes whose sickle status was not known (most states routinely test newborns). If sickle trait is present, adjustments in the training programs can allow the athlete to reach his or her full potential. Taking a few extra seconds to recover between sprints or repetitive training sets and having oxygen supplementation on the sidelines and at practice can decrease the buildup of lactic acid and minimize the risk of sickling.
Mr. Clark's story is a reminder that while a person may appear fit, there is always potential danger that lurks under the surface. There is a common theme that reasserts itself in medicine and in life; prevention is easier than dealing with disaster.
Medically reviewed by John A. Daller, MD
American Board of Surgery with subspecialty certification in surgical critical care
"Overview of the pulmonary complications of sickle cell disease"