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Update: Exertional Rhabdomyolysis Among Active Component Members of the U.S. Armed Forces, 2018–2022

Image of Cover3 650736. Exertional rhabdomyolysis is a pathologic muscle breakdown associated with strenuous physical activity.

Exertional rhabdomyolysis is a pathologic muscle breakdown associated with strenuous physical activity. A largely preventable condition, it persists as an occupational hazard of military training and operations, especially in high heat environments among individuals exerting themselves to endurance limits. During the 5-year surveillance period, unadjusted incidence rates of exertional rhabdomyolysis among U.S. service members declined by approximately 15%, from 43.1 cases per 100,000 person-years (p-yrs) in 2018 to 36.5 cases per 100,000 p-yrs in 2022. Consistent with prior reports, subgroup-specific rates in 2022 were highest among men, those younger than 20 years, non-Hispanic Black service members, Marine Corps or Army members, and those in combat-specific and “other” occupations. Recruit trainees had the highest rates of exertional rhabdomyolysis in 2021 and 2022, with incidence rates 10 times higher than all other service members. Prompt recognition of the symptoms of exertional rhabdomyolysis (muscular pain or swelling, limited range of motion, or the excretion of darkened urine after strenuous physical activity, especially in hot, humid weather) by health care providers is crucial to avoid the most severe consequences of this potentially life-threatening condition.

What are the new findings?

The 473 incident cases in 2022 of exertional rhabdomyolysis represent an unadjusted annual incidence rate of 36.5 cases per 100,000 p-yrs among the active component, the lowest rate observed between 2018 and 2022. Exertional rhabdomyolysis occurred most frequently from mid-spring until early autumn at installations that support basic combat/recruit training or major Army or Marine Corps combat units.

What is the impact on readiness and force health protection?

Exertional rhabdomyolysis is a potentially serious condition requiring vigilance for early diagnosis and aggressive treatment to prevent severe consequences. Service members who experience exertional rhabdomyolysis may be at risk for recurrence, which could limit their military efficacy and potentially predispose them to serious injury. The risk of exertional rhabdomyolysis can be reduced by command awareness of environmental conditions and troop fitness levels, with emphasis on graded, individual preconditioning for more strenuous training, and adhering to recommended work and rest ratios with appropriate hydration schedules, especially in hot, humid weather.

Background

Rhabdomyolysis is characterized by the breakdown of skeletal muscle cells and subsequent release of intracellular contents into the circulatory system. This damage to skeletal muscle is generally caused by high-intensity, protracted, or repetitive physical activity, usually after strenuous exercise at unaccustomed intensity or duration.1 Initiation of a new strenuous activity during high levels of environmental heat stress heightens risk of exertional rhabdomyolysis.1 Among members of the U.S. military, this condition is most commonly identified at recruit training and combat installations, where physiological adaptation and environmental acclimatization required for the first 90 days of basic training may predispose new recruits.2,3 Even carefully monitored athletes who are accustomed to intense training are at risk of exertional rhabdomyolysis,4 especially when exerting themselves to endurance limits.5 A history of heat illness and prior heat stroke have also been described as significant risk factors for recruits who sustained rhabdomyolysis,3,6 revealing the potential for comorbid conditions.

Rhabdomyolysis severity ranges from asymptomatic elevation in serum muscle enzyme levels to life-threatening disease associated with extreme enzyme elevations, electrolyte imbalances, acute kidney failure, disseminated intravascular coagulation, compartment syndrome, cardiac arrhythmia, and liver dysfunction.1,7-9 The characteristic triad of rhabdomyolysis symptoms are weakness, muscle pain, and red-to-brown urine due to high levels of myoglobin, which are accompanied by an elevated serum concentration of creatine kinase.7,8 Diagnostic criteria for exertional rhabdomyolysis include severe muscle symptoms (e.g., pain, stiffness, and/or weakness) with laboratory results indicating myonecrosis (usually defined as a serum creatine kinase level 5 or more times the upper limit of normal) following recent exercise.10

Each year, the MSMR summarizes the numbers, rates, trends, risk factors, and locations of exertional heat injury occurrences including exertional rhabdomyolysis. This report includes data from 2018 to 2022. Additional information about the definition, causes, and prevention of exertional rhabdomyolysis can be found in previous issues of the MSMR.2

Methods

The surveillance period ranged from January 2018 through December 2022 and includes all individuals who served in the active component of the Army, Navy, Air Force, or Marine Corps during that time. All data used to determine incident exertional rhabdomyolysis diagnoses were derived from routine Defense Medical Surveillance System (DMSS) records. These records document both ambulatory encounters and hospitalizations of active component members of the U.S. Armed Forces in fixed military and civilian (if reimbursed through the Military Health System [MHS]) treatment facilities worldwide. In-theater diagnoses of exertional rhabdomyolysis were identified from medical records of service members deployed to Southwest Asia or the Middle East whose health care encounters were documented in the Theater Medical Data Store.

For this analysis, a case of exertional rhabdomyolysis was defined as an individual with 1) a hospitalization or outpatient medical encounter with a diagnosis in any position of either “rhabdomyolysis” (International Classification of Diseases, 9th Revision [ICD-9]: 728.88; International Classification of Diseases, 10th Revision [ICD-10]: M62.82) or “myoglobinuria” (ICD-9: 791.3; ICD-10: R82.1) with a diagnosis in any position of 1 of the following: “volume depletion (dehydration)” (ICD-9: 276.5*; ICD-10: E86.0, E86.1, E86.9), “effects of heat and light” (ICD-9: 992.0–992.9; ICD-10: T67.0*–T67.9*), “effects of thirst (deprivation of water)” (ICD-9: 994.3; ICD-10: T73.1*), “exhaustion due to exposure” (ICD-9: 994.4; ICD-10: T73.2*), or “exhaustion due to excessive exertion (overexertion)” (ICD-9: 994.5; ICD-10: T73.3*).2 Each individual could be considered an incident case of exertional rhabdomyolysis only once per calendar year. 

To exclude secondary cases of rhabdomyolysis due to either traumatic injury, intoxication, or adverse drug reaction, medical encounters with diagnoses in any position of “injury, poisoning, toxic effects” (ICD-9: 800.*–999.*; ICD-10: S00.*–T88.*, except the codes specific for “sprains and strains of joints and adjacent muscles” and “effects of heat, thirst, and exhaustion”) were not considered indicative of exertional rhabdomyolysis.11 

For health surveillance purposes, recruit trainees were identified as active component members assigned to service-specific training locations during coincident service-specific basic training periods. Because of the lack of Army personnel data in November and December 2022, soldiers who started basic training during this period were not counted as recruits. Recruit trainees were considered a separate category of enlisted service members in summaries of exertional rhabdomyolysis by overall military grade.

In-theater diagnoses of exertional rhabdomyolysis were analyzed separately using the same case-defining criteria and incidence rules that identified incident cases at fixed treatment facilities. Records of medical evacuations from the U.S. Central Command (CENTCOM) area of responsibility (AOR) (i.e., Southwest Asia/Middle East) to a medical treatment facility outside the CENTCOM AOR were analyzed separately. Evacuations were considered case-defining if affected service members met the aforementioned criteria in a permanent military medical facility in the U.S. or Europe, from 5 days preceding until 10 days following their evacuation dates.

Medical data from sites using the new electronic health record for the Military Health System, MHS GENESIS, between July 2017 and October 2019 are not available in the DMSS—these sites include Naval Hospital Oak Harbor, Naval Hospital Bremerton, Air Force Medical Services Fairchild, and Madigan Army Medical Center. Medical encounter data for individuals seeking care at any of these facilities from July 2017 through October 2019 were not included in the current analysis.

Results

In 2022, there were 473 cases of rhabdomyolysis likely associated with physical exertion and/or heat stress (i.e., exertional rhabdomyolysis), with 35.3% (n=167) resulting in hospitalization (Table 1).

Table listing incident diagnoses and incidence rates of extertional rhabdomyolysis among the active component of the U.S. Armed Forces in 2022; click on table to open a 508-compatible PDF version

Consistent with prior annual reports, crude incidence rates remained highest among men, those younger than 20 years of age, non-Hispanic Black service members, Marine Corps or Army members, and those in combat-specific and “other” occupations. Recruit trainees continued to present the highest rates of exertional rhabdomyolysis in 2022, at a rate 10 times higher than officers and enlisted members. 

During the surveillance period, from 2018 through 2022, crude rates of exertional rhabdomyolysis declined by approximately 15% (Figure 1).

This graph uses stacked columns to depict the numbers of cases of exertional rhabdomyolysis diagnosed in active component service members of the U.S. Armed Forces during each year from 2018 through 2022. For each year, the height of the lower segment of the column corresponds to the number of cases detected through hospitalization records, and the upper segment represents the number of cases identified through records of ambulatory health care visits. A line connects points that reflect the crude annual incidence rates of exertional rhabdomyolysis in cases per 100,000 person-years (or p-yrs). The greatest total number of cases (556 in number) occurred in 2018 and the lowest number of cases (473 in number) was in 2022. In the years 2019 through 2021, case counts were 520, 508, and 517. The rates reflect the same trends as the case counts.  In each year from 2018 through 2022, rates were 43.1, 39.7, 38.4, 38.9, and 36.5 cases per 100,000 p-yrs.

This reduction was observed among all services except the Army (Figure 2).

This graph uses 4 horizontal lines representing each service branch connecting points which reflect annual incidence rates of exertional rhabdomyolysis among active component members of the U.S. Armed Forces for the years from 2018 through 2022. A separate line shows the rates for all services combined. The Marine Corps had the highest rates of any of the services each year. The lowest rate for the Marine Corps was 82.8 cases per 100,000 person-years (or p-yrs) in 2022. The next highest rates were those of the Army, for which the lowest rate was 48.2 cases per 100,000 p-yrs. The next lowest rates were those of the Air Force, for which the lowest rate was 14.3 cases per 100,000 p-yrs. The lowest of all rates were those of the Navy, for which the lowest rate was 11.6 per 100,000 p-yrs. For all 5 years of the surveillance period, the services’ annual rates retained the same ranking with respect to each other.

Since 2020, less than 40% of cases resulted in hospitalization, a notable decline from the proportions identified from inpatient data records in 2018 (47.3%) and 2019 (42.3%) (Figure 1). During 2018-2022, approximately three-quarters (75.9%) of cases occurred in the warmer months (May through October) (Figure 3).

This column chart depicts the cumulative number of cases by month of diagnosis for exertional rhabdomyolysis among active component service members over the 5-year period from 2018 through 2022. The 3 months with the highest cumulative number of cases over the 5-year period were July, August, and June, with 442, 441, and 359 cases, respectively. The months September and May were next in rank order, with 299 and 237 cases, respectively. December and February had the smallest numbers of cases, with 61 and 96 cases, respectively. Although the seasonal pattern of case counts was not surprising, cases were documented throughout the year.

Rhabdomyolysis by location

During the 5-year surveillance period, 12 installations diagnosed at least 50 cases each; combined, those 12 installations diagnosed more than half (56.7%) of all cases (Table 2).

Table listing incident cases of extertional rhabdomyolysis by installation (with at least 30 cases during the surveillance period) among the active component of the U.S. Armed Forces from 2018 to 2022; click on the table to open a 508-compatible PDF version

Four of these 12 installations support recruit/basic combat training centers: Marine Corps Recruit Depot (MCRD) Parris Island/Beaufort, SC; Fort Benning, GA; Joint Base San Antonio-Lackland, TX; and Fort Leonard Wood, MO; while 7 installations support large combat troop populations: Fort Bragg, NC; MCB Camp Lejeune/Cherry Point, NC; Marine Corps Base (MCB) Camp Pendleton, CA; Fort Hood, TX; Fort Shafter, HI; Fort Campbell, KY; Fort Carson, CO. From 2018 to 2022, MCRD Parris Island/Beaufort and Fort Bragg together accounted for about one-fifth (20.7%) of all cases (Table 2).

Rhabdomyolysis in Iraq and Afghanistan

Six cases of exertional rhabdomyolysis were diagnosed and treated in Iraq/Afghanistan during the 5-year surveillance period; half were diagnosed in 2018, with with 1 case each year from 2019 to 2021 and none in 2022 (data not shown). The majority of those deployed service members affected by exertional rhabdomyolysis were non-Hispanic Black (n=3) or non-Hispanic White (n=3), male (n=4), in the Army (n=5), enlisted (n=5), and in health care occupations (n=3). One active component service member was medically evacuated for exertional rhabdomyolysis during the surveillance period, in November 2020 (data not shown).

Discussion

The results of this report document a crude reduction of approximately 15% in exertional rhabdomyolysis rates from 2018 to 2022. Exertional rhabdomyolysis continues to occur most frequently from mid-spring through early autumn at installations that support basic combat/recruit training or major Army or Marine Corps combat units. Recruits can be exposed to environmental situations that require acclimatization to high heat and humidity during the warmer months, while Soldiers and Marines in combat units often perform rigorous unit physical training, field training exercises, and personal fitness training regardless of weather conditions.

The annual incidence rates of exertional rhabdomyolysis observed among non-Hispanic Black service members were higher than rates observed among members of other racial/ethnic groups. This observation has been attributed, at least in part, to increased risk of exertional rhabdomyolysis among individuals with sickle cell trait (SCT),12-15 for which the U.S. carrier frequency is approximately 1 in 13 Black/African Americans.16 A significant association between SCT and a risk of exertional rhabdomyolysis is supported by studies among U.S. service members.17,18 The rhabdomyolysis-related deaths of 2 SCT-positive service members (a Navy recruit and an Air Force member) in 2019 after physical training stress this potential risk.19,20 Although previous studies have established that SCT is associated with a 54% increase in risk of exertional rhabdomyolysis,17,18 its association with disease progression and severity is unclear and warrants further study. 

The findings of this report should be interpreted with consideration of its limitations. A diagnosis of “rhabdomyolysis” alone does not indicate cause. Ascertaining the probable causes of exertional rhabdomyolysis cases was attempted through a combination of ICD-9/ICD-10 diagnostic codes related to rhabdomyolysis with additional codes indicating effects of exertion, heat, or dehydration. Other ICD-9/ICD-10 codes were used to exclude cases of rhabdomyolysis that may have been secondary from trauma, intoxication, or adverse drug reactions. 

Recruit trainees were identified using an algorithm based on age, rank, location, and time in service, which was only an approximation and likely resulted in some misclassification of recruit training status. The imputation used to address the gap in Army personnel data from November and December 2022 is another potential source of misclassification, which may have resulted in an underestimation of Army recruits and periods of recruit training during the last quarter of 2022. Due to this data discrepancy, recruit rates should be interpreted with caution.

Management after treatment for exertional rhabdomyolysis, including the decision to return to physical activity and duty, is a persistent challenge for both athletes and military members.21 Service members who experience a clinically-confirmed exertional rhabdomyolysis event should be further evaluated and risk-stratified for recurrence before return to activity or duty.10,21,22 The Defense Health Agency publishes practice recommendations that provide a synopsis of care for initial management of exertional rhabdomyolysis, high-risk or recurrent exertional rhabdomyolysis, and inpatient care.23,24 The most severe consequences of exertional rhabdomyolysis are preventable with effective mitigation measures and hightened awareness of probability when environmental conditions favor muscular injury. Commanders and supervisors at all levels should ensure that guidelines for heat illness prevention are consistently implemented, maintain vigilance for early signs of exertional heat injury, and intervene aggressively when exertional rhabdomyolsis is suspected.10

References

  1. Rawson ES, Clarkson PM, Tarnopolsky MA. Perspectives on exertional rhabdomyolysis. Sports Med. 2017;47(suppl 1):33-49. doi:10.1007/s40279-017-0689-z
  2. Armed Forces Health Surveillance Branch. Update: exertional rhabdomyolysis among active component members, U.S. Armed Forces, 2014-2018. MSMR. 2019;26(4):21-26.
  3. Hill OT, Scofield DE, Usedom J, et al. Risk factors for rhabdomyolysis in the U.S. Army. Mil Med. 2017;182(7):e1836-e1841. doi:10.7205/MILMED-D-16-00076
  4. McKewon S. Two Nebraska football players hospitalized, treated after offseason workout. Omaha World-Herald. January 20, 2019. Accessed March 1, 2022. https://www.omaha.com/huskers/football/two-nebraska-football-players-hospitalized-treated-after-offseason-workout/article_d5929674-53a7-5d90-803e-6b4e9205ee60.html
  5. Raleigh MF, Barrett JP, Jones BD, Beutler AI, Deuster PA, O'Connor FG. A cluster of exertional rhabdomyolysis cases in a ROTC program engaged in an extreme exercise program. Mil Med. 2018;183(suppl 1):516-521. doi:10.1093/milmed/usx159
  6. Hill OT, Wahi MM, Carter R, Kay AB, McKinnon CJ, Wallace RF. Rhabdomyolysis in the U.S. active duty Army, 2004-2006. Med Sci Sports Exerc. 2012;44(3):442-449. doi:10.1249/MSS.0b013e3182312745
  7. Zutt R, van der Kooi AJ, Linthorst GE, Wanders RJ, de Visser M. Rhabdomyolysis: review of the literature. Neuromuscul Disord. 2014;24(8):651-659. doi:10.1016/j.nmd.2014.05.005
  8. Chavez L, Leon M, Einav S, Varon J. Beyond muscle destruction: a systematic review of rhabdomyolysis for clinical practice. Crit Care. 2016;20:135. doi:10.1186/s13054-016-1314-5
  9. Bosch X, Poch E, Grau JM. Rhabdomyolysis and acute kidney injury. N Engl J Med. 2009;361(1):62-72. doi:10.1056./NEJMra0801327
  10. O’Connor FG, Deuster P, Leggit J, et al. Clinical Practice Guideline for the Management of Exertional Rhabdomyolysis in Warfighters 2020. Bethesda, Maryland: Uniformed Services University. 2020.
  11. Armed Forces Health Surveillance Branch. Surveillance Case Definition: Exertional Rhabdomyolysis. Accessed March 1, 2023. https://www.health.mil/Reference-Center/Publications/2017/03/01/Rhabdomyolysis-Exertional
  12. Gardner JW, Kark JA. Fatal rhabdomyolysis presenting as mild heat illness in military training. Mil Med. 1994;159(2):160-163.
  13. Makaryus JN, Catanzaro JN, Katona KC. Exertional rhabdomyolysis and renal failure in patients with sickle cell trait: is it time to change our approach? Hematology. 2007;12(4):349-352. doi:10.1080/10245330701255254
  14. Ferster K, Eichner ER. Exertional sickling deaths in Army recruits with sickle cell trait. Mil Med. 2012;177(1):56-59. doi:10.7205/milmed-d-11-00106
  15. Naik RP, Smith-Whitley K, Hassell KL, et al. Clinical outcomes associated with sickle cell trait: a systematic review. Ann Intern Med. 2018;169(9):619-627. doi:10.7326/M18-1161
  16. Centers for Disease Control and Prevention. Data and Statistics on Sickle Cell Diseae. Accessed April 18, 2023. https://www.cdc.gov/ncbddd/sicklecell/data.html
  17. Nelson DA, Deuster PA, Carter R, Hill OT, Wolcott VL, Kurina LM. Sickle cell trait, rhabdomyolysis, and mortality among U.S. Army soldiers. N Engl J Med. 2016;375(5):435-442. doi:10.1056/NEJMoa1516257
  18. Webber BJ, Nye NS, Covey CJ, et al. Exertional rhabdomyolysis and sickle cell trait status in the U.S. Air Force, January 2009–December 2018. MSMR. 2021;28(1):15-19.
  19. Air Combat Command. U.S. Air Force Ground Accident Investigation Board Report. 20th Component Maintenance Squadron 20th Fighter Wing, Shaw Air Force Base, South Carolina. Fitness Assessment Fatality; 24 May 2019.
  20. Mabeus C. Autopsy reports reveal why two recruits died at boot camp. Navy Times. November 8, 2019. Accessed March 1, 2022. https://www.navytimes.com/news/your-navy/2019/11/08/autopsy-reports-reveal-why-two-recruits-died-at-boot-camp
  21. O’Connor FG, Brennan FH, Campbell W, Heled Y, Deuster P. Return to physical activity after exertional rhabdomyolysis. Curr Sports Med Rep. 2008;7(6):328-331. doi:10.1249/JSR.0b013e31818f0317
  22. Atias D, Druyan A, Heled Y. Recurrent exertional rhabdomyolysis: coincidence, syndrome, or acquired myopathy? Curr Sports Med Rep. 2013;12(6):365-369. doi:10.1249/JSR.0000000000000007
  23. DeHan PJ, Buchanan BK, DeGroot DW, O’Connor, FG. Initial Management of Exertional Rhabdomyolysis. Defense Health Agency. September 2022. Accessed April 25, 2023. https://www.hprc-online.org/sites/default/files/document/DHA_PR_Initial_Management_of_Exertional_Rhabdomyolysis_508.pdf
  24. DeHan PJ, Bartlett SI, Buchanan BK, DeGroot DW, O’Connor, FG. Inpatient Management of Exertional Rhabdomyolysis. Defense Health Agency. August 2022. Accessed April 25, 2023. https://www.hprc-online.org/sites/default/files/document/DHA_PR_Inpatient_Management_of_Exertional_Rhabdomyolysis_508.pdf

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