Exertional rhabdomyolysis: an analysis of 321 hospitalised US military service members and its relationship with heat illness.

INTRODUCTION: Exertional rhabdomyolysis is a syndrome of muscle breakdown following exercise. This study describes laboratory and demographic trends of service members hospitalised for exertional rhabdomyolysis and examines the relationships with heat illness. METHODS: We queried the US Armed Forces Health Surveillance Center's Defence Medical Epidemiology Database for hospitalised cases of rhabdomyolysis associated with physical exertion from January 2010 July 2013. Descriptive statistics reported means and medians of initial, peak and minimal levels of creatine kinase (CK). Correlations explored the relationship between CK, creatinine, length of hospital stay (LOS) and demographic data. RESULTS: We analysed 321 hospitalised cases of exertional rhabdomyolysis. 193 (60.1%) cases were associated with heat; 104 (32.4%) were not associated with heat; and 24 (7.5%) were classified as medical-associated exertional rhabdomyolysis. Initial, maximum and minimal CK levels were significantly lower in heat cases: CK=6528 U/L vs 19 247 U/L, p=0.001; 13 146 U/L vs 22 201 U/L, p=0.03; and 3618 U/L vs 10 321 U/L, p=0.023) respectively, compared with cases of rhabdomyolysis with exertion alone. Median LOS was 2 days (range=0-25). In the rhabdomyolysis with exertion alone group and the rhabdomyolysis with heat group, LOS was moderately correlated with maximal CK (Spearman's ρ=0.52, p<0.001, and Spearman ρ=0.38, p<0.001, respectively). There was no significant difference in median LOS between the rhabdomyolysis with exertion alone and rhabdomyolysis associated with heat groups (2 vs 2, p value=0.96). CONCLUSION: Most hospitalisations for exertional rhabdomyolysis were associated with heat illness and presented with lower CK levels than cases without associated heat illness. These data add evidence that rhabdomyolysis with heat illness is a different entity than rhabdomyolysis with exertion alone. Differentiating exertional rhabdomyolysis with and without heat should inform future research on rhabdomyolysis prognosis and clinical management.