Newswise — SILVER SPRING, MD, April 2, 2015 – The annual incidence rates of heat stroke, exertional rhabdomyolysis and exertional hyponatremia among service members in the active component rose slightly in 2014, according to newly released health surveillance reports.

The increase in rates of these three conditions – which are often associated with increased physical activity among service members, especially in warm weather – occurred after they had either stabilized or decreased in previous years, according to the reports in the March issue of the Medical Surveillance Monthly Report (MSMR)published by the Armed Forces Health Surveillance Center (AFHSC).

“The 2014 increase in cases of these conditions is a reminder that the threat of heat-related illness is present whenever service members are exerting themselves in warm or hot weather,” said Dr. Julie Pavlin, deputy director of the AFHSC. “It is the responsibility of commanders, unit leaders, and training cadre to recognize the threat, to train service members in preventive countermeasures, and to enforce their implementation.”

In 2014, 344 incident cases of heat stroke occurred among active component service members with an overall crude incidence rate of 0.25 per 1,000 person-years (p-yrs) (Table 1). The annual incidence rate (unadjusted) of cases of heat stroke in 2014 was slightly higher than in 2013, and similar to rates in 2011 and 2012 (Figure 1). There were more heat stroke–related reportable events (83) and ambulatory visits (167) in 2014 than in 2013 but fewer hospitalizations (94). The total of all heat stroke cases in 2013 was 324.

In 2014, incidence rates of heat stroke were highest among males and service members younger than 20 years of age, Asian/Pacific Islanders, Marine Corps and Army members, recruit trainees, and combat-specific occupations (Table 1). Heat stroke rates in the Marine Corps were 50% higher than in the Army; Army rates were more than nine-fold those in the other two services; the rate was 86 percent higher among males than females. There were only 10 cases of heat stroke among recruit trainees, but their incidence rate was more than 52 percent higher than the rates for other enlisted members and officers.

The annual incidence rates of exertional rhabdomyolysis – the breakdown of muscle from extreme physical activity – increased nearly 50 percent during 2010 through 2014 (Figure 1). In 2014, total incident cases rose to 403 (184 hospitalizations and 219 ambulatory visits) compared to 385 in 2013 (176 hospitalizations and 209 ambulatory visits).

The highest incidence rates of rhabdomyolysis occurred in service members who were male; younger than 20 years of age; black, non-Hispanic; members of the Marine Corps and Army; recruit trainees; and in combat-specific occupations (Table 1). In 2014, the 98 cases of exertional hyponatremia – a low concentration of sodium in the blood during or after prolonged physical activity – represented an increase of 30 percent compared to the number in 2013 (75 cases) This rise followed a decrease of more than 50 percent from 2010 through 2013 (Figure 1).

In 2014, among the services, the highest overall incidence rate of exertional hyponatremia was in the Marine Corps (14.8 per 100,000 p-yrs), although the Army had the most cases during the year (n=35) (Table 1). Eighty-two percent of cases (n=80) in 2014 affected males, but the rate during the year was higher among females (8.7 per 100,000 p-yrs) than males (6.8 per 100,000 p-yrs)(Table 1).

In 2014 and in past 16 years overall, the highest age group-specific incidence rates of exertional hyponatremia affected the youngest (less than 20 years) service members (Table 1). Also, overall rates were higher among white, non-Hispanic than other race/ethnicity groups of service members. Rates among recruit trainees were more than double in 2014 and nearly quadruple overall the rates among other enlisted members and officers.

Click here to read all the reports. The MSMR is the flagship publication for the AFHSC, featuring articles on evidence-based estimates of the incidence, distribution, impact, and trends of illness in service members and associated populations.