Delayed Onset Exercise Induced Hyponatremia During Multiday Mountain Bike Race

HISTORY: A 51-year-old male biker came for consultation a week after participating in a 4-day mountain bike race. Apparently, towards the end of the 3rd riding day, he felt confused and dizzy, had severe muscle spasms, weakness and imbalance and needed assistance to reach the finish line. He immediately entered a recovery cooling pool for 10 minutes and then walked to the race clinic where he received 1000 ml NaCl 0.9% IV and was discharged. During the same night, he felt dehydrated and drank 8 liters of water! In the morning he was examined again at the clinic as he was still feeling unwell, and was referred to the emergency department (ER). At the ER, he was found to be edematous, confused and emetic, lost his consciousness within few minutes and developed general convulsions. Blood laboratory studies showed sodium level of 119 mmol/L. The rider was sedated, intubated and admitted to the ICU with hypertonic sodium infusion. High blood pressure and fever were noted and chest X-ray demonstrated hilar opacity, therefore antibiotics and hypertensive medications were administered. Follow up blood tests showed elevation of CPK and liver enzymes, peaking at the 4th hospitalization day (CPK = 37,896, ALT = 177, AST = 596 U/L). After 24 hours, sedation was discontinued and the rider was discharged following four days of supervision. PHYSICAL EXAMINATION: Within normal limits 10 days after discharge. DIFFERENTIAL DIAGNOSIS: 1- Hyponatremia due to prolonged, intensive exercise in a hot climate + excessive drinking or pneumonia induced.2- Heat stroke with liver damage and rhabdomyolysis, followed by hyponatremia due to excessive drinking with additive muscle damage as a result of general convulsion. TESTS AND RESULTS: 1- Brain CT: No intracranial bleeding or edema.2- Follow up laboratory: Normal levels of electrolytes, CPK, liver enzymes and creatinine. FINAL WORKING DIAGNOSIS: His confusion & imbalance towards the end of the 3rd day raise the suspicion that the rider suffered from heatstroke, partially treated before referring to a medical facility. Dehydration & oliguria followed by excessive water intake induced severe hyponatremia. TREATMENT AND OUTCOME: The rider was instructed to monitor blood pressure and gradually return to activity with gradual heat exposure after 2 weeks of rest and with normal laboratory tests.