Exertional Sickling: Assessing and Addressing External Risk Factors for Athletes with Sickle Cell Trait

exertional sickling

In 2007, NFL safety Ryan Clark traveled to Denver, Colorado to play against the Broncos with his team, the Pittsburg Steelers. During the game, Clark experienced excruciating pain in his left side. After being rushed to the hospital Clark was diagnosed with a spleen problem that was a result of an exertional sickling crisis. Clark had surgery to remove both his spleen and gall bladder, lost 30 lbs. during his hospitalization, and was deactivated for the remainder of the season.

Upon regaining his health and returning to the Steelers the following season, Clark remained prohibited from playing all future games in Denver. Clark had previously been aware that he was a carrier of the trait for sickle cell anemia, however, he was unaware of the very serious risk his condition posed to his health, or that performing high-intensity athletics at increased elevation could trigger a life-threatening attack.

What was the factor that brought about Clark’s exertional sickling emergency? And how can we protect other athletes who are at risk of a similar event?

Understanding Sickle Cell Trait and the Risk of Exertional Sickling

An athlete who carries the sickle cell trait (SCT) may experience no general health crisis but are susceptible to catastrophic episodes of exertional sickling—a potentially life-threatening condition resulting from the sickling of red blood cells under conditions of intense exercise sustained for a few minutes.1



Risk factors for individuals with SCT can be broken into two categories: internal and external.

    • Internal risk factors include respiratory conditions, other illnesses, poor sleep, inadequate nutrition, and hypohydration.
    • External risk factors include altitude, hyperthermia of muscle tissue perpetuated by inappropriate work to rest ratios or physical effort greater than fitness level, and pressures to perform.

In the case of Ryan Clark, the primary risk factor was playing in Denver, which is known as “The Mile High City” due to the elevation being 5,280 feet (1609 meters) above sea level. Low atmospheric pressure at altitudes above 4,920 feet (1,500 meters) lowers a person’s oxygen saturation levels (i.e., oxygen attachment to hemoglobin), and individuals with SCT hemoglobin genes already have reduced oxygen carrying capacity.

For a person living with SCT, altitude significantly reduces oxygen levels resulting in a condition called hypoxemia, a below-normal level of oxygen in one’s blood, specifically the arteries.2 In Ryan’s case, we can easily speculate that severe hypoxemia resulted in the organ tissues becoming ischemic.

Examining Further External Risks

An additional example of the external risk factors for athletes with SCT is the tragic case of Ted Agu. A carrier of SCT, Agu was a defensive lineman for the University of California, Berkeley who died shortly after a strenuous off-season conditioning workout.

The workout consisted of steep hill sprints during which the players were directed to run up and down 10 times while holding a thick rope together. During the workout, Agu showed visible difficulty in completing the drill, falling to his knees several times before collapsing halfway up the hill on the last lap.

This example involves multiple risk factors, but a primary external risk factor was hyperthermic muscle tissue which occurs when an individuals’ muscle tissue and core body temperature increase due to metabolic heat generation. Experts theorize that as muscle tissue temperature increases, the warmed blood encourages red blood cells to change shape from round to sickle.3,4 When a workout is created that does not allow an individual with SCT to set their own pace, such as holding onto a rope to keep up with the other team members who are running the hill, we know exertional sickling is extremely likely.

An individual does not need to reach critical core body temperatures (>104.5F) which constitute exertional heat stroke for red blood cells to sickle. Any level of hyperthermia (>98F) may encourage red blood cells to sickle. Whether Agu’s exertional sickling crisis was due to inappropriate work to rest ratios or physical effort greater than the fitness level is difficult to assert, as not all details of the case are available in media articles.

Key Takeaways for Athletic Trainers and other Healthcare Providers

If you are the health care provider of an athlete with SCT, recognizing altitude as a major risk factor is critical. If traveling, the sports medicine team should determine if the destination is above 1500 meters. If so, there should be a meeting with the sports medicine team and the athlete to decide if the travel should be terminated for the upcoming trip.

It is important to note that athletes with SCT can certainly complete appropriately planned conditioning sessions, just as their counterparts do. However, they need to be allowed to set their own pace and allowed rest breaks whenever they need it. This will reduce the likelihood of exertional sickling. Creating a good working relationship between the sports medicine team and the human performance team will ultimately place the safety of the athlete with SCT at the forefront while still ensuring they make athletic performance improvements.

To become more knowledgeable about working with athletes with SCT, Clint Haggard and I offer two comprehensive courses that focus on the etiology and recognition of exertional sickling, as well as it’s treatment and prevention.

  1. Centers for Disease Control and Prevention. (2020, December 14). What is sickle cell trait? Centers for Disease Control and Prevention. Retrieved October 13, 2021, from https://www.cdc.gov/ncbddd/sicklecell/traits.html.
  2. Mayo Clinic Staff. (2018, December 1). Hypoxemia (low blood oxygen) causes. Mayo Clinic. Retrieved October 13, 2021, from https://www.mayoclinic.org/symptoms/hypoxemia/basics/causes/sym-20050930.
  3. O'Connor, F. G., Franzos, M. A., Nye, N. S., Nelson, D. A., Shell, D., Voss, J. D., Anderson, S. A., Coleman, N. J., Thompson, A. A., Harmon, K. G., & Deuster, P. A. (2021). Summit on exercise collapse associated with sickle cell trait: Finding the “Way ahead.” Current Sports Medicine Reports, 20(1), 47–56.
  4. Casa, D. J., & Stearns, R. L. (2017). Preventing sudden death in sport and physical activity. Jones & Bartlett Learning.